Is it better to clean or replace dirty fuel injectors?
 
Injector Operation
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.
Injector Service
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. |
