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Fuel system diagnosis
The fuel system diagnosis monitor the long term fuel
trim adaptions, 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 mo-
nitored 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 combus-
tion. 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 com-
bustion, 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.
