Misfire Diagnostics

Misfire diagnostic

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

The time between the 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 is misfiring or not. An adaptation of the flywheel signal is being made in order to eliminate the mechanical fault in the flywheel. Two camshaft revolutions are divided into five intervals for 5 cylinder engines and six intervals for 6 cylinder engines. The aim of this modification is to filter out the normal distortion present in the flywheel. By registering the time difference between the intervals and comparing this with the normal distortion in the flywheel, misfires can be gauged. In order for the engine control module (ECM) to register misfires, the flywheel must be adapted. Misfire diagnostics are shut off until the flywheel is adapted for the first time. This value is saved and then used in subsequent operating cycles.

Adaptation of the flywheel is done when:
- the engine speed (RPM) is between 2300 rpm and 3000 rpm
- the load should exceed 40 % of relative load (RL), which corresponds to normal forward travel on a smooth road at approximately 100 km/h.
Flywheel adaptation takes approximately 60 seconds.
Drive line oscillations, caused by uneven road surfaces for example, may lead to uneven engine operation. Drive line oscillations will be registered by the ABS system and the information sent to the engine control module (ECM). The engine control module (ECM) uses this information to differentiate between these oscillations and real misfiring. Misfire diagnostics are shut off in the event of drive line oscillations

Misfire diagnostics are also shut off when:
- leak diagnostic, when leak diagnostics are taking place
- diagnostic trouble codes (DTCs) in the flywheel sensor, mass air flow (MAF) sensor, engine coolant temperature (ECT) sensor and leak diagnostic.
Misfiring lights the malfunction indicator lamp (MIL). In the event of misfiring, if there is risk of damage to the three way catalytic converter, 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 ranges 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 ranges. The malfunction indicator lamp (MIL) lights 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 received in the next operating cycle.
If the misfire stops, the requirements for the rpm and load ranges must be met without any misfires before the engine control module (ECM) will begin counting down to extinguish the warning lamp and erase stored diagnostic trouble codes (DTCs) for misfiring.