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 be able to 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 (UART). 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 is set, this additional DTC will also set indicating that rough road data
was not available during the misfire calculation due to an ABS System malfunction.
Conditions for Setting the DTC
| • | DTCs P0100, P0102, P0103, P0122, P0123, P0335, P0336, P0742 not
set. |
| • | Vehicle speed above 5 mph. |
| • | Light to moderate engine load. |
| • | Engine misfire detected (DTC P0300 set). |
| • | An ABS system malfunction exists preventing the PCM from receiving
rough road detection data. |
Action Taken When the DTC Sets
| • | The PCM will illuminate the Malfunction Indicator Lamp (MIL) after
two consecutive drive trips that the misfire diagnostic runs and fails. This
DTC can only set if the MIL is already ON for misfire. |
| • | The PCM will record operating conditions at the time the diagnostic
fails. This information will be stored in the Freeze Frame and/or Failure
Records. |
Conditions for Clearing the MIL/DTC
| • | The PCM will turn the MIL OFF after three consecutive drive trips
that the misfire diagnostic runs and does not fail. |
| • | If no failures are reported by this or any other non-emission
related diagnostic. |
| • | A last test failed (Current DTC) will clear when the diagnostic
runs and does not fail. |
| • | PCM battery voltage is interrupted. |
Diagnostic Aids
Using Freeze Frame and/or Failure Records data may aid in locating an
intermittent condition. If the DTC cannot be duplicated, the information included
in the Freeze Frame and/or Failure Records data can be useful in determining
how many miles since the DTC set. The Fail Counter and Pass Counter can also
be used to 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 were noted. This will isolate
when the DTC failed. For intermittents, refer to
Symptoms
.
Test Description
Number(s) below refer to step numbers on the diagnostic table.
-
The
ABS system must be serviced before diagnosing for misfire, because actual
engine misfire may or may not exist. The following two examples illustrate
how this DTC can be set with or without actual engine misfire:
| • | There was actual engine misfire during an ABS system malfunction. |
| • | The vehicle was driven 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 misfire. |
