"Dirty" injectors are a commonly encountered problem. Dirty
is actually a misnomer because the problem isn't dirt but rather
a buildup of fuel varnish in the injector nozzle. The olefins
(heavy waxy substances) in gasoline form deposits that gradually
accumulate and restrict injector nozzles. When this happens, the
injector's normal cone-shaped spray pattern develops streamers
that inhibit proper fuel atomization. Deposits also restrict the
size of the opening and reduce the amount of fuel that's
delivered per squirt, which leans out the fuel mixture causing
hesitation, lean misfire, rough idle and similar driveability
Though dirty injectors can often be cleaned with fuel
additives, by feeding pressurized solvent into the fuel rail to
flush the injectors on the vehicle, or by removing the injectors
and cleaning them on special off-car cleaning equipment,
sometimes the injectors are too badly clogged to be fully
restored by cleaning.
HOW TO TELL IF INJECTORS ARE DIRTY
If a vehicle ix experiencing driveability symptoms that would suggest
a lean fuel condition (hard starting, rough idle, lean misfire at idle,
hesitation or stumble when accelerating, poor performance, etc.) the
following tests can be used to verify dirty injectors:
* Power balance test -- Compares rpm drop between cylinders to
identify weak cylinders. If compression is the same in all the
cylinders, a single weak cylinder would probably indicate a dirty or
dead injector. If all the injectors are dirty, this test won't help
* Fuel pressure drop test -- Measures drop in fuel pressure after
fuel line is pressurized and engine is turned off. A gauge is used
to measure the pressure drop when each injector is energized for the
same period of time (a few seconds). Any injector that shows less
of a drop than the rest is probably restricted.
* Off-car flow test. Injectors are removed from the engine and
mounted on a flow bench. The amount of fuel that flows through the
injectors into calibrated tubes is measured and compared. Flow
should not vary more than 5% cylinder to cylinder. Low flowing
injectors need to be cleaned or replaced.
Injector replacement is also necessary if an injector is worn
(which is not uncommon in high mileage engines) or has failed
electrically (shorted or open). A working injector will make a
buzzing noise as it cycles open and shut, but a dead injector
will remain silent.
Accurate diagnosis of injector problems is essential before
any parts are replaced to rule out other possibilities. A bad
wiring connector or computer circuit can render an injector
inoperative. Air leaks past the rubber O-ring seal at the base
of an injector can also mimic the symptoms of a clogged injector.
WHAT CAUSES IT
The following are all factors that can cause fuel injector
clogging and restrictions:
* Low quality gasoline -- Many discount brands of gasoline do
not contain adequate levels of detergent to keep fuel injectors
* Injector soak time - Worst case scenario would be 15
minutes of running followed by 45 minutes of hot soak, what
around-town driving does. Deposits don't form while the fuel is
flowing: only when it stops.
* Injector design - Pintle style injectors are more likely
to clog up than injectors with ball or disc style valves.
* Fuel system configuration and injector tip temperature -
High tip temperature means the formation of deposits, and the
hotter it is, the more rapid the progress. Low hood lines and
tightly packed engine compartments don't help.
* Geographical considerations - Injector failure rate is
higher where it's hot and dry.
* EGR and PCV - Deposits on injectors and valves are
promoted by EGR and PCV gases present in the intake passages at
shutdown, which condense onto sensitive surfaces. This appears
to be a big part of the mechanism that plugs injectors.
* Poor intake filtration - Leaking intake housing seals and
gaskets can allow dirty air into the intake system, increasing
* Fuel stress - The constant heating and pumping of fuel
through the pressure regulator and return line in a typical
multiport injection setup may cause subtle chemical changes that
can contribute to deposit formation.
* Engine operating temperatures and thermal characteristics
- Heat is a big part of the problem so the cooling system design
and function are important.