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Why Spark Plugs Need To Be Replaced

by Larry Carley copyright

One of the leading causes of hard starting is fouled or worn spark plugs. When a fuel injected engine that normally starts quite easily has to be coaxed to life, it often means the spark plugs are overdue for a change. As the electrodes wear, the voltage required to jump the gap and ignite the fuel mixture goes up. At the same time, accumulated deposits on the insulator can drain off voltage before it even has a chance to form a spark. So the engine fails to start or starts only reluctantly after prolonged cranking.

One of the reasons why spark plug sales take off when cold weather arrives is because many motorists put off changing the plugs until they absolutely have to. The spark plugs continue to rack up mile after mile until they have deteriorated to the point where they are causing noticeable starting and driveability problems.

Emission checks will catch a lot of bad spark plugs and force motorists to change plugs that need to be replaced. But in areas where emission checks are not required, the only incentives for changing the spark plugs are the driveability problems created by the plugs themselves. So many motorists today think they are saving money on maintenance by putting off a spark plug change until it is obvious the engine needs new spark plugs. Then and only then will they begrudgingly spend any money on a new set of spark plugs.


What motorists need to know is that spark plugs do NOT last forever, even the long-life 100,000 mile plugs. All spark plugs need to be changed sooner or later. Here's why:

REASON #1 To Replace Spark Plugs: New plugs maintain peak engine performance and efficiency. Every engine will misfire occasionally. But as the number of misfires per mile goes up over time, it increases exhaust emissions, wastes gas and reduces power. In the past, most motorist would not notice the gradual decline in ignition performance until it reached a point where it created a steady miss, caused the engine to run rough, buck or stall, or made it hard to start. Not so today. All 1996 and newer vehicles have an OBD II onboard diagnostic system that tracks ignition misfires. When the rate of misfires exceeds a certain limit and causes emissions to increase 50% over baseline levels, it illuminates a warning light. Too bad older vehicle do not have this watchdog system. So on older vehicles, replacing the spark plugs at the recommended service intervals for preventive maintenance will reduce the risk of misfires and maintain peak engine performance. For standard spark plugs, the service interval is typically every 45,000 miles. For platinum spark plugs, it is 100,000 miles.

A new set of plugs is not a cure-all for driveability and emissions problems, but in many cases a plug change can make a significant improvement. Changing the plugs can reduce hydrocarbon (HC) emissions up to several hundred parts per million, which may make the difference between failing and passing an emissions test.

REASON #2 to Replace Spark Plugs: New plugs improve cold starting. Bad plugs are often responsible for many cold weather "no start" service calls. Many times the battery has been run dead while cranking the engine because the plugs would not light the fire. When the old plugs are removed and examined, they are often found to be worn or dirty. Dirty spark plugs can cause fouling which results in misfiring. For more info on this subject, see Spark Plug Fouling.

New plugs reduce the voltage requirements on the ignition system, which decreases the chance of misfire while leaving more amps for the starter and injectors.

Wet fouled plugs can also prevent an engine from starting, but in many instances the fouling problem has nothing to do with plug wear or neglect. If an engine is flooded with fuel while it is being cranked, gasoline can soak the plugs and bleed off the ignition voltage before it forms a spark. Wet fouling tends to be more common on older vehicles that have carburetors because pumping the gas pedal can easily flood the engine with too much fuel. Flooding can also occur if the choke sticks, the float is set too high or the needle valve leaks. On fuel injected engines, wet fouling is less of a problem but can happen if a cold start injector leaks or there is a fuel calibration problem that creates an overly rich startup mixture. The cure in all cases is to wait for the plugs to dry out, or to remove the plugs and clean or replace them.

fouled spark plug Share

REASON #3 to Replace Spark Plugs: New plugs minimize the risk of catalytic converter failure. A single misfiring plug can dump enough raw fuel into the exhaust to overheat and damage the converter. The presence of higher than normal quantities of unburned gasoline in the exhaust will cause the operating temperature of the converter to soar, which may lead to a partial of complete meltdown of the converter substrate. This, in turn, may form a partial restriction or complete blockage in the exhaust that creates enormous backpressure and chokes off the engines ability to exhale. The engine may lack power, especially at higher speeds, and deliver terrible fuel economy. Or, it may stall and refuse to run after it is first started. Replacing the converter will solve the restriction problem. But unless the spark plugs are replaced, the new converter may soon die from the same ailment.


P> The spark plugs are the business end of the ignition system. Whether an engine has a conventional distributor or a direct ignition (distributorless) system, a good set of plugs is absolutely essential for peak performance.

The typical spark plug needs anywhere from 5,000 to 25,000 volts from the ignition coil before it will fire. The exact firing voltage depends on:

Fouled electrodes may not fire at all!

Reliable ignition, therefore, requires a hot spark from the coil, good plug wires to carry the juice, and spark plugs that are clean, in good condition and gapped properly. If any of these criteria are not met, the spark may not reach it intended destination causing the engine to misfire.

One way to tell if the plugs need changing is to look at a vehicle's odometer. If it has been more than the recommended number of miles (usually 30,000) since the spark plugs were last changed, it is time for a new set.

Another way to tell is to observe the secondary ignition pattern on an oscilloscope. If there is an open plug or wire, the plug will not fire causing the firing voltage to shoot up to the maximum output of the coil. Badly worn plugs or plugs that have been misgapped too wide will also increase the firing voltage dramatically (as can a bad rotor and/or ignition cables with excessive resistance). If the required voltage exceeds the maximum output of the system, the plugs may not fire. If the pattern shows initial secondary spikes approaching the upper voltage limits of the system, therefore, it is a sure sign that the plugs (and/or cap, rotor and cables) need attention.

A fouled plug (or shored ignition cable), on the other hand, will show an unusually low firing voltage.

Firing voltages should not vary by more than 3 kV cylinder to cylinder. A cylinder that shows an abnormally low firing voltage probably has a grounded spark plug (deposits bridging the electrode gap), or a shorted ignition cable. A cylinder that shows an abnormally high firing voltage compared to the others likely has an open ignition cable or a plug with a wide gap.

The plug firing time (spark firing line) portion of the secondary ignition display shows the duration of the spark in milliseconds (thousandths of a second). The average spark duration with the engine idling should be about 1.5 milliseconds.

A duration of less than 0.8 milliseconds would mean there either is not enough voltage to keep the spark going (low coil output), or the voltage is having trouble reaching its destination (excessive resistance in the plug wires).

A longer than normal spark (1.8 milliseconds or more) is an indication that the firing voltage is experiencing little resistance because a plug is fouled or grounded (or a plug wire is shorted) probably due to accumulated carbon deposits. Fouling can be a problem if a plug's heat range is too cold for the application (which can be solved by installing hotter plugs). But it may also be the result of excessive oil consumption due to worn valve guides or seals, worn rings, or even short trip stop-and-go driving.

Intermittent misfires can be caused by a variety of ignition, fuel or mechanical problems. Lean misfire occurs when there is too much air and not enough fuel, so the engine should be checked for air or vacuum leaks, dirty injectors, carburetion problems or a leaky EGR valve. If the misfire appears to "jump around" from cylinder to cylinder, a manifold vacuum leak or a leaky EGR valve may be the cause. But if the misfire is isolated to a single cylinder, a worn or fouled spark plug (or bad plug wire) is the most likely cause.


spark plug diagnosis chart
Click on image above to view Spark Plug Diagnosis Chart.

Examining the tips of the spark plugs as they are removed can reveal a great deal about the health and performance of an engine. The appearance and color of the deposits can reveal other problems that may need fixing:


When spark plugs are replaced, you might want to upgrade to a premium long-life platinum or iridium plug. These plugs cost a little more initially, but can actually save you money in the long run because they do not have to be replaced as often. Many of these plugs can go 100,000 miles or more. Such plugs would be a good choice for any vehicle where access to the spark plugs is a problem.

Another option is upgrading to "performance" spark plugs. These plugs typically have unique electrode configurations that increase spark exposure to the air/fuel mixture and have multiple edges to reduce the chance of misfire. Performance spark plugs are usually more expensive than standard or even long-life plugs, but may be a good alternative if you want the ultimate in ignition performance.

There are also spark plugs today that are specially designed for truck engines. Such plugs have increased fouling resistance and oversized electrodes for longer service life. Share

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Spark Plug Fouling

Don't Neglect the Spark Plugs

Original Equipment Spark Plugs, Are They Best?

Ford Motorcraft Spark Plug Breakage Problem (2004 - 2008 Ford Trucks w/5.4L V8, & 2005 - 2008 Mustang GT with 4.6L V8)

Bosch Platinum +4 Spark Plugs

Don't Use Ordinary Spark Plugs with Waste Spark DIS Ignition Systems

Spark Plug Wires

Analyzing Ignition Misfires

Spark Plugs & Ignition Performance

Distributorless Ignition Systems

Coil-Over-Plug Ignition Systems

Diagnosing An Engine that Won't Crank or Start

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