The Fuel Monitor is one of the Continuous tests and runs whenever the engine is running. It is entirely in software and monitors the system Fuel Trims.
OBD enables the software to adapt to a deterioration in the sensors and the catalysts over a period of time by utilising Long Term Fuel Trims (adjustments). These can be summarised as follows -
NOTE, on the OBD Vehicle Explorer software the Short Term Fuel Trim is called RTFT in the DATA page, and SHRTFT on the Enhanced Data.
Entry conditions are :
Rich: if the LTFT <25% and STFT <10% - P0172 Bank 1 Rich: P0175 Bank 2 Rich.
On occasion a fuel pressure problem may cause difficult starting. A difficult re-start (when hot or cold, after the car has been stood only for a few minutes) and LTFT is persistently negative at idle, whether or not a P0171 and/or P0174 has been recorded - then suspect the fuel pressure regulator is faulty - causing fuel rail over-pressure.
If the engine is difficult to start when cold and the LTFT is showing a consistent positive LTFT - whether or not P0172 and/or P0175 is recorded then suspect that the pressure regulator is not pressurising the fuel rail properly and the INJectors are unable to provide enough fuel.
Experience has shown that the following may be contributory causes:
Slow switching and/or flatlining HO2S sensors can push up the LTFT to compensate. The HO2S sensors 11 (and 21 on 24V engines) should be monitored and checked for periods of inactivity, slow switching, or low and high voltages.
Leaks in the inlet system can admit unmetered air which leans the mixture. This will also cause a rolling/uneven idle. However, experience has shown that a very small leak in the inlet can cause uneven idle, without being enough to upset the fuel trims and without setting any DTCs. It requires a larger leak to drive the Fuel Trims high enough to set a DTC. Points to check are:
Minor leaks which are enough to effect idle but not enough to set DTCs have shown to be:
In general, on an engine with a smooth, steady idle an unmetered inlet leak can be ruled out as a cause of DTCs. For more information see Poor Idle
A dirty MAF sensor can mislead the PCM by detecting less air entering the system than there actually is (by up to 30%). This may push the LTFT to rich to compensate and may cause a sooty exhaust pipe.
Dirty/gummed INJectors can deliver fuel as a dribble rather than a spray, and this effects combustion efficiency. There is debate about whether this causes a shift to Rich or Lean, but dirty INJectors are undesirable in any case, so regular Injector Treatment should be included when fuelling.
Consistent Positive or Negative Fuel Trims
The Long Term Fuel Trims can also be effected by the Fuel Pressure Regulator. This should be providing petrol pressure in the fuel rail set at about 3 Bar. If the engine is difficult to start when recently run and the LTFTs are showing consistent negative trims at idle then suspect that the fuel pressure regulator is overpressurising the fuel rail. Thus too much fuel is injected and the LTFT swings to negative to try to correct this. Because the INJectors can only lean the mixture by a small amount (by shorter opening times) the LTFT swing over to negative and stays there. There can also be slow switching HO2S DTCs - this is not a failure of the HO2S sensors. The pressure regulator must be replaced.
If the engine starts poorly when cold and the LTFTs show a positive trim at idle, suspect the fuel pressure regulator is failing to maintain a proper fuel rail pressure. In this case not enough petrol is being injected into the inlet to provide a good combustion - the engine will often struggle to start and the LTFT will continue to swing to positive trims at idle.
A failing MAF sensor can also cause Fuel System DTCs. See Testing the MAF If lean running DTCs (P0171 and or P0174) are set, (and both banks the same on a 24V) and there is a consistent very high Long Term Fuel Trim (25% is often found) then log the MAF reading carefully at idle and at higher revs. No load idle reading on the 24V should be ITRO 0.657 llbs/min - less than that and the LT fuel trims will react to compensate. Experience has shown that a reading of 0.235 at idle is enough to provoke the LTFT to maximum without causing a DTC for a failing MAF. If this is seen then the MAF must be replaced even though no MAF fault has been registered.
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