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 control module will compare this change in crankshaft speed to the previous cylinder. If the crankshaft speed change is more than a maximum allowable speed, the misfire is detected. Misfire may occur in a specific cylinder or in all cylinders randomly.

When an engine is misfiring, brief decelerations in crankshaft rotational speed will be detected by the CKP. The control module determines which cylinder has misfired based upon the camshaft position (CMP) sensor input. Misfire data is stored for each cylinder in separate accumulators. After 100 combustion events, the misfire totals are compared to a calibrated maximum number. If the misfire is excessive, this diagnostic trouble code (DTC) will set.

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

If the VCM determines that the engine misfire is significant enough to have a negative impact on emissions, the VCM turns ON the malfunction indicator lamp (MIL) after the misfire has been detected on 2 non-consecutive trips under the same operating conditions. If the misfire is severe enough that catalytic converter damage could result, the MIL flashes while the misfire is present.

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:

1. 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 crossfiring 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.

2. Check for contaminated and 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.

For more information, refer to fuel supply system.

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

2. If DTCs P0337 (Crankshaft Position Sensor Circuit Low Input) or P0338 (Crankshaft Position Sensor Circuit - High Input) are set, this could result in a misfire condition.

5. 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.

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.