Misfire Monitor Diagnosis

MISFIRE MONITOR DIAGNOSTIC OPERATION
The misfire monitor diagnostic is based on crankshaft rotational velocity (reference period) variations. The PCM determines crankshaft rotational velocity using the crankshaft position sensor and camshaft position sensor. When a cylinder misfires the crankshaft slows down momentarily. By monitoring the crankshaft and camshaft position sensor signals, the PCM can calculate when a misfire occurs.

CATALYST DAMAGING MISFIRE
For a non-catalyst damaging misfire, the diagnostic is required to monitor a misfire present for between 1000-3200 engine revolutions. For catalyst damage misfire, the diagnostic responds to the misfire within 200 engine revolutions.

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

TCC DISABLING MISFIRE
On 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 crankshaft rotation. When the TCC has disabled as a result of misfire detection, the TCC will re-enabled 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 reevaluate the system.