Misfire Detection

As part of the CARB/OBD II regulations the Engine Control Module must determine if misfire is occurring and also identify the specific cylinder(s). The ECM will determine severity of the misfire event, and whether it is emissions relevant or catalyst damaging. In order to accomplish these tasks the ECM monitors the crankshaft for acceleration losses during firing segments of cylinder specific firing order. If the signal is implausible an erroneous reference mark can be obtained by the ECM which will result in a misfire fault being set.

Possible causes of cylinder misfire faults (actual field findings):

• Vehicle ran low or out of fuel
• Poor fuel quality (ex. water in fuel, customer uses an additive, etc.)
• Low/high fuel pressure
• Ignition coil
• Fouled spark plug(s)
• Restricted/contaminated fuel injector(s)
• Crankshaft position sensor
• Poor combustion due to low compression or high leakage
• Blocked/restricted catalyst