Possible Causes
NOTE: The listed causes may not be a complete list of all potential problems and there may be other causes.
- Faulty ignition coil
- Intake vacuum leak
- Faulty camshaft position sensor
- Ignition coil harness is open or shorted
- Poor electrical connection in the ignition coil circuit
Description
Under CARB/OBD regulations, the Engine Control Module (ECM) must determine if a misfire is occurring and also identify the specific cylinder(s) and the severity of the misfire event, and whether it is an emissions problem or if it damages the catalyst. To accomplish these tasks, the control module monitors the crankshaft for acceleration losses during the firing segments of each cylinder based on the firing order.
The misfire/engine roughness calculation is derived from differences in the period duration (T) of individual gear segments. Each segment period consists of an angular range of 120° crankshaft angle that begins 78° before top dead center (TDC).
If the expected period duration is greater than the allowed value, a misfire fault for the affected cylinder is stored in the ECM‘s fault memory. Depending on the measured misfire level, the control unit will illuminate the Service Engine Soon light, may cut fuel to the particular cylinder, and may switch lambda operation to open loop. All misfire faults are weighted to determine if the misfires are emissions-related or damaging to the catalyst.
How to Repair
Check the “Possible Causes” listed above. Visually inspect the wiring harness and associated connectors. Check for damaged components and look for broken, bent, pushed out, or corroded connector pins.
Possible Symptoms
- Engine light on (or Service Engine Soon warning light)