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:

1. Check the ignition control (IC) circuit for an intermittent short to ground.
2. Ensure that the spark plug wires are securely attached to the spark plugs and the distributor cap.
3. Ensure that the coil wire is securely attached to the distributor cap and the coil.
4. Check the wire routing to ensure that cross-firing is not occurring.
5. If the misfire occurs when the weather is damp, the problem could be due to worn plug wires.
6. 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.
7. Check for sticking intake or exhaust valves on engines which misfire when the engine is cold.
8. Check the Heated Oxygen Sensor (HO2S) for abnormal voltage readings.
9. 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. See INTERMITTENT CONDITIONS  under SELF-DIAGNOSTIC SYSTEM.