Fuel System Diagnosis

Fuel System Diagnosis

The fuel system diagnosis monitor the long term fuel trim adaptations, to check if any of the adaption points has reached it's limits (rich or lean), and no more adaption is possible. This will not immediately lead to higher emissions, because the short-term fuel trim can take care of additional faults. The long term fuel trim is calculated from the front linear oxygen sensor, and there are 6 times 6 (depending on load and engine speed) different adaptation points. Each point is monitored in order to check if it is higher/lower than the threshold value.

Below are some faults that illustrate cases, which could cause higher emissions:
- Fault leading to lean A/F mixture.
- Air leakage after MAF sensor.

If there is an air leakage after the MAF sensor, this will lead to unmeasured air is added to the combustion. Short term and long term fuel trim will adjust fuel amount to homogenous A/F mixture, and if the leakage is large enough, the diagnosis will detect a lean fault. Greatest influence of this fault is at low load. Fault leading to low fuel pressure.

If for example there is a fault which decreases the fuel pressure from required pressure, this could also affect the short term and long term fuel trim, and if this difference is a large deviation from the required fuel pressure, then the diagnosis will detect a lean fault. Greatest influence of this fault is at high load.
- Fault leading to rich A/F mixture.
- MAF sensor which is rich.

If the MAF sensor measure more air than is actually passing the sensor, then this will result in a rich combustion, and the consequence if the fault is great enough, the diagnosis will detect a rich fault.

Other fault leading to rich A/F mixture

If the fuel pressure regulator is broken, injectors are broken or there is another fault that will result in a rich A/F mixture, then the diagnosis will detect rich.