Is it better to clean or replace dirty fuel
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
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
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.