Misfire Monitor Diagnostic Operation

The misfire monitor diagnostic is based on the crankshaft rotational variations, or reference period. The control module determines the crankshaft rotational velocity using the Crankshaft Position (CKP) Sensor and the Camshaft Position (CMP) sensor.

When a cylinder misfires, the crankshaft slows down momentarily. By monitoring the crankshaft and camshaft position sensor signals, the control module can calculate when a misfire occurs.

For a non-catalyst damaging misfire, the diagnostic is required to monitor a misfire present for between 1000-3200 engine revolutions.

For a catalyst damage misfire, the diagnostic responds to the misfire within 200 engine revolutions.

Rough roads may cause a false misfire detection. A rough road applies torque to the drive wheels and the drive train. This torque can intermittently decrease the crankshaft rotational velocity. The control module detects this as a false misfire.

On the automatic transmission equipped vehicles, the Torque Converter Clutch (TCC) will disable whenever a misfire is detected. Disabling the TCC isolates the engine from the rest of the drive line and minimizes the effect of the drive wheel inputs on the crankshaft rotation.

When the TCC has disabled as a result of misfire detection, the TCC will re-enable after approximately 3200 engine revolutions if no misfire is detected. The TCC remains disabled whenever the misfire is detected, with or without a DTC set. This allows the misfire diagnostic to re-evaluate the system.

During a transmission high temperature condition, the misfire diagnostic will disable and the TCC will operate normally. This avoids further increasing the temperature of the transmission.