The following procedures should be used to diagnose a
lean/rich condition or a high/low fuel trim scanner reading. The steps
should be followed in the sequence presented to avoid diagnostics errors.
1. Determine if the ECM is in control. While pre-loading the
engine, look at these PIDs—O2 sensor , fuel trims, and MAF or MAP readings
and at the same time make sure that the O2 sensor keeps switching at all
times except deceleration and WOT. The use of a graphing display is highly
recommended because of the ease of graphical visual recognition. Check for
any codes present.
2. Check for proper fuel pressure and volume. A low fuel
supply can upset the vehicle’s fuel injection performance in ways that might
make other engine components appear faulty. Check fuel pump, fuel filter,
and the voltage/ground reaching the pump. A resistance in the fuel pump
circuit ( less than battery voltage reaching the pump ) can greatly alter
the fuel pump’s rotational speed ( RPM ) and fuel flow rate.
3. Determine if proper spark is present. Check for high
firing line KV demand, spark line KV, and spark duration. Typical firing
line KV are in the range of 8 to 14 KV, spark line KV at around 2 to 3 KV,
and spark duration at 1.3 mS minimum for 2 air gap systems ( distributor ),
1.0 mS for a 2 air gap DIS waste spark system and 1.5 mS minimum for 1 air
gap systems ( COP). As a rule of thumb, the spark should jump a minimum of
4. Determine if EGR is stuck open. An EGR stuck open will
skew the MAF sensor reading, tricking the ECM into reacting as if there is
less air coming into the engine and creating a lean condition. In case of a
MAP system, the ECM will react as if there is less vacuum or a bigger load
present, therefore, increasing fuel pulsation. An EGR stuck open will make a
MAF engine run lean and a MAP engine run rich because of the way
these sensors respond to changing conditions.
5. Check the Fuel quality. Certain additives, water,
alcohol, etc in the mixture will alter the O2 sensor’s response and have an
adverse effect on emissions.
NOTE: When the ECM is not in control ( O2
not switching ) and the problem is always present ( Idle ) look for primary
fuel control sensors like ECT, IAT,TPS off range, and O2 sensor stuck ( lean
or rich ). MAP & MAF sensors can also cause idle fuel problems if the
reading are off range. A bad air intake duct hose, vacuum hose or clogged
vacuum port can also be the culprit since they will upset the MAP/MAF
readings. If the problem only happens upon acceleration then the MAP, MAF or
TPS sensor are the most likely cause since they are responsible for fuel
control at that time. Always check for vacuum and/or fuel leaks.
Fuel problems, whether lean or rich, are usually a result of
the following order of possibilities.
a) ECM not in control.( MAF/MAP/TPS/ECT problems, intake air
duct hose, MAP vacuum hose, PCV, etc. )
b) Improper pressure & volume.( fuel flow restrictions,
clogged injector or filter, kinked fuel hose, defective fuel reg. )
c) Exhaust leaks.( air leaking into the exhaust before the
O2 sensor. )
d) Vacuum leaks.( intake gaskets, brake booster, PCV, etc )
e) Fuel leaking into the engine.( Bad fuel reg., injector,
f) Diluted engine oil.
g) Water in fuel or bad fuel.