Misfire Diagnostics

Misfire diagnostics

Misfiring
If the fuel / air mix does not completely ignite in the combustion chamber, the engine misfires. The engine control module (ECM) detects the misfire by registering deviations in the rotation of the flywheel.

Deviation in flywheel rotation is due to:
- driveline oscillations
- normal variations caused by uneven combustion
- flywheel mechanical tolerances
- misfiring, which is caused by:
- incorrect fuel / air mixture
- poor ignition sparking
- insufficient compression.

Diagnostics
The mechanical tolerances and oscillations in the drive line disrupt the signal. This makes it difficult to determine if a misfire has occurred or if the detection is incorrect.
The signal from the flywheel is adapted to filter out the irregular rotation. This eliminates the mechanical variations in the flywheel.
Misfires can be detected by registering the time difference between the intervals and comparing this with the filtered time difference in the flywheel. If the flywheel signal has not been adapted, the diagnostic is active. However this cannot identify misfires as accurately as an adapted flywheel signal. The adaptation is carried out at different loads and engine speed ranges. For the values to be set in the different ranges, the deviation in speed must be within certain fixed limits.
The misfire diagnostic is shut off when certain component faults have been detected by the engine control module (ECM).
"Rough Road" detection (drivetrain oscillations caused by, for example, extremely uneven roads) is not activated on this engine.

Diagnostic trouble code (DTC) management
Diagnostic trouble codes (DTC) for misfiring will light the malfunction indicator lamp (MIL).
The engine control module (ECM) determines how much the engine is misfiring by counting the number of misfires during a certain number of engine revolutions.

Misfire affecting emission levels
If the Engine control module (ECM) detects a certain number of misfires during 1,000 engine rpms this is interpreted as misfires that affect emissions. Trouble code is stored and MIL-light is activated. Immediately in connection with engine start, that is, during the engine's first 1,000 revs after start, the misfire diagnosis is four times as sensitive as later on in the driving cycle.

Catalytic converter damaging misfire
If the engine control module (ECM) detects a certain number of misfires during 200 engine revolutions, this is interpreted as misfiring which damages the three-way catalytic converter (TWC). The number of misfires that the engine control module (ECM) allows before interpreting it as misfiring depends on the load and engine speed range. the malfunction indicator lamp (MIL) flashes when the misfire occurs, and shines constantly once the diagnostic trouble code (DTC) is stored.
The engine control module (ECM) does not store specific diagnostic trouble codes (DTCs) for emission or catalytic converter damaging related misfires.
The engine control module (ECM) registers and stores the engine speed and load range and whether the engine is warm or cold when misfiring is detected.
If the misfire stops, the requirements for the engine speed, load parameters and engine temperature must be met without misfiring before the engine control module (ECM) will begin counting down to extinguish the malfunction indicator lamp (MIL) and erase the stored diagnostic trouble codes (DTCs) for misfiring
The number of misfires can be read out.