Misfire Diagnostics

MISFIRE DIAGNOSTIC

Overview
If the fuel/air mixture does not ignite in the ignition stroke the engine is said to be misfiring. The engine control module (ECM) detects misfires by registering the time between two segments of the flywheel.

The time between two segments varies depending on
misfiring
driveline oscillations
normal variations caused by uneven combustion flywheel mechanical tolerances.

The mechanical tolerances and drive line oscillations disrupt the signal and it is difficult to detect whether the engine really is misfiring or not. By registering the time difference between the sectors and comparing this with the time differences in an earlier rotation misfires can be gauged. To eliminate mechanical tolerances one of the 5 sectors on the flywheel is used as a reference point for the timing.

Shutting off the misfire diagnostic

Misfire diagnostics are shut off in the event of
driveline oscillations.

Drive line oscillations, caused by uneven road surfaces for example, may lead to uneven engine operation. Drive line oscillations will be posted by the ABS system and this information is sent to the engine control module (ECM). The engine control module (ECM) uses this information in order to differentiate between oscillations and actual misfiring and the diagnostic is shut off. diagnostic trouble codes (DTCs) in the flywheel sensor, mass air flow (MAF) sensor, ABS and camshaft position (CMP) sensor.

The malfunction indicator lamp (MIL) lights in the event of misfiring
The malfunction indicator lamp (MIL) lights in the event of misfiring. If there is risk of damage to the three way catalytic converter due to misfiring, the malfunction indicator lamp (MIL) will flash and then switch to a constant light.

The engine control module (ECM) registers and stores the rpm and load parameters within which the misfire occurred. For the diagnostic trouble code (DTC) to be stored, the misfire must occur a further two times within the same rpm and load parameters. The malfunction indicator lamp (MIL) is lit if the diagnostic trouble code (DTC) for misfire is stored in the previous operating cycle and a new diagnostic trouble code (DTC) for misfire is stored in the next operating cycle.

If the misfire stops, the requirements for the rpm and load parameters must be met without misfiring before the engine control module (ECM) will begin counting down to extinguish the warning lamp and erase stored diagnostic trouble codes (DTCs).