Misfire Monitor Diagnosis

Misfire Monitor Diagnostic Operation

The misfire monitor diagnostic is based on crankshaft rotational velocity (reference period) variations. The control module 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 control module can calculate when a misfire occurs.

For a non-catalyst damaging misfire, the diagnostic will be required to report a misfire present within 1000-3200 engine revolutions.

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

Rough roads may cause false misfire detection. A rough road will cause torque to be applied to the drive wheels and 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 drive wheel inputs on crankshaft rotation.

When the TCC has been disabled as a result of misfire detection, it will be re-enabled after approximately 3200 engine revolutions if no misfire is detected. The TCC will remain disabled whenever misfire is detected. This allows the misfire diagnostic to reevaluate the system.

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