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 serial data communication between the ABS System
and the PCM is malfunctioning and the PCM can not receive
the rough road data, the misfire diagnostic continues
to run. However, if a misfire DTC sets, this additional
DTC also sets indicating that the rough road data was not available
during the misfire calculation due to a serial data malfunction.
Conditions for Running the DTC
| • | The vehicle speed is above 10 mph. |
| • | The engine speed is below 3200 RPM. |
| • | The engine load is less than 60 percent. |
| • | Engine misfire is detected (DTC P0300 set). |
Conditions for Setting the DTC
| • | A serial data malfunction exists preventing the PCM from receiving
rough road detection data. |
| • | The above conditions met for 20 seconds. |
Action Taken When the DTC Sets
| • | The powertrain control module (PCM) stores the DTC information into memory
when the diagnostic runs and fails. |
| • | The malfunction indicator lamp (MIL) will not illuminate. |
| • | The PCM records the operating conditions at the time the diagnostic fails.
The PCM stores this information in the Failure Records. |
Conditions for Clearing the DTC
| • | A last test failed, or current DTC, clears when the diagnostic
runs and does not fail. |
| • | A history DTC will clear after 40 consecutive warm-up cycles,
if no failures are reported by this or any other non-emission related diagnostic. |
| • | Use a scan tool in order to clear the DTC. |
Diagnostic Aids
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. |
