High-Probability Causes:
* Ignition System: Even after a "complete check," this is the most common source of a misfire. Double-check:
* Spark Plugs and Wires: Are they the correct type, properly gapped, and in good condition? Wires can be cracked or arcing. Look for carbon tracking on the insulators.
* Distributor Cap and Rotor: Inspect for cracks, corrosion, or worn contact points.
* Ignition Coil: Test the coil's primary and secondary windings. A weak coil can cause misfires, especially under load.
* Ignition Module (or Electronic Ignition Control Unit): This component controls the ignition timing and spark. It can fail intermittently, leading to a misfire.
* Ignition Switch: A failing ignition switch can deliver inconsistent power, resulting in a misfire.
* Fuel System:
* Fuel Filter: A clogged fuel filter restricts fuel flow, causing a lean condition and misfires. Replace it.
* Fuel Pump: A weak fuel pump may not provide sufficient pressure. Test the fuel pressure.
* Fuel Injectors: Clogged or faulty injectors can cause a misfire in specific cylinders. A fuel pressure test may show good overall pressure but weak spray from a single injector. Injector flow testing is ideal.
* Fuel Pressure Regulator: A faulty regulator can cause incorrect fuel pressure.
* Vacuum Leaks: These can disrupt the engine's air/fuel mixture, leading to a misfire. Inspect all vacuum lines for cracks or disconnections. Pay close attention to the PCV system.
* Engine Sensors: Several sensors feed information to the engine control module (ECM). A faulty sensor can cause the ECM to send incorrect signals, leading to misfires. This includes:
* Mass Airflow Sensor (MAF): Measures the amount of air entering the engine.
* Throttle Position Sensor (TPS): Monitors the throttle position.
* Crankshaft Position Sensor (CKP): Essential for ignition timing.
* Oxygen Sensor (O2 Sensor): Monitors exhaust gases and impacts fuel mixture.
* Compression Test: Low compression in one or more cylinders indicates a problem with the valves, piston rings, or cylinder head gasket. This is a crucial test that should've been part of any complete check.
Less-Likely (but still possible) Causes:
* Camshaft Position Sensor (CMP): Similar to the CKP sensor, a bad CMP affects the ignition timing.
* Exhaust Restrictions: Severely clogged exhaust manifolds or catalytic converters can impede exhaust flow and cause misfires.
Troubleshooting Steps:
1. Re-examine the areas already checked: It's easy to overlook something.
2. Perform a compression test: This pinpoints cylinder issues.
3. Check for vacuum leaks: Use a vacuum gauge and listen for hissing sounds.
4. Test fuel pressure: A fuel pressure gauge is essential for verifying fuel delivery.
5. Scan for diagnostic trouble codes (DTCs): Even if the check engine light isn't on, there might be stored codes. A code reader is needed.
6. Check for any erratic engine behavior: Does the misfire happen at specific RPMs or under load? This can help isolate the problem.
If the problem persists after these steps, consult a qualified mechanic specializing in older vehicles. They'll have the tools and experience to diagnose and repair more complex issues. Mention all the steps you've already taken. Providing more details about the "complete check" would also be helpful.
