Circuit Description

PCM detects engine misfire by detecting variations in crankshaft deceleration between firing strokes. For accurate detection of engine misfire, PCM must be able to distinguish between crankshaft deceleration caused by actual misfire or deceleration caused by rough road conditions.

ABS system can detect if vehicle is on a rough road based on wheel acceleration/deceleration data supplied by wheel speed sensors. If ABS system detects rough road above a predetermined threshold, this information is sent to PCM via serial data. PCM can then take rough road into account when calculating misfire. Even if ABS is malfunctioning and cannot detect rough roads, misfire diagnostic will continue to run. However, if a misfire DTC is set, this additional DTC will also set indicating that rough road data was not available during misfire calculation due to ABS system malfunction.

DTC will set when following conditions are present:

DTCs P0101, P0102, P0103, P0335, P0336, P0742, P1120, P1121, P1220 or P1221 not set.
Vehicle speed greater than 10 MPH.
Engine load less than 60 percent.
DTC P0300 set.
Engine speed is less than 3200 RPM.
An ABS system malfunction exists preventing rough road detection data.

NOTE: ABS system must be diagnosed/serviced before diagnosing misfire. Actual engine misfire may or may not exist.