Misfire Diagnostics

MISFIRE DIAGNOSTICS

The engine is said to misfire if ignition does not take place within a specified period. The ECM is equipped to detect misfiring by recording the interval between two segments on the flywheel/carrier plate. This interval may vary depending due to the following causes:

^ Misfiring
^ Flywheel mechanical tolerances
^ Driveline oscillations
^ Normal variations caused by uneven combustion
^ Poor road

Since the mechanical tolerance and driveline oscillation factors also disturb the signal, it is difficult to ascertain whether or not the disturbance is due to misfiring. Modulation of the flywheel signal is carried in order to eliminate mechanical faults in the flywheel.

The purpose is to even out the signal so that a mechanical fault in the flywheel cannot be registered as a misfire.

The modulated signal still contains some disturbance due to driveline oscillations and normal physical variations in the engine.

The signal is modulated when:
^ the speed is between 1200 and 3000 rpm;
^ the engine braking fuel cut-off function has been active for 30 ignitions.

Flywheel signal modulation is completed after 40 seconds of accumulated fuel shut-off within the specified engine speed band.

The acceleration sensor signal is used by the ECM to determine the degree of vertical movement of the car, for example, on a bumpy road. Since this may also cause uneven engine running, the ECM uses the signal to distinguish the phenomenon from actual misfiring.

B5254S: Misfiring results in lighting of the MIL lamp. If there is a risk of TWO damage, the MIL will flash as long as misfiring continues, after which it will shine with a steady light.

The control module registers and stores the engine speed and load band within which misfiring occurred. If misfiring ceases, this entire rpm and load band must be completed without any misfiring in order for the control module to start the countdown for extinguishing the MIL and erasing the DTCs.