Circuit Description
The powertrain control module (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 antilock brake system (ABS)
can detect if the vehicle is on a rough road based on wheel acceleration
or deceleration data supplied by the wheel speed sensors. If
the ABS 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
diagnostic trouble code (DTC) sets, this additional DTC also sets
indicating that 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 16 km/h (10 mph). |
| • | The engine speed is below 3,200 RPM. |
| • | The engine load is less than 60 percent. |
| • | An engine misfire is detected, and DTC P0300 is set. |
Conditions for Setting the DTC
A serial data malfunction exists preventing the PCM from receiving
rough road detection data for 20 seconds or more.
Action Taken When the DTC Sets
| • | The control module stores the DTC information into memory when
the diagnostic runs and fails. |
| • | The malfunction indicator lamp (MIL) will not illuminate. |
| • | The control module records the operating conditions at the time
the diagnostic fails. The control module stores this information in the Failure
Records. |
| • | The driver information center, if equipped, may display a message. |
Conditions for Clearing the DTC
| • | A current DTC Last Test Failed clears when the diagnostic runs
and passes. |
| • | A history DTC clears after 40 consecutive warm-up cycles,
if no failures are reported by this or any other non-emission related diagnostic. |
| • | Clear the DTC with a scan tool. |
Diagnostic Aids
| • | If you cannot duplicate the DTC, use the Freeze Frame/Failure
Records in order to locate an intermittent condition in the following ways: |
| - | Use the information included in the Freeze Frame/Failure
Records in order to determine how many miles since the DTC set. |
| - | Use the Fail Counter and Pass Counter in order to determine
how many ignition cycles the diagnostic test reported as a
pass or a fail. |
| - | Operate the vehicle within the same Freeze Frame conditions
that you observed in order to isolate when the DTC failed. Use the following
conditions for your tests: |
Test Description
The number below refers to the step number on the diagnostic table.
-
Service the
ABS before diagnosing for a misfire, because an actual engine misfire may
or may not exist. The following 2 examples illustrate how this
DTC can be set with or without an actual engine misfire:
| • | There was an actual engine misfire during an ABS malfunction. |
| • | The vehicle operator drove on a rough road, the ABS 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. |
