Misfire Diagnostics

Misfire diagnostics

Misfiring
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
- 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 rotation must be within certain fixed limits.
Drive line oscillations, caused by extremely 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 or mass air flow (MAF) sensor
- When DSTC, traction control and/or ABS is active.

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 1000 engine revolutions, this is interpreted as misfiring which affects emissions. If misfiring is detected in the first 1000 engine revolutions a diagnostic trouble code (DTC) is stored and the malfunction indicator lamp (MIL) lights. However, if misfiring is detected after the first 1000 engine revolutions, four misfires need to be detected before a diagnostic trouble code (DTC) is stored.

Catalytic converter damage
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 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 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) for misfiring.