Circuit Description
The PCM detects engine misfire by detecting variations in crankshaft
deceleration between firing strokes. For accurate detection of engine misfire
the PCM must distinguish between crankshaft deceleration caused by actual
misfire and deceleration caused by rough road conditions. The ABS Brake
System can detect if the vehicle is on a rough road based on wheel acceleration/deceleration
data supplied by the wheel speed sensors. If the ABS system detects rough
road above a predetermined threshold, this information is sent to the PCM
via serial data. The PCM can then take the rough road into account when
calculating misfire. Even if the ABS is malfunctioning and cannot detect
rough roads, the misfire diagnostic will continue to run. However, if a
misfire DTC sets, this additional DTC also sets indicating that rough road
data was not available during the misfire calculation due to an ABS System
malfunction.
Conditions for Setting the DTC
| • | DTCs P0101, P0102, P0103, P0335, P0336, P0742 P1120, P1121, P1220,
P1221 not set. |
| • | The vehicle speed is above 10 mph. |
| • | The engine load is less than 60%. |
| • | Engine misfire is detected (DTC P0300 set). |
| • | The engine speed is less than 3200 RPM. |
| • | An ABS system malfunction exists preventing the PCM from receiving
rough road detection data. |
Action Taken When the DTC Sets
| • | The PCM illuminates the Malfunction Indicator Lamp (MIL) on the
second consecutive drive trip that the misfire diagnostic runs and fails.
This DTC can only set if the MIL is already ON for misfire. |
| • | The PCM records the operating conditions at the time the diagnostic
fails. The PCM stores this information in the Freeze Frame and/or Failure
Records. |
Conditions for Clearing the MIL/DTC
| • | The PCM turns OFF the malfunction indicator lamp (MIL) after
3 consecutive ignition cycles that the diagnostic runs and does not
fail. |
| • | A last test failed, or current DTC, clears when the diagnostic
runs and does not fail. |
| • | A history DTC clears after 40 consecutive warm-up cycles,
if no failures are reported by this or any other emission related diagnostic. |
| • | Use a scan tool in order to clear the MIL and the DTC. |
Diagnostic Aids
Using the Freeze Frame and/or Failure Records data may aid in locating
an intermittent condition. If you cannot duplicate the DTC, the information
included in the Freeze Frame and/or Failure Records data can help determine
how many miles since the DTC set. The Fail Counter and Pass Counter can
also help determine how many ignition cycles the diagnostic reported a pass
and/or a fail. Operate vehicle within the same freeze frame conditions (RPM,
load, vehicle speed, temperature etc.) that you observed. This will isolate
when the DTC failed. For an intermittent condition, refer to
Symptoms
.
Test Description
The numbers below refer to the step numbers on the diagnostic table.
-
Service the ABS system before diagnosing for a misfire, because an actual
engine misfire may or may not exist. The following two examples illustrate
how this DTC can be set with or without an actual engine misfire:
| • | There was an actual engine misfire during an ABS system malfunction. |
| • | The vehicle operator drove on a rough road, the ABS system could
not detect this due to a malfunction, and the PCM interpreted the variations
in crankshaft speed caused by the rough road as a misfire. |
