Misfire Diagnostic

Misfire Diagnostic

If the fuel/air mixture does not burn correctly, then the generated torque will be less than intended and the engine rpm will drop suddenly, (decelerate) the engine is said to be misfiring. The control module can detect misfiring by measuring the time between successive segments on the flywheel /carrier plate.

If there is a misfire then there will be a step change in the size of these successive time measurements, if there is a misfire the lost torque will be noticed as a slowing down of the flywheel rotation. The prerequisite for reliable misfire detection is accurate segment period measurement. However, the period between two top dead centers (TDC), at constant speed, is also subject to variations due to manufacturing tolerances and off center installation. These inaccuracies are systematic, so they can be "learned" during fuel cut off periods and used for compensation. By this way, the systematic error introduced by the tolerances of the target flywheel is largely eliminated. The segment time can vary due to the following reasons:
- Misfiring
- Flywheel mechanical tolerances
- Driveline oscillations
- Normal variations caused by uneven combustion
- Poor roads.

Since mechanical tolerances and driveline oscillation interfere with the signal, it is difficult to ascertain whether or not this interference is due to misfiring. To eliminate mechanical faults in the flywheel the flywheel signal is adapted. Two crankshaft revolutions are divided into five periods, (on a 5-cylinder engine), if the engine has no external load all five periods should be equal. This is to even out the signal, so that a mechanical fault in the flywheel is not registered as misfiring. After adaptation there is some interference in the signal due to oscillations in the drive train and normal engine irregularities. The flywheel signal is adapted when:
- Engine speed is between two targets
- The fuel shut-off system is operating and has been active for 100 revolutions.

DTC is stored for misfiring which leads to increased emissions and diagnostic trouble code is stored for misfiring which could cause damage to the TWC. The engine control module registers and stores the engine speed and load range in which the misfiring occurred. The MIL lights if misfiring occurs again within the same engine speed and load range. If misfiring ceases, the engine must go through this entire engine speed and load range without a misfire before the engine control module starts counting down to put out the MIL and erase the misfire codes.