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Fuel Delivery System — Keeping Injectors Clean


Clean fuel injectors are a must for peak engine performance, fuel economy and emissions. If the injectors are dirty and cant deliver their normal dose of fuel, then performance, fuel economy and emissions all suffer. Dirty injectors cant flow as much fuel as clean ones, nor can they deliver 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 running in closed loop, but it cant fix the underlying problem.


Symptoms that may indicate dirty injectors include lean misfire, rough idle, hesitation and stumbling on light acceleration, a loss of power, and higher hydrocarbon (HC) and carbon monoxide (CO) emissions.

On 1996 and newer vehicles with OBD II, dirty injectors may cause enough misfires to trigger a fault code and turn on the Check Engine light. The OBD II system will likely have a P0300 code indicating a random misfire problem, or a P030X code where "X" is the number of a specific cylinder that is misfiring. If the injectors are at fault, cleaning or replacement will be needed to eliminate the misfire.


It doesnt take much of a restriction in an injector to lean out the fuel mixture. Only an 8 to 10 percent restriction in a single injector is enough to cause a misfire. When this occurs, unburned oxygen enters the exhaust and makes the oxygen sensor read lean. On older multiport systems that fire the injectors simultaneously, the computer compensates by increasing the on-time of all the injectors when the O2 sensor reads lean, 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 detonation. When the engine is under boost at high rpm, it needs all the fuel the injectors can deliver. If the injectors are dirty and cant keep up with the engines demands, the fuel mixture will lean out. Thats bad news because lean mixtures under high boost are a prescription for detonation and preignition.


All vehicles are vulnerable to injector clogging, but the ones that are most vulnerable and most likely to experience drivability and emission problems are older cars and trucks with pintle-style multiport injectors. Many newer injector designs are more resistant to clogging.

Lower levels of gasoline detergents combined with stop-and-go city driving add up to a tremendous opportunity for fuel system cleaners. These products can help keep fuel systems clean and can also be used to treat engines with dirty injectors, throttle bodies, intake manifolds, intake valves, pistons and combustion chambers.

Fuel system cleaners typically fall into one of two categories: those that are formulated to prevent fuel varnish and other deposits, and those that are formulated to clean dirty fuel systems by washing away existing deposits. Always refer to the product label for application and usage directions.


Products that are designed for preventive maintenance typically recommend adding the cleaner to 10 or more gallons of gasoline every other fill up or every 1,000 miles. The idea is to keep the fuel system clean by supplementing the detergent additives that may or may not already be in the gasoline.

Products that are designed to remove existing deposits are usually much more concentrated and contain stronger detergents or solvents. These should be used sparingly or as needed if the fuel system is already dirty.

Some products are designed to deliver a double whammy. One can of cleaner is added to the fuel tank to loosen and remove deposits in the fuel injectors while a second can of "top cleaner" is fed into the intake manifold through a vacuum hose connection while the engine is idling to directly attack deposits in the manifold, intake valves and combustion chambers.


There are also aerosol products that can also be used to remove surface deposits in throttle bodies and carburetors. 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. Caution: Most of these products are extremely flammable and must be used with care.

Cleaners that are added to the fuel tank obviously take awhile to work their magic. For a light to moderate buildup of deposits, many products work quite well. Some are more effective than others and may deliver results more quickly. But for really heavy deposits or injectors that are badly clogged, stronger measures may be needed if one or two tank additive treatments fail to correct the problem.


ON- & OFF-CAR INJECTOR CLEANINGMany experts say injectors should be professionally cleaned every 30,000 miles, or when the spark plugs are replaced. They also say it is a good idea to clean the intake valves and combustion chambers at the same time. Heavy carbon 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. Valve deposits can also restrict airflow and hurt performance. Combustion chamber deposits increase compression and the risk of engine-damaging detonation (spark knock).

A professional on-car injector cleaning treatment requires special equipment to feed solvent directly to the injectors through the fuel rail. Some DIY-type products that use an aerosol can of solvent are available, but most professionals use equipment that attaches to shop air to deliver the cleaner. Cleaning typically takes about 10 to 15 minutes and is done with the engine idling. 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 and may not respond to repeated cleaning attempts. So in many cases, off-car cleaning or injector replacement becomes necessary.


Some technicians prefer to clean injectors off the vehicle because they believe off-car cleaning is more effective and allows them to better evaluate the actual condition of each injector. To do this, the injectors have to be removed from the engine and placed in a special cleaning machine that flushes the injectors and compares fuel-delivery rates. Injectors that fail to respond to cleaning can be easily identified for further cleaning or replacement. Comparing the flow rates and spray patterns of each injector also allows the tech to spot problem injectors. Speed shops often use this type of equipment to closely match the flow rates of injectors to boost engine performance.


As a rule, there should be less than 5 to 7 percent variation in the flow rates between injectors. Performance shops like to see less than one percent variation between injectors! If an injector doesnt flow as much fuel as its companions, it will lean out the fuel mixture causing a loss of power and an increased risk of misfire, detonation and preignition.

When injectors are completely clogged and fail to respond to cleaning, or they are worn, shorted or open, replacement is the only option. Replacing an entire set is costly, so another alternative is "remanufactured" fuel injectors. Reman injectors are used injectors that have been cleaned and flow tested.


Other items that may also have to be replaced to restore like-new performance include a new fuel filter, air filter, PCV valve, spark plugs and plug wires. The oil should also be changed following an on-car injector flushing treatment or a "decarbon" treatment of the combustion chambers.

Warning! Cheap Gas Makes Matters Worse!

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.

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, up to 85 percent of the gasoline sold today 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.

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