Injectors include a precision-ground needle valve and are controlled by an electro-magnetic solenoid that is turned on and off by an electric control unit. Fuel is injected only during the “on” time and is metered by the size of the opening, duration of “on” time, and fuel pressure.
Try cleaning them first. If this is not successful, they must be replaced.
Because of their construction, fuel injectors tend to “gum up” after 15,000 to 30,000 miles of driving. Fuel spraying from the injector must pass through a very small opening in the discharge nozzle. This is necessary to create a cone-shaped spray pattern that breaks the fuel up into a fine mist for proper atomization.
Some newer style injectors are more clog resistant than their predecessors, but all are vulnerable to some extent.
Every time the injector sprays fuel, a small amount remains in the nozzle. As it evaporates, it leaves behind a wax-like residue that forms hard varnish deposits.
The rate at which deposits build up depends on the quality of gasoline burned, whether or not the gas has detergent in it (and what kind), and the number of thermal cycles the engine experiences per miles driven. Short-trip driving builds up deposits more quickly than continuous driving.
As deposits build up in injectors, they restrict the discharge orifice and break up the normal cone-shaped spray pattern. The spray pattern may develop “legs” (streamers of unatomized fuel) or turn into a continuous stream of unatomized fuel like a fire hose.
Liquid fuel does not burn as efficiently as atomized fuel, so it has a “leaning effect” on the air/fuel mixture. Accumulated deposits in the discharge orifice also restrict the total amount of fuel delivered per squirt, which further compounds the leaning effect. This can result in the appearance of driveability problems such as hard starting, hesitation, poor fuel economy, loss of power, and elevated exhaust emissions.
An engine with dirty injectors will usually show a wide variation in RPM between cylinders when doing a power balance test. There will also be a lot of variation in peak firing voltages between cylinders on a scope.
For do-it-yourselfers, there are two options – use a fuel additive to clean the injectors, or buy a can of pressurized solvent that’s designed for on-car injector cleaning. Fuel additives can only do so much, so badly-clogged injectors usually need to be pressure flushed with solvent.
With on-car cleaning, pressurized solvent is run through injectors to flush out deposits. To do this, the fuel pump is temporarily disconnected so solvent can be fed directly into the test valve fitting on the fuel rail.
When the engine is started, the solvent becomes the temporary “fuel supply” while injectors are cleaned.
The resulting improvement in performance is usually quite noticeable. But on-car cleaning doesn’t always do the trick, especially if an injector is badly plugged.
Unless injectors are removed and tested, there is no easy way to spot marginal injectors (those with defective spray patterns) or ones that don’t deliver as much fuel as the others (mismatched injectors can reduce horsepower and increase emissions).
Off-car cleaning involves more work, but results are often worth it. For one thing, injectors that don’t respond to on-car cleaning can often be restored to like-new performance with off-car cleaning.
Some available cleaning equipment can reverse flush injectors, doing a thorough cleaning job. Most off-car cleaning equipment also allows the mechanic to observe and measure injector flow patterns so bad ones can be identified.
Flow rating also allows injectors to be more closely matched for improved engine performance.