P0300

DTC P0300 Engine Misfire Detected




Circuit Description
The Crankshaft Position (CKP) Sensor is the primary input to determine if misfire is occurring. Engine misfire is detected by monitoring crankshaft speed variations between cylinders. If a crankshaft deceleration occurs during a combustion or power stroke, the VCM will compare this change in crankshaft speed to the previous cylinder. If the crankshaft speed change is greater than a maximum allowable amount, the VCM will have detected a misfire. Misfire may occur on a specific cylinder or may be random and affect all cylinders.

When an engine is misfiring, brief decelerations in crankshaft rotational speed will be detected by the CKP and the signal is input to the VCM. This signal is monitored by the VCM which can determine which cylinder has misfired from the Camshaft Position CMP Sensor input. The VCM stores the misfires for each cylinder in separate accumulators. After 100 combustion events, these accumulated misfire totals are checked by the VCM to determine if any have exceeded a calibrated maximum number. If the misfire is excessive, a DTC P0300 will be set.

Conditions for Running the DTC

IMPORTANT: If start-up ECT is below -7 °C (20 °F), misfire detection is delayed until ECT is greater than 21 °C (70 °F). If start-up ECT is greater than -7 °C (20 °F), misfire detection begins after a 5 second delay.

- No active VS sensor DTCs
- No active TP sensor DTCs
- No active MAF sensor DTCs
- No active CKP sensor DTCs
- No active CMP sensor DTCs
- The fuel level is greater than 10 percent.
- The engine speed is between 450 - 5,000 RPM
- The system voltage is between 11 - 16 volts
- The throttle position is steady within 2 percent for 100 ms.

Conditions for Setting the DTC
The VCM detects a deceleration in the crankshaft speed characteristic of either an emission type misfire or a catalyst damaging type misfire.

Action Taken When the DTC Sets
- The VCM illuminates the MIL (Malfunction Indicator Lamp) if a failure is detected during 2 consecutive key cycles.
- The VCM will set the DTC and records the operating conditions at the time the diagnostic fails. The VCM stores the failure information in the scan tools Freeze Frame and/or the Failure Records.

Conditions for Clearing the MIL/DTC
- The VCM turns OFF the MIL after 3 consecutive drive trips when the test has Run and Passed.
- A history DTC will clear if no fault conditions have been detected for 40 warm-up cycles (coolant temperature has risen 22 °C (40 °F) from the startup coolant temperature and the engine coolant temperature is more than 70 °C (158 °F) during the same ignition cycle).
- Use the scan tool Clear Information function.

Diagnostic Aids
The Misfire Index counts the number of misfires. The scan tool can monitor the Misfire Index. There is a current and history misfire counter for each cylinder. Use the current misfire counter in order to determine which cylinder is misfiring.

Many different condition could cause an intermittent misfire. Check for the following conditions:
- Check the IC control circuit for a intermittent short to ground.
- Check the spark plug wires and the coil wire for the following conditions:
- Ensure that the spark plug wires are securely attached to the spark plugs and the distributor cap.
- Check the wire routing in order 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. In order to test for this condition, spray the wires with water and with the engine running, watch for spark to jump from the wires. If a spark is visible, replace the wires.
- Check for contaminated fuel or a low fuel level and the following conditions:
- Check the fuel condition and quality. Dirty or contaminated fuel could cause a misfire condition.
- If the fuel level is low, contaminants in the bottom of the fuel tank could enter into the fuel metering system.
- Check HO2S for abnormal voltage readings.
- Check for a vacuum leak as a possible cause of the engine misfire.
- Sticking intake or exhaust valves can cause a misfire when the engine is cold.
- For more information, refer to fuel supply system.

Steps 1 - 4:

Steps 5 - 8:

Steps 9 - 14:

Steps 15 - 16:

Test Description
The numbers below refer to the step numbers on the diagnostic table.
2. If DTC's P0337 (Crankshaft Position Sensor Circuit Low Input) or P0338 (Crankshaft Position Sensor Circuit - High Input) are set, This could result in a misfire condition.
3. In order to duplicate the conditions under which the misfire occurred, it may be necessary to drive the vehicle and monitor the scan tool DTC Set This Ignition Cycle variable.
4. If there is not a current misfire, performing the CKP System Variation Learn procedure may correct an intermittent condition
6. If all of the misfire counters are increasing, then look for a problem with a component effecting all cylinders. Examples: coil wire, distributor cap and rotor, ignition coil and fuel.
7. When checking the spark at the spark plug wires, the spark should be consistent. A few sparks then nothing is no spark.
9. At this point, the ignition system is OK and the problem may be in the fuel system or an engine mechanical problem.