clean dirty fuel injectors
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How to Troubleshoot & Clean Dirty Fuel Injectors


Clean fuel injectors are a must for peak engine performance, fuel economy and emissions. If the injectors are dirty and can't deliver their normal dose of fuel, performance, fuel economy and emissions are all going to suffer. Dirty injectors can't flow as much fuel as clean ones, nor can they delivery the correct spray pattern that is so essential for clean, efficient combustion. The fuel feedback control system will compensate for the leaning effect once it is in closed loop, but it can't correct the underlying condition that is causing the problem.

The injectors need to be cleaned, if an engine is experiencing any of the classic symptoms of dirty injectors, such as lean misfire, rough idle, hesitation and stumbling on light acceleration, a loss of power, and higher hydrocarbon (HC) and carbon monoxide (CO) emissions.

Lean misfire may also trigger a misfire code and turn on the Check Engine light on 1996 and newer vehicles with OBD II systems. The code often will be a P0300 random misfire code, or you may find one or more misfire codes for individual cylinders, depending on which injectors are most affected.


It doesn't take much of a restriction in an injector to lean out the fuel mixture. A restriction of only 8% to 10% in a single fuel injector can be enough to cause a misfire. When this occurs, unburned oxygen enters the exhaust and makes the O2 sensor read lean. On older multiport systems that fire the injectors simultaneously, the computer compensates by increasing the "on" time of all the injectors, which can create an overly rich fuel condition in the other cylinders.

In turbocharged engines, dirty injectors can have a dangerous leaning effect that may lead to engine-damaging detonation. When the engine is under boost and higher rpms, it needs all the fuel the injectors can deliver. If the injectors are dirty and can't keep up with the engine's demands, the fuel mixture will lean out, causing detonation to occur.

All vehicles are vulnerable to injector clogging, but the ones that are most vulnerable and most likely to experience such driveability and emissions problems are older ones with pintle-style multiport injectors. Later injector designs are more resistant to clogging.

In the early pintle-style injectors, the nozzle's shape and orifice size determine how much fuel flows through the injector and the shape of the spray pattern. Most pintle-style injectors are designed to produce a cone-shaped spray pattern. But, if fuel deposits accumulate in the nozzle area, it can restrict fuel delivery and break up the spray pattern, causing a lean fuel condition and many of the problems just mentioned.

Where do the deposits come from? Mostly from the fuel itself. Gasoline is a mixture of many different hydrocarbons, including olefins, which are heavy, waxy compounds. The heavier the hydrocarbon, the more energy it yields when it burns. When the engine is shut off, the injectors undergo heat soak. Fuel residue in the injector nozzles evaporates, leaving the waxy olefins behind. Because the engine is off, there is no cooling air flow through the ports and no fuel flow through the injectors to wash it away, so heat bakes the olefins into hard varnish deposits. Over time, these deposits can build up and clog the injectors.

The formation of these deposits is a normal consequence of engine operation, so detergents are added to gasoline to help keep the injectors clean. But if a vehicle is used primarily for short-trip driving, the deposits may build up faster than the detergents can wash them away.

On four-cylinder engines, the #2 and #3 injectors are in the hottest location and tend to clog up faster than the end injectors on cylinders #1 and #4. The same applies to the injectors in the middle cylinders in six and eight cylinder engines. The hotter the location, the more vulnerable the injector is to clogging from heat soak. Throttle body injectors are less vulnerable to heat soak because of their location high above the intake manifold plenum.


To save a few pennies per gallon and to increase the competitive and/or profit margin of gasoline, some suppliers have cut back on the amount of detergent they add to their fuel or have switched to cheaper and less effective additives.

Commonly used deposit-control additives include polysibutylamine, polyisbutylene succinimide and polyisobutylene phenylamine. But these same additives also can build up on intake valve stems causing them to stick. To prevent this from happening, additional additives called "fluidizers" must also be added to the fuel. But, over time, these can contribute to the formation of combustion chamber deposits that raise compression and the engine's octane requirements.

fuel injector
Dirty injectors lean out the fuel mixture and contribute to lean misfire, hesitation and even detonation.
Cleaning should restore like-new performance.

One of the best additives is polyetheramine. It keeps injectors, valves and combustion chambers clean without the help of any additional fluidizers - but it costs more than twice as much as the other commonly used additives.

How much additive does it take to provide an adequate level of protection? Industry sources say the recommended level is about 1,000 parts per million (ppm) of dispersant-detergent in the fuel - which costs the gasoline supplier less than a penny a gallon. Even so, as much as 85% of the gasoline that is being sold contains only one-tenth of the recommended dosage, or only 100 ppm of additive. Consequently, using cheap gas contributes to the formation of injector deposits.

clean fuel injectors
Cleaning injectors off-car takes more time and effort, but usually delivers much better results than on-car cleaning.


The benefits realized by injector cleaning obviously will vary depending on the condition of the injectors prior to cleaning and how badly they were clogged. Injectors that are really dirty should show more of a noticeable improvement in performance than ones that have only a light accumulation of deposits. Either way, performance, fuel economy and emissions should all be better after a cleaning.

Most high-mileage engines as well as engines that are used mostly for short trip stop-and-go driving are the most likely prospects for injector cleaning. Some experts recommend cleaning the injectors every 25,000 to 30,000 miles to keep them flowing at peak efficiency.


Another component that also may need to be cleaned to remove fuel varnish is the throttle body. Fuel vapor rising up through the intake manifold can accumulate and vaporize around the throttle plate and air bypass circuits, causing a change in the idle air/fuel mixture. Sometimes you can see the deposits, and sometimes you can't. Either way, cleaning the throttle body and intake tract also may be necessary to fully restore engine performance, idle quality and emissions. An aerosol cleaning solvent works well here.

The intake valves and combustion chambers should also be cleaned when you do the injectors to remove deposits that may also be contributing to driveability and emissions problems. Deposits on the backs of intake valves can act like a sponge and absorb fuel, causing a momentary hesitation when the throttle is suddenly opened. Combustion chamber deposits increase compression and the risk of engine-damaging detonation (spark knock).

Engines that burn oil typically will have heavy intake valve and combustion chamber deposits that do not respond well to normal levels of detergent in gasoline. Additional cleaner is needed, which can be added to the fuel tank or run directly through the injectors.

To remove carbon deposits from the intake valves and combustion chambers, use a "top cleaner" type of product and follow the instructions, or use equipment that is designed to clean the upper engine (such as a Motorvac Decarbon machine).

clean fuel injectors
Direct injection fuel injectors have very precise spray patterns and are even more sensitive to deposits than regular injectors.

injector spray patterns
Injector spray patterns can show if any are misshapen or contain streamers of unvaporized liquid.

Some experts recommend replacing the spark plugs after doing an on-car injection cleaning or decarbon treatment. The residue that is loosened and washed away by the solvent may increase the risk of plug fouling. Changing the oil and filter is also a good idea following a cylinder decarbon treatment because some of the solvent will get past the rings and end up in the crankcase.


Should you clean the injectors in place or remove them and use some type of injector cleaning machine? It depends.

The easiest route is to clean the injectors in place because you do not have to remove the injectors (which can be a real chore on some import engines). Running cleaner through the injectors while the engine is running also removes many of the deposits on the valves and inside the combustion chambers. This eliminates the need for an extra cleaning step if the engine is full of carbon deposits. The job takes only 10 to 15 minutes, and you can usually tell right away if the treatment addressed the problem (engine runs smoother, idle misfire gone, etc.).

When doing the cleaning procedure itself, you must use pressurized equipment to feed the solvent directly into the fuel rail while the engine is running. This means you either have to disable the fuel pump and plug the fuel return line, or install a U-tube so the fuel will recirculate right back to the tank. Disabling the fuel pump can set a fault code on some cars, requiring you to clear the code after the job is done.

Easy as it is, there are some limitations with on-car injector cleaning. One is that badly clogged injectors may not pass enough solvent during a normal cleaning cycle to be thoroughly cleaned. Some baked-on deposits can be very difficult to remove, requiring you to prolong or repeat the cleaning process. And if on-car cleaning does not work? You will have to remove the injectors and have them cleaned on an injector cleaning machine - or replace them.

Another limitation with on-car injector cleaning is that you may have to do some additional tests to confirm that the injectors responded well enough to your cleaning efforts. A test drive may be needed to see if the driveability symptoms have been eliminated, or you may have to check emissions to make sure HC and CO levels are back to normal. A power balance test is another way to confirm engine performance and check for weak cylinders (there should be less than a 10% power variation between cylinders). An injector pressure drop test will tell you if the injectors are flowing evenly or not.

There may be some risk to the vehicle's fuel system when using concentrated solvent to clean the injectors in place. Most equipment suppliers say to disconnect and plug the fuel return line so that solvent does not circulate back to the fuel tank. Strong solvents may attack rubber and plastic components in the fuel pump, regulator and fuel lines, creating additional problems that you don't need.

On-car injector cleaning also involves some risk to the person who's performing the service. You have to disconnect pressurized fuel lines, make sure there are no fuel leaks, and feed high-pressure solvent (which is just as flammable as gasoline) into the engine while the engine is running. Safety precautions should always include eye protection, making sure there are no open sources of ignition (sparks) nearby, and avoiding direct exposure with the cleaning solvent.


Injectors that are really dirty may not respond well to on-car cleaning. You may have to use a more powerful solvent and/or longer cycle time to loosen the baked-on deposits. That is where an off-car injector cleaning machine really helps.

Off-car injector cleaning is a more expensive service because of the labor involved to remove the injectors (which can be considerable on some applications), and it requires special equipment that can cost anywhere from $4,000 to $10,000.

fuel injector flow testing

Many shops charge between $25 and $35 per injector for off-car cleaning - which makes it more costly than on-car cleaning. But it also can motorists a lot of money because off-car cleaning is a lot cheaper than replacing the injectors with new ones (which can cost hundreds of dollars a set!).

Off-car injector cleaning can often restore dirty injectors that fail to respond to on-car cleaning. That is why some shops do only off-car cleaning. They do not want to have to clean the injectors twice. Off-car cleaning takes more time (typically 30 to 45 minutes after the injectors have been removed), and most machines have an ultrasonic bath that can be used to soak badly clogged injectors. Some machines also reverse-flush the injectors, which provides an added measure of cleaning.

Another reason for using off-car cleaning equipment is that the injectors can be flow-tested after they have been cleaned to verify their performance. The injectors typically are mounted on a test manifold and energized to spray solvent into clear graduated cylinders. By comparing the volume of fuel delivered, it is easy to see if all the injectors are flowing evenly.

As a rule, you should see less than 5% to 7% variation between injectors (some performance engine builders aim for 1% or less variation between injectors!). If an injector is not passing as much liquid as its companions, you can subject it to more cleaning. And, if it fails to respond to additional cleaning, there is no guesswork about which injector needs to be replaced.

Flow-testing also allows you to compare the actual flow rate of each injector to factory specifications. If the flow is within specifications, you know the injector should perform properly when it is reinstalled back in the engine. Flow-testing also is a good way to make sure the injectors are the right ones for the engine (one or more injectors may have been previously replaced by someone else).

A flow test on the cleaning equipment allows you to see each injector's spray pattern. If you see a normal, cone-shaped mist, you know the injector is flowing properly. If you see streamers of unvaporized liquid in the spray pattern, you know additional cleaning is needed or the injector needs to be replaced.

book More Fuel Injection Articles:

How Electronic Fuel Injection Works

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Troubleshooting Electronic Fuel Injection Problems

Intake Valve Deposits in Gasoline Direct Injection Engines

Gasoline Fuel Treatments

Troubleshooting Hesitation Problems

Fuel System Diagnostics: Finding the Best Approach

Diagnosing Returnless Electronic Fuel Injection Systems

Electric Fuel Pumps

Fuel Pump Diagnosis

How To Replace an In-Tank Electric Fuel Pump

Bad Gasoline Can Cause Performance Problems

Watch Out for Bad Gas!

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