Powertrain Controls Diagnosis

SYSTEM DESCRIPTION
The powertrain control module (PCM) uses information from the crankshaft position (CKP) sensors and the camshaft (CMP) sensor in order to determine when an engine misfire is occurring. By monitoring variations in the crankshaft rotation speed for each cylinder, the PCM is able to detect individual misfire events. A misfire rate that is high enough can cause three-way catalytic converter (TWC) damage. The malfunction indicator lamp (MIL) will flash ON and OFF when the conditions for catalytic converter damage are present.

CONDITIONS FOR RUNNING THE DTC

IMPORTANT: If the start-up engine coolant temperature (ECT) is less than 7°C (20°F), misfire detection is delayed until engine coolant temperature (ECT) is more than 21°C (70°F). If the start-up ECT is more than 7°C (20°F), misfire detection begins after a 5 second delay.

^ DTCs P0101, P0102, P0103, P0116, P0117, P0118, P0128, P0335, P0336, P0341, P0343, P0500, P0502, P0503, P1114, P1115, P1121, P1122, P1220, P1258, P1336, or P1345 are not set.
^ The fuel level is more than 10 percent
^ Then engine speed is between 450-5,000 RPM.
^ The system voltage is between 11-16 volts.
^ The throttle position (TP) is steady within 2 percent for 100 ms.

CONDITIONS FOR SETTING THE DTC
The powertrain control module (PCM) detects a deceleration in the crankshaft speed that is characteristic of either an emission type misfire or of a catalyst-damaging type misfire.

ACTION TAKEN WHEN THE DTC SETS
^ The control module illuminates the malfunction indicator lamp (MIL) on the second consecutive ignition cycle that the diagnostic runs and fails.
^ The control module records the operating conditions at the time the diagnostic fails. The first time the diagnostic fails, the control module stores this information in the Failure Records. If the diagnostic reports a failure on the second consecutive ignition cycle, the control module records the operating conditions at the time of the failure. The control module writes the operating conditions to the Freeze Frame and updates the Failure Records.

CONDITIONS FOR CLEARING THE MIL/DTC
^ The control module turns OFF the malfunction indicator lamp (MIL) after 3 consecutive ignition cycles that the diagnostic runs and does not fail.
^ 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 emission related diagnostic.
^ Clear the MIL and the DTC with a scan tool.

DIAGNOSTIC AIDS
The misfire index counts the number of misfires. The scan tool can monitor the misfire index. There is a current and a history misfire counter for each cylinder. Use the current misfire counter in order to determine which cylinder is misfiring. Use the history misfire counter for misfires that are not currently present.

Many different conditions could cause an intermittent misfire. Check the following conditions:
^ Check the ignition control (IC) circuit for an intermittent short to ground.
^ Ensure that the spark plug wires are securely attached to the spark plugs and the distributor cap.
^ Ensure that the coil wire is securely attached to the distributor cap and the coil.
^ Check the wire routing to ensure that cross-firing is not occurring.
^ If the misfire occurs when the weather is damp, the problem could be due to worn plug wires.
^ Check the fuel for the following conditions:
- Condition and quality - Dirty or contaminated fuel could cause a misfire condition.
- The fuel level - If the fuel level is low, the fuel pump may draw air into the fuel rail, causing a stumble and a possible misfire condition. Check the fuel trim number in the Freeze Frame in order to determine if this has occurred. This condition would be likely if the short term fuel number was more than +20.
- Restrictions - A restricted fuel filter can also cause a misfire.
^ Check for sticking intake or exhaust valves on engines which misfire when the engine is cold.
^ Check the heated oxygen sensor (HO2S) for abnormal voltage readings.
^ Check for a vacuum leak as a possible cause of the engine misfire.

An intermittent may be caused by any of the following conditions:
^ A poor connection
^ Rubbed through wire insulation
^ A broken wire inside the insulation

Thoroughly inspect any circuitry that is suspected of causing the intermittent complaint. Refer to Testing for Intermittent and Poor Connections in Diagnostic Aids. If a repair is necessary, refer to Wiring Repairs in Diagnostic Aids.

TEST DESCRIPTION

DTC P0300 - Steps 9-13:

DTC P0300 - Steps 9-13:

DTC P0300 - Steps 14-18:

The numbers below refer to the step numbers on the diagnostic table,
5. If the Misfire Current Counters are incrementing, but the engine is NOT misfiring, this indicates a mechanical condition. For example, an accessory drive belt could cause this condition.
7. The spark should be consistent when you test the spark at the spark plug wires. The presence of few sparks then nothing indicates no spark.