Misfire diagnostic: Overview

If the fuel/air mixture does not ignite in the ignition stroke the engine is said to be misfiring. The engine control module (ECM) detects misfires by registering the time between two segments of the flywheel.

The time between two segments varies depending on 

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

The mechanical tolerances and drive line oscillations disrupt the signal and it is difficult to detect whether the engine really is misfiring or not. By registering the time difference between the sectors and comparing this with the time differences in an earlier rotation misfires can be gauged. To eliminate mechanical tolerances one of the 5 sectors on the flywheel is used as a reference point for the timing.