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The higher speed at which an engine idles during warm-up. When first started, a cold engine needs more throttle opening to idle properly. On carbureted engines without computer idle speed control, a set of cam lobes on the choke linkage provides a fast idle speed (850 to 1200 rpm) during engine warm-up.



See Diagnostic Trouble Code and Trouble Codes. Click Here for more information about Fault Codes.



A principle of fuel system design wherein a signal from an oxygen sensor in the exhaust system is used to give a computer the input it needs to properly regulate the carburetor or fuel injection system in order to maintain a balanced air/fuel ratio. Also, a signal to a computer that reports on the position of a component, such as an EGR valve. Typically, the feedback device is a variable resistor.



An electronic carburetor that controls the air/fuel mixture according to commands from the engine control computer, typically through the operation of a duty solenoid.



The order in which the spark plugs fire. The firing order will vary depending on the engine application. The firing order must be correct otherwise the engine may not start or run prioperly. See Chevy Firing Orders, and Ford Firing Orders


A refrigerant that may ignite or burn. This includes butane, propane, isobutane and certain other hydrocarbons. Flammable refrigerants are considered dangerous because of their risks to service personnel as well as the occupants of a vehicle should there be a refrigerant leak into the passenger compartment or during a collision. Flammable refrigerants are not approved for use in mobile A/C systems by the EPA.



The name given to fault codes or trouble codes that are read by counting flashes of the Check Engine light or ABS warning light. Though not available on some systems, flash codes provide essential diagnostic information for troubleshooting problems. To read any codes that may be stored in the control module�s memory, the computer system must first be put into a special diagnostic mode by grounding a terminal on the vehicle�s diagnostic connector. The codes are then flashed out via the light. By carefully counting the flashes and pauses, a numeric code is deciphered that tells you which diagnostic chart to refer to in the service manual. A series of step-by-step checks must then be made to isolate the faulty component.



The process of using a chemical to remove sludge, dirt, rust or metallic debris from inside A/C system components, the purpose of which is to clean the system, restore proper refrigerant flow and prevent clogging. Or, refers to reverse flushing the cooling system to remove accumulated scale deposits and old coolant.



A large heavy wheel on the end of the crankshaft that helps the engine maintain momentum when the clutch is engaged. The flywheel also helps dampen engine vibrations. The flywheel should be resurfaced when the clutch is replaced to restore a smooth surface. Oil or grease on the surface of the flywheel can make the clutch slip and chatter. On some vehicles, the ignition timing marks are on the flywheel and are observed by peering through a hole in the bellhousing. The teeth along the edge of the flywheel are for the starter to engage when the engine is cranked. Nicked, broken or missing teeth can cause starting problems, so a damaged flywheel should be replaced. On vehicles equipped with an automatic transmission, the flywheel is lightweight stamped steel and resembles a spoked wheel. This is because the torque converter is quite heavy and provides the momentum. See Flywheel Resurfacing.



An alignment job that includes all four wheels, not just the front two. All vehicles can benefit from a four-wheel alignment, not just those with front-wheel drive or independent rear suspensions. The rear wheels have just as much influence over directional stability as those at the front, and that is why many vehicles need to have all four wheels aligned. Many problems such as a steering pull to one side, uneven tire wear on the rear tires or poor tracking can be caused by misaligned rear wheels. See Wheel Alignment and Fixing Common Alignment Problems.



A method of driving a vehicle by applying engine torque to all four wheels. Various schemes are used for 4WD including part-time, full-time and variable four-wheel drive. The primary advantage of four-wheel drive is increased traction, which is especially useful for off-road excursions or severe weather driving, but is of little practical value for normal driving. Because of the added friction in the drivetrain, a four-wheel drive vehicle typically gets significantly lower fuel mileage than a front- or rear-wheel drive vehicle. To help cut the drag, most 4WD drivetrains have a transfer case that allows the driver to select either two- or four-wheel drive depending on driving conditions. In trucks, you will often find locking hubs on the front wheels that can be locked in the "on" or free-wheeling position as needed. Some performance cars have full-time variable four-wheel drive and use a computer-controlled transfer case to route power between the wheels.



A system that uses all four wheels to steer the car. Turning the rear wheels in the opposite direction to the front at slow speeds can allow faster maneuvering and a much tighter turning radius. Turning the rear wheels in the same direction as those at the front at high speed allows sudden lane changes with much greater stability. Turning the rear wheels in the same direction as the front when parking makes parallel parking much easier.



The angle of a truck's frame with respect to the ground. The angle affects front caster. For every degree of change in the frame angle, caster also changes one degree. Raising the rear of a truck increases the frame angle (positive) while lowering the rear decreases it (negative).


Operating data such as sensor values, engine rpm, coolant temperature, vehicle speed, etc. that are captured or stored by a scan tool or by the OBD II system when a fault occurs. Can be used for diagnostic purposes, especially with intermittent faults.




An expansion plug located in the side of an engine block that is supposed to protect the block against freeze damage. Water expands when it turns to ice, and if the coolant does not have enough antifreeze protection it can freeze and crack the engine block. The freeze plugs (there are several) are supposed to pop out under such conditions to relieve the pressure on the block. Freeze plugs can often be a source of troublesome leaks as a result of internal cooling system corrosion. Ease of replacement depends on accessibility.



A registered trademark of the DuPont Corporation for their family of CFC refrigerants, which includes R-12.



A means of driving a vehicle by applying engine power to the front wheels instead of the rear wheels. There are advantages and disadvantages to front-wheel drive. On the plus side, the advantages go mostly to the vehicle manufacturers because it makes it easier for them to package a vehicle engine/drivetrain/body combination more efficiently. In other words, the same basic engine/drivetrain package can be installed under a variety of "different" model cars. The same basic engine/transaxle package Chrysler developed for their Omni and Horizon (which they basically copied from Volkswagen) can be found under all their current front-wheel drives ranging from the mini-vans to the sports sedans. Thus a manufacturer can save a bundle in tooling and development when he wants to bring a new front-wheel drive model to the showroom floor. As far as FWD being superior to RWD, it is mostly hype. Some people will argue that front-wheel drive handles better than rear-wheel drive while others will argue exactly the opposite. Porsche and Mercedes seem to be unimpressed by FWD, and most race cars are rear-wheel drive. On the negative side, some front-wheel drive cars have a tendency to "torque steer" (See Torque Steer), and transaxle problems can be very costly to repair because it often involves pulling the engine. See Front-Wheel Drive Guide .



A device installed in the fuel line to trap contaminants before they reach the engine. A plugged fuel filter can cause the engine to stall. The fuel filter should be replaced periodically for preventive maintenance. See Fuel Filters.



A method of fuel delivery whereby fuel is sprayed into the intake manifold or intake ports through a nozzle. Originally developed by the Robert Bosch Corp. Fuel injection replaced carburetors back in the 1980s because it allows faster and easier starting, better emissions and fuel economy. Different types include Throttle Body Injection (TBI), Multiport Injection (MPI) and Gasoline Direct Injection (GDI). Throttle Body Injection has one or two injectors mounted in a throttle body on the intake manifold. Multiport Injection has a separate fuel injector for each of the engine's cylinders mounted in the intake manifold ports. Gasoline Direct Injection also has a separate injector for each cylinder, but the injectors are mounted in the head and spray fuel directly into the cylinders. See Fuel Injection Basics.



A pump that moves fuel from the fuel tank to the engine. On older vehicles with carburetors, a low pressure engine-mounted mechanical fuel pump is used. On newer vehicles with electronic fuel injection, a high pressure electric fuel pump is used. If the fuel pump fails, the engine will stop and not restart. See Electric Fuel Pumps, Fuel Pump Diagnosis and Mechanical Fuel Pumps



Refers to fuel mixture adjustments made by the ECM or PCM in response to engine operating conditions. Fuel trim numbers above the zero (neutral) point indicate a lean fuel mixture while negative numbers indicate a rich mixture. Short term fuel trim varies with changes in throttle position and engine load, while long term fuel trim is a learned value that compensates for an engine's tendency to run rich or lean. Fuel trim numbers that deviate more than 10 to 15 points from the norm typically indicate a problem. For more information about using fuel trim to diagnose a lean fuel condition, read this article on Fuel Trim by Wells Manufacturing (PDF file, requires Adobe Acrobat to read).



A fuse is a protective link in a wiring circuit that is designed to burn out in case of an overload. The fuse has a tiny wire inside it that is designed to melt if the current exceeds a certain value. When the wire melts, it breaks the circuit and protects against damage or fire. Most fuses are located in the fuse box under the dash, although "in-line" fuses may be hidden elsewhere. "Fuse links" which are short sections of special wiring are also used to protect wiring circuits. The locations of both in-line fuses and fuse links can be looked up in a wiring diagram for the vehicle. When replacing a blown fuse, try to determine why the fuse blew. Always replace a fuse with one of the same rated capacity. Never substitute one of a higher capacity because the circuit may not be able to handle it. See Fuses & Relays.



A type of shock absorber that is pressurized with nitrogen gas to reduce internal foaming and cavitation. Considered to be a premium grade shock, gas shocks are often used as original equipment on sports sedans and even mini-vans. A gas shock usually provides noticeably better ride control and flatter cornering. They are well worth considering if you are in need of replacement shocks. See Diagnosing Ride Control Complaints .



A means of sealing the mating surfaces between various components. Gaskets are used between the various parts of the engine to keep oil, coolant, air and fuel in their respective places. Rubber, cork or combination cork/rubber gaskets are often used to seal the oil pan, valve covers, water pump and timing chain cover. Metal gaskets are used between the cylinder head and engine block, and metal or asbestos gaskets are used to seal intake and exhaust manifolds. Over time, cork gaskets tend to become brittle and break. This allows oil to leak out of the engine (See Oil Consumption). Tightening the cover bolts will sometimes stop a leak but usually the gasket must be replaced. Some late model engines use various chemical sealers (such as RTV silicone) in place of conventional gaskets. Leaks can be repaired by either applying fresh sealer or substituting a conventional gasket. See Preventing Repeat Head Gasket Failures.



When condensation builds up inside a fuel tank during the winter, water sometimes gets into the fuel line where it freezes in the low spots. The ice effectively blocks the flow of fuel and makes the car impossible to start. The only cure is to get the vehicle inside a warm garage where it can thaw out. There are several ways to prevent gas line freeze. One is to keep the fuel tank full so there is little room for condensation to form. Another is to dump an alcohol-based additive in the fuel tank at every fill to absorb moisture.



A mixture of various liquid hydrocarbons derived from crude oil. It is a non-renewable resource upon which we are overdependent and for which we will pay dearly if and when supplies run short. Depending on how it is refined and what is added to it, the fuel's quality can vary greatly (See Octane and Gasohol). Tetraethyl lead used to be used as a fuel additive to boost low-grade gasoline to a higher octane rating, but MBTE or ethanol is used now. Gasoline is highly flammable and should always be treated with respect. Never smoke when working on the fuel system (or when filling the fuel tank) and never use it as a cleaning solvent. See Bad Gasoline Can Cause Performance Problems, Bad Gas Update 2006, Fuel Octane Ratings & Recommendations and Alternative Fuels.



A type of fuel injection system that sprays fuel directly into each of the engine's cylinders under extremely high pressure (up to 2200 PSI or higher). Gasoline Direct Injection allows better fuel control, fuel economy and performance than multiport injection. See gasoline Direct Injection.



Same as "manifold gauge set." One, two or three pressure gauges attached to a manifold (a pipe with several inlet & outlet connections) used to measure A/C system pressures.



A gradual increase in the average temperature of the Earth blamed on human activity. The primary causes are the emission of carbon dioxide (CO2) by burning fossil fuels (coal-fired factories and vehicle emissions), deforestation and methane emissions (cow farts) from livestock and decaying crops. An international agreement called the Kyoto Protocol is supposed to limit CO2 emissions. The effects of global warming are uncertain, but may include climate change, melting of polar ice (rising sea level) and species extinctions (including possibly our own!). See Global Warming.



A gas that contributes to a gradual warming of the Earth's climate (Global Warming) as a result of increased heat retention. Certain gases (primarily carbon dioxide from the burning of fossil fuel, but also CFCs) increase the retention of heat from the sun in the atmosphere. See SUVs versus Trees.



A phenomenon in which voltage is generated by the action of a magnetic field acting on a thin conducting material, commonly used to control the primary circuit of an electronic ignition system. Named for the American scientist, Edwin Hall (1855-1938). The principle is used in Hall effect crankshaft position sensors and ignition pickups to produce a very clean on-off signal.



The name given to either of the two driveshafts that run from the transaxle to the wheels in a front-wheel drive vehicle. Halfshafts may be of solid or tubular construction, and of equal or unequal length side-to-side. See Front-Wheel Drive Guide.



A type of headlight that produces more light than an ordinary headlight. A halogen bulb burns brighter because it has a thinner filament. To keep the filament from melting, however, the gas mixture inside the bulb is altered slightly by adding a small amount of halogen gas (bromine, chlorine, fluorine, iodine or astatine) and sometimes krypton. See Headlights.



Same as "discharge pressure" in an A/C system, the amount of pressure in the compressor discharge line.



A system that maintains intake air at a more or less constant temperature by blending outside or underhood air with heated air picked up from a shroud over the exhaust manifold. A typical version uses a vacuum motor to power a door in the air cleaner snorkel, and a thermostatic bleed valve to control the signal to the vacuum motor. Also called Thermostatic Air Cleaner (TAC).A malfunction that prevents the door from closing can cause hesitation and stumbling when the engine is cold. An air temperature control flap stuck shut will overheat the air/fuel mixture, possibly causing detonation and elevated CO levels (due to a rich air/fuel ratio, as warm air is less dense than cold air). See Emission Guide.



A water-to-air heat exchanger that provides heat to the passenger compartment airstream. Hot coolant from the engine circulates through the tubes in the heater core.



A channel in an intake manifold through which exhaust gas flows in order to heat the manifold, thus aiding in fuel vaporization. Commonly used in V6 and V8 engines.



A control valve between the exhaust manifold and exhaust pipe on one side of a V8 or V6 engine that restricts the flow of exhaust causing it to flow back through the heat riser channel under the intake manifold. This aids fuel evaporation and speeds engine warm up. A heat riser valve stuck open will slow engine warm-up and may cause hesitation and stalling when the engine is cold. A valve stuck in the closed position will greatly restrict the exhaust system and cause a noticeable lack of power and drop in fuel economy.



Auxiliary springs that increase a suspension's load carrying capacity. These are typically bolt-on springs with a progressive action that do not come into play until the vehicle is loaded or the suspension deflects past a certain point. May be leaf or coil springs.



The refrigeration lines between the compressor outlet and orifice tube or expansion valve. The two longest high pressure lines are the "discharge" and "liquid" lines.



Same as "discharge side" in an A/C system, the part of the refrigeration circuit between the compressor outlet and orifice tube or expansion valve.



Same as "discharge pressure" or amount of pressure in refrigerant liquid line.



A unit of measure for quantifying power output. Invented by James Watt, the term was originally used to describe how much effort a horse exerted when lugging coal out of a coal mine. One horsepower was the amount of effort one horse put forth in raising 33,000 lbs. one foot in one minute. Engine horsepower ratings are determined on special equipment (See Dynamometer), and are usually expressed as so much "brake" horsepower (the amount of horsepower the engine actually delivers after internal friction and parasitic loses are taken into account). For more information on this subject, see Horsepower & Torque.



A temperature-sensitive carburetor valve that opens when the inlet air temperature exceeds a certain level. This allows additional air to enter the intake manifold to prevent overly rich air/fuel ratios.



A wheel that is centered or located on the hub by a machined center hole, as opposed to "lug centered" wheels that are located by the position of the lug nuts alone.



The amount or percentage of moisture in the air. This affects the perceived cooling performance of the A/C system, and also causes condensation to form on the inside of the windows on cold or rainy days.



Acronym for "Heating, Ventilation and Air Conditioning."



Refers to attracting or absorbing moisture. Refrigerants and lubricating oils and brake fluid will all pick up moisture in service. Moisture is undesirable because it can freeze and form ice that may block an A/C orifice tube, promote the formation of corrosion inside an A/C system and the brake system, and lower the boiling temperature of brake fluid (increasing the risk of pedal fade if the brakes get too hot). See Brake Fluid: A Hot Topic.



The large hex nut on the outer end of a front-wheel drive halfshaft that holds the shaft within the wheel hub. Most vehicle manufacturers recommend replacing this nut if it is removed for CV joint service.



A hydrocarbon (HC) is any kind of substance that contains hydrogen and carbon. Gasoline is a hydrocarbon. So is oil. When gasoline burns inside an engine, there is always a tiny amount that�s left over. If an engine is misfiring because of a fouled spark plug or a leaky valve, or if it has worn rings or valve guides and uses oil, quite a bit of unburned HC can pass through into the exhaust. Unburned HC is a major source of air pollution and is the primary source of smog in most urban areas. Various means are used to reduce the amount of HC an engine produces, the primary one of which is the catalytic converter. The converter reduces HC emissions by "reburning" and converting it into harmless water vapor. See Emission Guide.



A derogatory term for an instrument panel warning light. They are called idiot lights because they are for idiots who do not know how to read or understand gauges. Actually they are not all bad. An idiot light won't give you any indication that a problem is developing until it happens. On the other hand, a light commands more immediate attention than a gauge. The "ultimate" instrumentation should include both: gauges to give an accurate indication of coolant temperature, oil pressure and charging current, and lights to catch the driver's attention when readings approach the "danger zone." See Warning Lights and Troubleshoot Electronic Instrument Cluster



An electrically-operated valve which allows air to bypass the throttle plate in a fuel injected engine to regulate engine idle speed. See also Idle Surge (cause & cure)



A plastic device pressed over a carburetor's idle mixture screw which limits the amount of adjustment available during service. Also designed to discourage tampering that could increase emissions.



Adjusting the engine idle speed. Idle is not adjustable on many late model engines with computerized idle speed controls. How To Diagnose & Repair Carburetor Problems.



The air/fuel ratio that is delivered through the carburetor when the engine is idling. It can be adjusted by turning the idle mixture adjustment screw(s) on the carburetor. The screw opens up a little passage that lets more or less fuel into the engine. On most late model vehicles, the idle mixture screws have caps that allow only limited adjustment, or they are sealed to prevent tampering. The relative richness or leanness of the idle mixture has a big effect on tailpipe emissions at idle. See How To Diagnose & Repair Carburetor Problems.



This refers to how fast the engine runs when idling. It can usually be adjusted by turning a screw on the carburetor throttle linkage, or by turning an air bypass screw on a fuel injection throttle body. On most newer cars, however, it is computer-controlled and non-adjustable. The idle speed is programmed into the computer, and the computer regulates idle speed by opening and closing the idle air control valve. See also Idle Surge (cause & cure)



An electromagnetic device mounted on carburetor linkage that maintains the proper throttle opening for specified idle speed while the ignition is on, but allows the throttle to close farther when the ignition is switched off. This shuts off the engine's air supply to prevent engine run-on.



A pivot point in a parallelogram steering system that follows the motions of the pitman arm. A worn idler arm bushing typically causes steering wander (looseness) and toe wear.



The component in the ignition system that turns low voltage into high voltage to fire the spark plugs. When 12-volts passes through the coil's primary windings, it creates a strong magnetic field. Then when the current is shut off (by the ignition module or the opening of the contact points in older ignition systems), the magnetic field causes a surge of high voltage (as much as 40,000 volts) in the coil's secondary windings. The high voltage passes to the distributor, then on to the spark plugs where it jumps the plug gap and fires the plugs. Coil problems include shorts or opens in the internal wiring, and cracks around the high voltage terminal. See Distributor Ignition Systems,Distributorless Ignition Systems, Coil-Over-Plug Ignition Systems and Multi-Coil Ignition Systems. and Ignition Coil Diagnosis & Testing



The electronic control for the ignition system. The module receives a signal from a magnetic pickup or Hall effect sensor in the distributor. The module uses this signal to open and close the ground circuit to the ignition coil to fire the spark plugs. The ignition module itself may be located inside the distributor (GM and some imports), on the distributor housing (some Ford applications) or in the engine compartment. Some modules also control timing advance and retard. If a module goes bad, it usually goes completely dead. The engine won't run because there is no trigger voltage to the ignition coil.



The various components that control the igniting of fuel in the engine's cylinders. The ignition system has two parts: the primary side (the distributor and electronic control module), and the secondary side (the ignition coil, distributor cap, rotor, spark plug wires and spark plugs). In distributorless ignition systems, there is no distributor. Each cylinder has its own ignition coil, or coils are shared between paired cylinders that are opposite one another in the firing order. See Distributor Ignition Systems,Distributorless Ignition Systems, Coil-Over-Plug Ignition Systems and Multi-Coil Ignition Systems.


I/M 240

Stands for "Inspection/Maintenance", an "enhanced" emissions testing program with a tailpipe test that lasts 240 seconds. The test is done on a dynamometer to simulate actual driving conditions. The vehicle is put through a "driving trace" as it accelerates, decelerates and cruises at various speeds. The emissions are collected at the tailpipe and analyzed by a computer to computer the total amount of pollutants in grams per mile (gpm) that are being emitted. The test measures carbon monoxide (CO), unburned hydrocarbons (HC) and oxides of nitrogen (NOX). The I/M 240 test also includes a check of the vehicle's evaporative emissions control system to make sure that the fuel system is not leaking fuel vapors into the atmosphere, and a flow test of the canister purge control valve. See Diagnosing OBD II Emission Problems and Emissions Testing.



An anti-theft module that disables the ignition if an attempt is made to start the engine without a computer-coded key or fob. The module may be part of a keyless entry system.



The sum of the camber and SAI angles in a front suspension. This angle is measured indirectly and is used primarily to diagnose bent suspension parts such as spindles and struts. See Wheel ALignment.



A filter which may be installed in a fuel line, power steering pump discharge line, or A/C compressor discharge line to trap debris that might cause damage. See Fuel Filters.



The component in a diesel or gasoline fuel injection system that squirts fuel into the engine. In gasoline engine applications, the injector is usually electrically triggered. Varnish and dirt can build up in the nozzle opening restricting the flow of fuel. Injectors can be cleaned by using various fuel additives. In most diesel engines, the injectors are mechanical and deliver fuel under very high pressure directly into the cylinders. Clogging and leaking are two common problems with diesel injectors. Dirty injectors can be cleaned with fuel additives but leaky injectors must be replaced. See Troubleshooting Fuel Injectors.



The CV joint closest to the transaxle in a front-wheel drive car. See Front-Wheel Drive Guide (free download).



A heat exchanger that is added to a turbocharged engine to cool the air after it leaves the turbo. This increases air density and means more air can be pumped into the engine. The result is roughly a 10 to 15 percent improvement in horsepower. See Turbocharger Diagnosis & Repair.



When you drive over a bump and the suspension is momentarily compressed, that is called jounce. When it springs back, that is called "rebound." See Diagnosing ride control complaints .



A technique of starting one vehicle using another vehicle's battery. A pair of jumper cables are required to connect the terminals of both batteries together (positive to positive, negative to negative). The safest technique is first connect the positive terminals on both batteries to one another, and then to connect the negative terminal on the good battery to a ground (such as the engine block or frame) on the vehicle with dead battery. The final jumper connection usually sparks so keeping the spark away from the discharged battery avoids any danger of blowing up the battery. Once the jumper cables have been connected, the engine should be run at fast idle to help charge the dead battery for a couple of minutes. Then the first attempt to start the car should be made. If it does not start within 15 seconds, stop and wait a minute before trying again. This gives the starter a chance to cool off. Continuous cranking can ruin the starter and drain the good battery. See Battery Safety & Jump Starting.



A pin that serves as the pivot or hinge for the steering knuckle, used primarily on trucks with I-beam axles and older vehicles that do not have ball joints.



The angle formed by a line that runs through the king pin in the steering knuckle on a truck with an I-Beam axle. It�s the same as the steering axis inclination (SAI).



The amount of heat required for a change in physical state (phase change). The latent heat of vaporization is the amount of heat required to change a liquid into a vapor.



Also called axial runout, it is the amount of sideways motion or wobble in a wheel or tire as it rotates. It is usually measured by holding a dial indicator against the face of the rim or tire sidewall. A wheel with too much lateral runout will wobble back and forth as it rotates creating a shimmy that feels like dynamic imbalance problem. See Curing wheel & tire vibrations and Wheel Balancing.



A "Liquid Crystal Display" is a type of electronic display that forms opaque or dull-colored letters or numbers on various backgrounds. LCD displays are popular for digital dashboards, but they are not as readable in direct sunlight as LED displays.



A type of spring made out of a flat strip or individual leaves. Most are steel, but some are made of lightweight composite materials.



A condition caused by an air/fuel mixture that is too lean to sustain combustion. Lean misfire causes one or more cylinders to pass unburned fuel into the exhaust system causing a big increase in hydrocarbon (HC) emissions. Symptoms include a rough idle and hesitation or stumble on acceleration. Lean misfire is often caused by vacuum leaks or an EGR valve that is stuck open. See Engine Hesitation and Random Misfire.



A "Light Emitting Diode" is an electronic light bulb of sorts that produces colored light. You will find LEDs used in the center high mounted stop light on many vehicles, and used as indicator lights in some instrumentation. LED headlights and trim lights are also common on late model vehicles. LEDs are also used in some vehicle speed sensors and in some electronic ignitions.



Also called "followers" or "tappets," they are the components that ride on the cam lobes and help "lift" the valves open. There are two basic types: solid and hydraulic. Hydraulic lifters are hollow and fill up with oil to take up slack in the upper valve train. Low oil pressure, loss of pressure from the lifters or plugged oil holes in the lifters can result in a "clattering" sound that is referred to as "noisy lifters." Hydraulic lifters do not require periodic adjustment but solid lifters do to maintain the correct amount of valve lash.



In Clutch Cycling Orifice Tube (CCOT) systems, the line connecting the evaporator to the orifice tube. In systems using TXVs (expansion valves), the line connecting the receiver-dryer to the TXV valve inlet.



A type of electronic EGR system that uses a small motor to move the EGR valve's pintle in small steps, which provides precise control of gas flow. See Exhaust Gas Recirculation (EGR).



An evaporative emissions system component mounted above the fuel tank that prevents liquid gasoline from entering the vent lines.



A type of nut that is used to prevent another nut or threaded component from loosening and backing off.



Same as "suction line" in an A/C system, the line from the evaporator outlet to the compressor inlet. Refrigerant vapor passes through this line as it circulates back to the compressor.



Same as "suction side" of an A/C system, the side between the evaporator inlet and compressor inlet where refrigerant exists as a vapor.



A special kind of oversized shock absorber that is used as part of the vehicle's suspension. When used on the front suspension, it replaces the upper control arm and ball joint. Some struts have coil springs around them while others do not. Some struts have replaceable internal components that can be repaired by dropping in a new cartridge. See Diagnosing ride control complaints .



The "Check Engine" on the instrument panel light that comes on when the onboard diagnostic system detects a fault. The MIL light will come on to alert the driver if a fault may cause vehicle emissions to exceed federal emission limits. A vehicle with an illuminated MIL lamp will NOT pass an OBD II plug-in emissions test. The only way to turn off the lamp on an OBD II vehicle is to clear the fault codes with a scan tool. If the fault that set the code is not corrected, the MIL lamp will probably come on again after a period of time. See Check Engine Light Diagnostics, More on Fault Code Diagnostics and Trouble-Codes.



The temperature of in the intake stream or manifold. Used by the computer to calculate air density and to regulate the air/fuel mixture. The MAT sensor may be a separate component or incorporated into an airflow sensor. See Air Temperature Sensors.



The amount of vacuum created in the intake manifold by the pumping action of the engine's pistons. Vacuum is highest at idle and lowest at wide open throttle. Vacuum is measured in inches or millimeters of mercury. See Finding & Fixing Engine Vacuum Leaks.



Refers to a manifold absolute pressure sensor, a variable resistor used to monitor the difference in pressure between the intake manifold at outside atmosphere. This information is used by the engine computer to monitor engine load (vacuum drops when the engine is under load or at wide open throttle). When the engine is under load, the computer may alter spark timing and the fuel mixture to improve performance and emissions. See MAP sensors and Sensor Guide.



A device used in many fuel injected engines to measure the amount of air entering the engine so the computer can control the air/fuel mixture. Located ahead of the throttle body, the MAP sensor uses a heated wire or filament to measure airflow. See Mass Airflow Sensors or Sensor Guide.



When you step on the brake pedal, it pushes a piston inside the master cylinder which produces hydraulic pressure inside the brake system. The brake fluid reservoir is located on top of the master cylinder, and you will find both mounted on the firewall in the engine compartment on the driver's side of the vehicle. When the piston seals in the master cylinder eventually wear out, the cylinder may start to leak fluid and/or lose pressure. A brake pedal that gradually sinks to the floor is a sure sign of a bad master cylinder. The unit can be rebuilt or replaced (Note: aluminum master cylinders should never be honed because doing so removes the protective anodizing from the inside of the cylinder). See Master Cylinder.



Mounting a tire on a rim so the low spot of the rim lines up with the high spot on the tire. This reduces overall runout for a smoother running tire and wheel assembly. See Curing wheel & tire vibrations and Wheel Balancing.



A type of strut suspension where the coil spring is mounted between the lower control arm and chassis instead of around the strut. Typical applications include late model Mustangs and Camaros.



A self-diagnostic check that runs when certain enable criteria are met. OBD II vehicles have a variety of monitors to check various sensors and systems. The monitors must be "ready" (have run) before the vehicle will be accepted to take an OBD II plug-in emissions test. See OBD II Diagnostics.



Acronym for "Motorist Assurance Program," an organization that has developed voluntary uniform inspection guidelines and a code of ethics for the auto repair industry. See Motorist Assurance Program (MAP) website.



Abbreviation for Multi-port Fuel Injection, a type of fuel injection system that has one injector for each engine cylinder. Each injector sprays its fuel directly into the intake port in the cylinder head. Multi-port fuel injection is considered to be the "hot" setup because it provides better cylinder-to-cylinder fuel distribution for more power.



The lifeblood of the engine, it not only lubricates the engine but also cools the crankshaft bearings and pistons. As an engine runs, combustion blowby into the crankcase contaminates the oil with moisture, soot and unburned fuel. Moisture is the worst culprit because it forms acids and sludge. Additives in the motor oil (nearly a third of a can of oil is additives) fight the contaminants and give the oil special lubricating properties. The oil itself never wears out but the additives do. That is why the oil must be changed periodically to replenish the additives (See Oil Consumption). Changing the oil also gets rid of accumulated acids, sludge and moisture. The oil filter traps dirt (but not moisture) so it too should be replaced at every oil change. Use the recommended viscosity and type of oil listed in the owner's manual (See Viscosity). The difference between competing brands of motor oil is mostly advertising hype. Any oil of the proper viscosity that conforms to the highest America Petroleum Institute rating should be safe to use. Straight weight or non-detergent oils in late model engines is not recommended. See What Every Motorist Should Know About Motor Oil, Motor Oils & Lubricants, How Often Should You Change Your Oil? and API Motor Oil Classifications.



Abbreviation for Miles Per Gallon. A vehicle's fuel economy is determined by a number of factors including the size of the engine, the type of carburetion used, the weight of the vehicle, the type of transmission used (manual or automatic), the final drive ratio, the size and type of tires used, tire inflation pressures, aerodynamic streamlining of the body, the driving habits of the driver, the kind of road surface and terrain upon which the vehicle is driven, the speed at which its driven, and environmental factors such as temperature, wind and humidity. Taking all these into consideration, it is no wonder the EPA says "the mileage you get may vary from the official EPA estimates." See Fuel Saving Tips from the Car Care Council and Advice On Gas-Saving Gadgets.



MPGe is the abbreviation for Miles Per Gallon Equivalent, which is the equivalent amount of energy consumed by an electric vehicle compared to that of a gasoline powered vehicle. The MPGe fuel economy rating is calculated by figuring out the number of miles an electric vehicle can travel using the same amount of energy in a gallon of gallon of gasoline.



The device in the exhaust system that quiets the exhaust. A muffler is nothing more than a steel can full of baffles. Some (the so-called "glass-pack" mufflers) use Fiberglass batting to soak up noise. Mufflers rust out because exhaust is roughly 50 percent water vapor. The further the muffler is located from the engine, the more prone it is to rapid rust-through because the water vapor has more time to cool and condense. The best mufflers use metal that is galvanized on both sides. "Aluminized" mufflers or those that use galvanized metal on the outside only are not as rust-resistant. The worst (cheapest) mufflers are those with no protection at all.



A vehicle that neither understeers or oversteers. It responds predictably and evenly to steering inputs when cornering.



Abbreviation for the National Highway Traffic and Safety Administration. This is the government agency that is responsible for making and policing safety rules for all vehicles. NHTSA is the agency that can order a vehicle manufacturer to issue a safety recall. See National Highway Traffic & Safety Admin. (NHTSA) website.



Abbreviation for Oxides of Nitrogen. The "N" stands for Nitrogen, the "O" for Oxygen, and the "X" is scientific notation for all the various combinations of the two. NOX is formed inside an engine when combustion temperatures exceed 2500 degrees Fahrenheit. NOX is considered to be a serious air pollutant because it is so irritating. NOX emissions are minimized by the EGR valve, and by the catalytic converter in 1981 and newer model cars. See Exhaust Gas Recirculation (EGR) and Emission Guide.



Onboard Diagnostics II, A second generation emissions diagnostic system required on all 1996 and newer vehicles (though some 1994 and 1995 model year vehicles were equipped with early versions of the system). The OBD II system monitors vehicle emissions, and illuminates the Check Engine or Malfunction Indicator Lamp (MIL) if it detects a problem that causes emissions to exceed the federal limits by 50% or more. The OBD II system also stores diagnostic trouble codes to help technicians diagnose the cause of the emissions problem. OBD II cars and light trucks also have a standard diagnostic connector. See All About OBD II and OBD II Diagnostics.

See also for tons of OBD information.



This is a measure of a fuel's resistance to detonation (See Detonation). The higher the number, the better the fuel. Typical unleaded regular octane ratings range from 86 to 88. Premium grade unleaded fuels start around 89 and go as high as 93 or 94. By comparison, leaded premium fuels of a decade ago often started at 95 and went to over 100. The octane rating of gasoline can be boosted by additional refining and/or adding "octane boosting" chemicals such as benzene, alcohol or tetraethyl lead. Lead is a great octane booster but it ruins catalytic converters and oxygen sensors. Because of this, leaded fuel was phased out. See Fuel Octane Ratings & Recommendations.



A condition where the steering wheel is not centered or is crooked when traveling straight ahead. The condition contributes to toe wear because anytime the wheels are steered off dead center, they toe out slightly which increases side slippage and scrubbing. The underlying cause of off-center steering is often rear axle or toe misalignment, but it can also be caused by failing to center the steering prior to adjusting toe. If accompanied by a lead or pull to one side, the underlying cause may be cross camber, cross caster, uneven tire pressure or mismatched tire sizes side to side. See Correcting Steering Pulls.



The position of the backside of the wheel center section with respect to the centerline of the rim. If the center is closer to the back of the wheel, is has "negative" offset. If the center is closer to the outside face or front of the wheel, it has "positive" offset. Most wheels on FWD cars have positive offset.



All engines use a small amount of oil over time. It gets past the piston rings and valve guide seals and is burned in the combustion chamber. A small amount escapes through the PCV system and a few drops usually managed to seep through a gasket or seal. The question is at what point should one consider oil consumption to be a problem? Any engine that consumes less than a quart of oil every 3000 miles is in excellent mechanical condition. If it uses less than a quart in 1500 miles, it is still in pretty good condition. But once oil consumption exceeds a quart every 1000 miles, it signals the engine is approaching retirement. Blue smoke in the exhaust or oil consumption on the order of a quart or more every 500 miles indicates serious oil burning problems (usually due to worn or broken piston rings, a cracked piston, or worn valve guides and/or seals). Sometimes a leaky seal or gasket can make an otherwise good engine use oil. The most frequent leak points are valve cover gaskets, crankshaft end seals and oil pan gaskets. Tightening the valve cover or pan bolts can sometimes stop a leak but usually the only cure is to replace the gasket See Causes of High Oil Consumption.



A heat exchanger for cooling oil. Most automatic transmissions are equipped with an oil cooler that is located inside the radiator. Since the radiator usually runs close to 200 degrees, the amount of "cooling" this kind of setup provides is questionable. An aftermarket oil cooler that can be installed outside the radiator can provide much better cooling, and is recommended for towing or high performance applications. Except for air-cooled engines (older Volkswagen Beetles for example) and race cars, most engines do not use an oil cooler for the engine. The engine's cooling system is usually adequate to keep oil temperatures within safe limits.



The amount of pressure created in an engine's oil system by the oil pump. A certain amount of oil pressure is needed to circulate oil throughout the engine and to maintain adequate lubrication. Low oil pressure or loss of pressure is dangerous because it can lead to expensive engine damage. A low oil level in the oil pan, oil leaks, dirty oil, diluted oil (with gasoline), too low a viscosity oil, a plugged oil pickup screen or oil filter, a worn oil pump or worn main bearings can all contribute to low oil pressure. Complete loss of oil pressure usually results from a broken oil pump drive shaft (if the pump is driven off the camshaft). Unless the engine is shut off immediately, it will be ruined. Oil pressure is monitored by a sending unit mounted on the engine block. Oil pushes against a spring-loaded diaphragm, which in turn is connected to a resistor or set of contacts that trigger a warning light if pressure drops below about 4 or 5 psi. See Troubleshooting Low Oil Pressure and Oil Pump Diagnosis .



Software in the engine control module or powertrain control module that runs self-diagnostic checks on the control module, sensors and other related systems. When a fault is found, the software sets a diagnostic trouble code and turns on the MIL lamp. See OBD II Help.



Part of some vehicle's evaporative emissions (EVAP) control system, a vapor recovery system traps and stores fuel vapors when the vehicle is refueled.



A mode of operation in a computerized engine management system that occurs after a cold start. During open loop, the computer provides a fixed air/fuel ratio that is richer than normal to improve cold drivability until the engine warms up. See also Closed Loop. See Understanding Engine Management Systems.



A metering device located just ahead of the evaporator on the high pressure side of an A/C system that restricts the flow of refrigerant into the evaporator. A small hole (the orifice) allows only a certain amount of refrigerant to pass through the device. The creates a pressure drop that allows the refrigerant to evaporate inside the evaporator.



The CV joint closest the wheel in a front- or rear-wheel drive vehicle. See Front-Wheel Drive Guide (free download).



This refers to a type of engine design that positions the camshaft in the cylinder head over the valves. It is a popular design on many four-cylinder, V6 and even some V8 engines. On engines that use a rubber belt to drive the overhead cam, the belt usually needs to be replaced somewhere around 60,000 to 90,000 miles (see the owners manual for specific recommendations).



When the temperature of the coolant exceeds the normal operating temperature range of the engine, it is said to be overheating. A number of things can cause this to happen. Idling for long periods of time in traffic during hot weather can cause overheating because the water pump does nott turn fast enough to circulate sufficient coolant through the system (put the transmission in neutral and rev up the engine to help cool it off). A defective thermostat can stick shut and prevent the coolant from circulating to the radiator (replace the thermostat). A leak that allows the coolant level to drop can result in overheating (fix the leak, then refill the cooling system). A defective or inoperative cooling fan can cause the engine to overheat. as can a slipping or broken fan belt (find the fault and fix it). If an engine overheats, turning on the heater can sometimes help increase cooling capacity enough to cool it down. In most cases, though, the engine should be shut off and allowed to cool. Running it hot can damage the engine. NEVER open the radiator cap on a hot engine. Steam and coolant can spray out under considerable force and burn you! Add coolant to the coolant reservoir, or wait until the engine has cooled to open and cap to add coolant directly to the radiator. See Overheating.



A condition where a tire contains more air pressure than the recommended amount for the tire size and load. Overinflation reduced rolling resistance but also increases ride harshness and the risk of tire damage. A tire's inflation pressure should never exceed the maximum rating on the tire sidewall. See Tire inflation tips.



A type of shock absorber that is equipped with a helper spring to keep the suspension from sagging when a vehicle is heavily loaded.



A handling trait wherein a vehicle tends to overrespond to changes that are made in the direction of the steering wheel. The rear end on a vehicle that oversteers will tend to spin around when the vehicle is turned sharply (see Understeer).



Any reaction in which a chemical joins with oxygen, as rusting or combustion.



A two-way catalytic converter or the chamber in a three-way converter that oxidized unburned hydrocarbons (HC) and carbon monoxide (CO) to reduce pollution. See Catalytic Converters.



See NOX.



A gaseous element given the chemical symbol O, and occurring as O2, which makes up approximately 20% of the earth's atmosphere. Necessary for combustion, and measured by an exhaust analyzer to identify lean fuel mixtures.



A component in the engine's computer control system that monitors the amount of oxygen in the exhaust. The computer uses this information to change the relative richness or leanness of the air/fuel mixture. Located in the exhaust manifold, the O2 sensor resembles a small spark plug on the outside. But inside it has a special zirconium element that produces a varying voltage once it gets hot. The lower the oxygen content, the higher the sensor's output voltage. The oxygen sensor is vulnerable to contamination and may be damaged by lead, silicone or phosphorus. A contaminated O2 sensor or one that has become sluggish with age can cause an increase in fuel consumption and emissions. On some older vehicles, replacement may be recommended at 50,000 miles for preventive maintenance. See Oxygen Sensors.



A molecular form of oxygen that contains three atoms of oxygen instead of the normal two. It is formed naturally by sunlight and electrical discharge. It has a pungent odor and a strong oxidizing effect. Ozone is broken down by natural chemical reactions, including reacting with chlorine which is present in R12 refrigerant.



Destruction of ozone in the ozone layer attributed to the presence of chlorine from manmade CFCs and other forces. The layer is thinning because ozone is being destroyed at a faster rate than it is being regenerated by natural forces. See Alternative Refrigerants.



A region in the stratosphere 12 to 35 miles up where the air is very cold and thin, and ozone is found in high concentrations. The ozone layer is continually replenished by solar radiation and screens out about 95 to 99% of the sun's UVC ultraviolet radiation.



A type of polyaklylene glycol lubricant used as a compressor oil mainly in original equipment R134a A/C systems. Various viscosities of PAG oils are specified by the vehicle manufacturers for specific A/C applications. Click Here for a list of PAG oil recommendations.



A type of steering linkage that uses a pitman arm, idler arm and center link to steer the front wheels. Used primarily on trucks and older rear-wheel cars, the system is so named because the center link always moves parallel to the axle.



A mechanical brake for locking the rear wheels when parking. When you pull on the parking brake handle or step on the parking brake pedal, it pulls a pair of cables that extend to the rear brakes. The cables work a lever mechanism that binds the rear shoes against the drums, or on rear disc brake-equipped vehicles locks the pads (or a pair of mini-shoes) against the rotor. The most common problem associated with the parking brake is corrosion in the cable sleeves, which can prevent the rear brakes from releasing once the brake has been applied. The best way to prevent this from happening is to use the parking brake frequently. See Parking Brake Service.



Solid particles, mostly carbon, found in vehicle exhaust. These types of emissions are associated primarily with diesel engines, and can be caused by a misadjusted or mistimed injection pump. See Emission Guide.



An emission control device in the exhaust system of a diesel engine that captures particulates before they can enter the atmosphere.



The Positive Crankcase Ventilation valve is an emissions control device that routes unburned crankcase blowby gases back into the intake manifold where they can be reburned. The PCV system is one of the oldest emission control devices, and also one of the most beneficial. Besides totally eliminating crankcase emissions as a source of air pollution, the constant recirculation of air through the crankcase helps remove moisture which otherwise would cause sludge to form. Thus the PCV valve extends the life of the oil and engine. The PCV valve requires little maintenance. The valve and filter should be replaced somewhere around 30,000 to 50,000 miles (see the vehicle owners manual for service intervals). See Positive Crankcase Ventilation (PCV)



Abbreviation for Port Fuel Injection, another name for a multi-port fuel injection system. The system uses one injector for each engine cylinder. Fuel is sprayed directly into the intake port for better cylinder-to-cylinder fuel distribution and more power. See Understanding Today's Fuel Systems.



The arm connected to the steering box sector shaft that moves side to side to steer the wheels.



A type of compressor oil that is compatible with both R134a and R12 refrigerants, as well as residual mineral oil that may still be in the A/C system. POE oil is often used when retrofitting an older R12 A/C system to R134a.



A means of controlling crankcase blowby emissions and removing moisture condensation from the crankcase to prolong oil life. See Positive Crankcase Ventilation (PCV)



Engine vacuum that is available above the throttle plates of a throttle body or carburetor. Used to advance ignition timing on older carbureted engines when the throttle is opened above its idle position.



A valve which passes or blocks the passage of vacuum to a vacuum-operated component, such as a distributor advance mechanism or EGR valve. The operation of the valve may be controlled by engine temperature or an electric solenoid.



Most vehicles use a vacuum booster to increase the pedal force applied to the master cylinder. Some use a hydraulic power unit that does they same thing with hydraulic pressure rather than vacuum. Power brakes require no special maintenance, but if the booster goes bad pedal effort will be noticeably higher. A loose or leaky vacuum hose to the booster unit is often all that is wrong. But if the booster itself is bad, it must be replaced. See Troubleshooting Power Brakes.



A means of hydraulically assisted steering. A belt-driven power steering pump creates system pressure. The pressurized fluid is then routed into a cylinder that helps push the wheels one way or the other when the steering wheel is turned. The two most common power steering complaints are noise and leaks. A slipping drive-belt on the power steering pump can produce a loud squeal, especially when turning sharply. A bad valve or bearings in the pump itself can make a growling noise. Leaks most often occur at hose couplings or on the power cylinder seals. In power rack & pinion steering units, internal leaks can be a major problem (which require replacing the entire unit with a new or rebuilt assembly). The only required maintenance for this system is to check the level of the power steering fluid periodically. If low, check for possible leaks, then add fresh fluid to the pump reservoir. Running the system low can ruin the pump. See Variable-Assist Power Steering Systems.



A damaging engine condition wherein the air/fuel mixture ignites spontaneously due to hot spots in the combustion chamber. Causes include engine overheating, spark plugs that are the wrong heat range (too hot), sharp edges on the combustion chamber, low octane gasoline, a lean fuel mixture, or carbon buildup in the combustion chamber. Preignition can burn holes in pistons and contribute to detonation.



A thrust load applied to a bearing such as a wheel bearings to minimize axial or sideways play. The amount of preload is critical with an adjustable wheel bearing because too little can contribute to steering wander while too much may cause premature bearing failure. Sealed wheel bearings are not adjustable. See Wheel Bearing & Seal Service.



A method of preventing problems by maintaining wear items according to a regular schedule. Lubricating, adjusting and replacing all wear items before they can cause problems contributes to trouble-free driving and longer vehicle life. Regular fluid checks, and fluid and filter changes are the most important items on any preventive maintenance checklist. See Preventive Maintenance Guidelines Chart.



Abbreviation for Pounds per Square Inch. Usually used when referring to tire inflation pressures, cooling system pressure or turbocharger boost pressure.



A method for controlling various functions by switching the current on and off. Varying the time (duty cycle) of the signal increases or decreases the operating speed or opening of the device being controlled (such as the fuel injectors, cooling fan, A/C compressor, etc.)

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