Misfire Diagnostics

Misfire diagnostics
If the fuel / air mixture does not ignite completely in the ignition stroke the engine is misfiring. 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
- Misfire, caused by
- Incorrect fuel / air mixture
- Poor ignition spark
- 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.
Two camshaft revolutions are divided into five intervals for 5 cylinder engines and six intervals for 6 cylinder engines.
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 rotation must be within certain fixed limits.
Drive line oscillation, caused by uneven road surfaces for example, may lead to uneven engine operation. Drive line oscillations are registered by the accelerator in the brake control module (BCM) which sends this information to the engine control module (ECM).
The misfire diagnostic is shut off when:
- The brake control module (BCM) transmits information to the engine control module (ECM) about driveline oscillations
- A fault has been detected by the engine control module (ECM) in the engine speed (RPM) sensor, mass air flow (MAF) sensor or engine coolant temperature (ECT) sensor
- When one of the DSTC (Dynamic stability and traction control), traction control or Anti-lock Brake Systems (ABS) are active.
The following applies from model year 2005-
The injector for that cylinder will be disabled if the engine control module (ECM) detects misfiring in a cylinder. This will cut the fuel supply to the cylinder.

Diagnostic trouble code (DTC) management
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. If the engine control module (ECM) detects a particular number of misfires below 1000 rpm it is interpreted as emission related misfires. If the misfires exceed a certain value during 200 engine revolutions, this is interpreted as misfiring which damages the three-way catalytic converter (TWC).
Diagnostic trouble codes (DTC) for misfiring will light the malfunction indicator lamp (MIL). If there is risk of damage to the three way catalytic converter, the malfunction indicator lamp (MIL) will flash during misfiring 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.
In the event of emissions related misfiring, the malfunction indicator lamp (MIL) lights during the second operating cycle and a diagnostic trouble code (DTC) is stored.
If the misfire stops, the requirements for the rpm and load ranges must be met without misfiring before the engine control module (ECM) will begin counting down to extinguish the warning lamp and erase the stored diagnostic trouble codes (DTCs) for misfiring.