Misfire Diagnostics

MISFIRE DIAGNOSTICS





If the fuel/air mixture does not combustion any one cycle or does not burn as it should, the engine is said to be misfiring. The Engine Control Module (ECM) can detect misfiring. This is done by recording the time between two segments on the flywheel/carrier plate.

This 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 drive train oscillation interfere with the signal, it is difficult to ascertain whether or not this interference is due to misfiring. To eliminate mechanical flywheel faults the flywheel signal is adapted. Two crankshaft revolutions are divided into five periods, if the engine is unloaded all five periods should be equal.

The intention is to even out the signal so that flywheel tolerances are not registered as misfiring.

After adaption there is still some interference in the signal. This is because of oscillations in the drive train and normal engine irregularities.

The flywheel signal is adapted when:

^ Engine speed is between 1,200 rpm and 3,000 rpm
^ The fuel shut-off system is operating during engine braking and has been active for 30 ignitions.

Flywheel signal adaption finishes after 40 second's accumulated fuel shut-off within that engine speed range.

The accelerometer signal is used by the engine control module (ECM) to determine how much the car is moving vertically and longitudinally, on a bumpy road for example. These irregularities can cause uneven engine running. The engine control module (ECM) uses this signal to differentiate irregularities from real misfiring. The misfire diagnosis is disconnected when driving on bumpy roads.





Misfiring lights the Malfunction Indicator Lamp (MIL). If there is a risk of catalytic converter damage the malfunction indicator lamp (MIL) flashes for as long as the misfiring continues, it then lights continuously.

The engine control module (ECM) registers and stores the engine speed and load range in which the misfiring occurred. The malfunction indicator lamp (MIL) lights if misfiring occurs again within the same engine speed and load range. If misfiring ceases, one must go through this entire engine speed and load range without a misfire before the engine control module (ECM) starts counting down to put out the malfunction indicator lamp and erase the misfire codes.