Misfire Isolation
- Start engine. Disconnect IAC motor. Using insulated pliers, remove one spark plug wire from a spark plug, and ground it against engine.
- Note engine RPM as wire is grounded. Reconnect spark plug wire. Repeat procedure for all cylinders. Stop engine, and reconnect IAC motor.
- If engine speed drops equally (within 50 RPM) on all cylinders, refer to ROUGH, UNSTABLE OR INCORRECT IDLE symptom. If there is no engine RPM drop or no excessive variation on one or more cylinders, check spark on respective cylinder(s).
Possible Causes & Corrections:
- Check for restricted exhaust system. See EXHAUST SYSTEM BACKPRESSURE under MECHANICAL INSPECTION in BASIC TESTING - GASOLINE article in the ENGINE PERFORMANCE section.
- Check for bent push rods, broken valve springs or worn camshaft lobes.
- Check ignition wires for short or faulty insulation.
- Check distributor cap (if equipped) for moisture, dust or cracks. Finely mist spark plug wires with water to check for shorts.
- Connect Spark Tester (ST-125) to spark plug, and check for adequate spark.
- Check ignition system for faulty grounds.
- Ensure EST wiring harness is not routed too close to wiring. EST wiring harness routed too close to wiring may cause induced voltage signals.
- Check ignition coil connections.
- Remove spark plugs, and check for incorrect heat range, wear, cracks, wetness, improper gap or heavy deposits.
- Check for proper pick-up coil (HEI distributor) resistance.
- Check for poor or contaminated fuel.
- Check for improper fuel pressure.
- Ensure EGR valve does not stick open.
- Check PCM for proper ground circuits.
- Ensure TP sensor does not stick or bind. TP sensor voltage should be less than 1.25 volts at idle.