Misfire Monitor Diagnostic Operation

The misfire monitor diagnostic is based on crankshaft rotational velocity, reference period, variations. The control module determines the crankshafts rotational velocity by using the Crankshaft Position (CKP) sensor and the Camshaft Position (CMP) sensor. When a cylinder misfires, the crankshaft slows down momentarily. By monitoring the CKP and CMP sensor signals, the control module can calculate when a misfire occurs.

For a non-catalyst damaging misfire, the diagnostic will report the misfire within 1,000-3,200 engine revolutions.

For catalyst damaging misfire, the diagnostic will respond to the misfire within 200 engine revolutions.

Rough roads may cause a false misfire detection. A rough road will cause torque to apply to the drive wheels and the drive train. This torque can intermittently decrease the crankshaft rotational velocity. This may be detected as a false misfire.

On automatic transaxle equipped vehicles, the Torque Converter Clutch (TCC) will be disabled 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 been disabled as a result of a misfire detection, the TCC will re-enable after approximately 3,200 engine revolutions if no further misfire is detected. The TCC will remain disabled whenever a misfire is detected. This allows the misfire diagnostic to re-evaluate the system.

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