Misfire Monitoring: Notes

Misfire detection must determine if misfire is occurring, identify specific cylinder(s) and the severity of misfire, and whether it is emissions relevant or catalyst damaging. See Fig 1. To do this, control module monitors crankshaft for acceleration losses during firing segments of each cylinder based on firing order. Process of misfire detection continues well after diagnostic drive cycle requirements have been completed. Misfire detection is ongoing and is only discontinued under certain conditions. See MISFIRE DETECTION DISABLING CONDITIONS table. See Fig 2-Fig 3 .

MISFIRE DETECTION DISABLING CONDITIONS

Condition	Specification
Engine Speed	Less Than 512 RPM
Engine Load	Varying/Unstable
Throttle Angle	Varying/Unstable
Timing	Timing Retard Request Active
Engine Start Up	Up To 5 Seconds After Start
A/C	Up To .5 Seconds After A/C Activation
Decel Fuel Cut-Off	Active
Rough Road Recognition	Active
ASC Control	Active

Fig 1: Identifying OBD-II Misfire Faults

Courtesy of BMW OF NORTH AMERICA, INC.

Fig 2: Diagnosing Engine Misfire (1 Of 2)

Courtesy of BMW OF NORTH AMERICA, INC.

Fig 3: Diagnosing Engine Misfire (2 Of 2)

Courtesy of BMW OF NORTH AMERICA, INC.