Engine management system - V8 (Misfire detection): Description and operation

Misfire detection

Due to increasing legislation, all new vehicles must be able to detect two specific levels of misfire.

Conditions The ECM is able to carry out misfire detection as part of the OBD system using the following component parts: l Flywheel reluctor adaptation. l Calculation of engine roughness. l Detection of excess emissions misfire. l Detection of catalyst damaging misfire.

Function The flywheel/ reluctor ring is divided into four segments 90° wide. The ECM misfire detection system uses information generated by the CKP to determine crankshaft speed and position. If a misfire occurs, there will be an instantaneous slight decrease in engine speed. The ECM misfire detection system is able to compare the length of time each 90° segment takes and is therefore able to pinpoint the source of the misfire.

For the ECM misfire detection system to be calibrated for the tolerances of the reluctor tooth positions, the flywheel/ reluctor ring must be 'adapted' as follows: l 1800 - 3000 rev/min = speed range 1. l 3000 - 3800 rev/min = speed range 2. l 3800 - 4600 rev/min = speed range 3. l 4600 - 5400 rev/min = speed range 4.

The ECM carries out flywheel/ reluctor ring adaptions across all the above speed ranges and can be monitored by TestBook. The test should be carried out as follows: l Engine at normal operating temperature. l Select second gear (for both automatic and manual transmission vehicles). l Accelerate until engine rev limiter is operational. l Release throttle smoothly to allow engine to decelerate throughout the speed ranges. l Repeat process as necessary until all adaptations are complete.

TestBook is able to retrieve the following misfire detection fault codes: