Misfire Diagnostic

Misfire Diagnostic

With the crankshaft sensor the segment time deviation between two following ignitions is measured.

The crankshaft is divided into 5 or 6 segments depending on engine cylinder type. Each segment corresponds to a specific ignition/cylinder. To avoid incorrect segment time deviations, due to manufacturing tolerances, a crankshaft adaptation must be accomplished. Nowadays the crankshaft adaptation is performed during fuel-on, unlike previously fuel-off. Misfire detection is shut-off for loads below the Zero Load-line at engine speeds up to 3000 rpm, and also shut-off for loads under the 4"Hg-line from 3000 rpm up to redline. It is also shut-off during rough road operation, which is determined by signal from the ABS control unit.

Misfire detection is enabled when the engine speed has reached 150 rpm below warm idle speed plus two crankshaft revolutions or after nine ignitions, depending on which occurs first. For detection of emission related misfires, the number of misfires which have occurred within the first interval of 1000 engine revolutions or the 4th exceedance (for the rest of DCY) over the emission threshold value after the first 1000 engine revolutions are relevant. If the number is so high that the exhaust emission standard is exceeded by a factor of 1.5, then the emission related misfire rate has been reached and exceeded a fault code will be stored. If misfires occur and the threshold is exceeded in the following DCY, MIL illuminates.

For detection of catalyst damaging misfires, the number of misfires that have occurred during an interval of 200 engine revolutions are relevant. If the number of misfires are so high that the catalyst is endangered (by various number of misfires depending on actual engine operating range), then the cat. Damaging misfire rate has been reached and exceeded a fault code will be stored. MIL will blink with one Hertz as long as the engine has catalyst-damaging misfires.