Misfire Detection: Notes

As part of the CARB/OBD regulations the engine control module must determine if misfire is occurring and also identify the specific cylinder(s) and the severity of the misfire event, and whether it is emissions relevant or catalyst damaging. In order to accomplish these tasks the control module monitors the crankshaft for acceleration losses during firing segments of each cylinder based on firing order.

Misfire Detection Example: M54 (6 Cyl.) with Siemens System The misfire/engine roughness calculation is derived from the differences in the period duration (T) of individual increment gear segments. Each segment period consist of an angular range of 120° crank angle that starts 78° before Top Dead Center (TDC).

If the expected period duration is greater than the permissible value a misfire fault for the particular cylinder is stored in the fault memory of the ECM. Depending on the level of misfire rate measured the control unit will illuminate the "Service Engine Soon" light, may cutoff fuel to the particular cylinder and may switch lambda operation to open-loop. All misfire faults are weighted to determine if the misfire is emissions relevant or catalyst damaging.