Misfire Diagnostic

If the fuel/air mixture does not ignite in the ignition stroke it can be said that the engine is misfiring. The flywheel is divided into 5 sectors where every sector corresponds to a special cylinder. The engine control module (ECM) detects misfires by registering the time between two sectors of the flywheel.

The time between the two segments varies depending on: 

• misfiring
• driveline oscillations
• normal variations caused by uneven combustion
• flywheel mechanical tolerances

1. The mechanical tolerances and drive line oscillations disrupt the signal and it is difficult to detect whether the engine is misfiring or not. To eliminate mechanical faults/damage to the flywheel the flywheel signal is adapted. Two crankshaft rotations are split into 5 intervals for 5 cylinder engines. The purpose of adaptation is to filter out the tolerances in the flywheel and resonance in the engine. By registering the time deviations between the sectors on the flywheel misfiring can be detected. In order for the engine control module (ECM) to be able to register misfires, the flywheel must be adapted. Misfire diagnostics are shut off until the flywheel is adapted for the first time. This adaptation value is saved and then used in subsequent operating cycles.

Adaptation of the flywheel is done when: 

• engine speed is between 2300 RPM and 3000 RPM
• the load exceeds 40% of relative load

Flywheel adaptation takes approximately 60 seconds.

1. Drive line oscillations, caused by uneven road surfaces for example, may lead to uneven engine operation (false misfiring). This is registered by the ABS module which sends this information to the engine control module (ECM) which uses this information in order to separate the oscillations from genuine misfiring. The misfire diagnostic is shut off when oscillations occur in the drive line caused by uneven road surfaces.

Misfire diagnostics are also shut off when: 

• leak diagnostic, when leak diagnostics are taking place
• diagnostic trouble codes (DTCs) in the engine speed (RPM)/position sensor, mass air flow (MAF) sensor, engine temperature sensor and the leak diagnostic and ABS.

1. 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.
2. The engine control module (ECM) registers and stores which engine speeds and load ranges the misfiring occurred. Misfiring must occur within the same engine speed and load range again for a diagnostic trouble code (DTC) to be set. 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 received in the next operating cycle.
3. If the misfire stops, the requirements for the RPM and load parameters must be met before the engine control module (ECM) will begin counting down to extinguish the warning lamp and erase stored diagnostic trouble codes (DTCs) for misfiring.