Engine timing, the relationship between the crankshaft and camshaft is maintained either by a timing chain or timing belt. The Engine Management System does not control engine timing but it does monitor the relative position and speed of these shafts by monitoring signals generated by sensors. The Engine Management System (EMS) uses signals generated by the camshaft position sensor to synchronize fuel injection to the valve sequence and for the on-board diagnostic procedure for misfire detection. The EMS energizes the injector at or near the time the intake valve opens. For misfire diagnosis, the EMS compares the number of camshaft sensor reference pulses and the number of crankshaft position sensor reference pulses received. If the EMS receives an incorrect number of pulses, Diagnostic Trouble Codes (DTC’s) should be stored in the EMS.
If the camshaft position sensor or circuit is faulty, most engines will start. Some engine management systems will then default to a pre-programmed injector firing sequence. All injectors may be energized simultaneously or all of the injectors on one bank may be energized at the same time. If the CMP data is required for misfire detection and reliable CMP data is not present, misfire detection would probably be suspended.
The following symptoms can be caused by an intermittent wiring connection or faulty signal to the EMS:
Note: For procedures on the position sensors, please refer to Driveability and Emissions.