Diagnostic Aids

Misfire index counts number of misfires. Scan tool can monitor misfire index. There is a current and a history misfire counter for each cylinder. Use current misfire counter to determine which cylinder is misfiring. Use 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 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 fuel for the following conditions:
  • Condition and quality. Dirty or contaminated fuel could cause a misfire condition.
  • Fuel level. If fuel level is low, fuel pump may draw air into fuel rail, causing a stumble and a possible misfire condition. Check fuel trim number in freeze frame to determine if this has occurred. This condition would be likely if short term fuel number was more than 20 percent.
  • 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 Heated Oxygen Sensor (HO2S) for abnormal voltage readings.
• Check for a vacuum leak as a possible cause of 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 TROUBLE CODE DETERMINATION  under ON-BOARD DIAGNOSTICS under SELF-DIAGNOSTIC SYSTEM.