Powertrain Controls Diagnosis (BI-Fuel)

DTC P0300

System Description

The powertrain control module (PCM) uses information from the crankshaft position (CKP) sensor and the camshaft position (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 misfire events.

Conditions for Running the DTC

- DTCs P0101, P0102, P0103, P0106, P0107, P0108, P0117, P0112, P0113, P0116, P0117, P0118, P0121, P0122, P0123, P0125, P0128, P0315, P0335, P0336, P0341, P0342, P0343, P0502, P0503, P1114, P1115, P1121, P1122, and P1258 are not set.
- If the start-up engine coolant temperature (ECT) is less than -7°C (20°F), misfire detection is delayed until 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.
- The fuel level is more than 10%.
- The engine coolant temperature (ECT) is less than 130°C (266°F).
- The engine speed is between 400 and 5200 RPM.
- The system voltage is between 11 and 18 volts.
- The throttle position is steady.
- The A/C compressor clutch is steady.

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 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

Important: When installing new gasoline fuel injectors on KL6 equipped vehicles, remove and discard the gasoline fuel injector lower seal (O-ring). If the gasoline fuel injector is installed with the lower seal in place, the flow of compressed natural gas (CNG) fuel through the fuel injector retainer (adapter tee) will be restricted. Restricted CNG fuel flow through the retainer (adapter tee) will result in a cylinder misfire while operating on CNG.

The misfire index counts the number of misfires. The scan tool can monitor the misfire index. There is a current misfire counter 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 can cause an intermittent misfire. Inspect the following conditions:
- Inspect 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 ignition coil.
- If the misfire occurs when the weather is damp, the condition could be due to worn spark plug wires.
- Test or inspect the fuel for the following conditions:
- Inspect for contaminated fuel.
- Inspect the fuel filter for a restriction. A restricted fuel filter can cause a misfire.
- Inspect for sticking intake or exhaust valves.
- Inspect the heated oxygen sensor (HO2S) for abnormal voltage readings.
- Inspect for a vacuum leak as a possible cause of the engine misfire.

Test Description

The numbers below refer to the step numbers on the diagnostic table.

5. This step determines if the engine is misfiring while operating on the gasoline fuel system.
7. 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.
8. This step verifies that the Crankshaft Variation Learn Procedure is performed. A crankshaft variation that is not learned may cause the misfire counters to increment.
10. 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.
14. A misfiring cylinder may cause more than one misfire counter to increment. Diagnose the fuel injector circuit of the cylinder that displays the highest level of misfire.
20. This step verifies that the catalytic convertors are operating correctly.