Ignition System:
* Worn Distributor Cap and Rotor: These are common wear items. Cracks or corrosion can cause misfires at all speeds.
* Worn or faulty Ignition Coil: A weak coil can't provide sufficient spark energy, leading to misfires.
* Worn Spark Plugs and/or Wires: Plugs can foul, gap incorrectly, or become damaged. Worn wires can create poor spark delivery. Check for cracks or corrosion.
* Ignition Module (or Control Unit): This electronic component controls the ignition timing. Failure can cause erratic engine operation.
Fuel System:
* Clogged Fuel Filter: Restricts fuel flow, leading to inconsistent fuel delivery to the injectors/carburetor.
* Vacuum Leaks: Leaks in vacuum lines affect fuel/air mixture and timing, causing rough running.
* Fuel Pump Issues: A weak or failing fuel pump may not supply enough fuel at all engine speeds.
* Carburetor Problems (if applicable): If it has a carburetor, issues like clogged jets, a faulty accelerator pump, or incorrect float level can cause poor running. This is very common on older vehicles.
Engine Mechanical Issues:
* Worn Valves: Worn or improperly adjusted valves will affect compression and cause a rough idle and rough running under load.
* Low Compression: This could stem from several issues, including worn piston rings, head gasket leaks, or valve problems. A compression test is essential to diagnose this.
* Camshaft Issues: A worn camshaft or faulty lifters can cause irregular valve timing and a rough run.
Other Possibilities:
* Incorrect Timing: If the ignition timing is significantly off, it can cause rough running.
* Sensor Problems (if equipped with electronic fuel injection): Various sensors (like oxygen sensor, mass airflow sensor, etc.) could be failing and sending incorrect signals to the engine control unit (ECU).
Troubleshooting Steps:
1. Check the basics: Start with the easiest and cheapest checks: Inspect the spark plugs, wires, distributor cap and rotor for wear or damage. Replace as needed.
2. Listen for unusual noises: Are there any knocking, ticking, or other sounds coming from the engine? This can help pinpoint mechanical issues.
3. Check fuel pressure (if applicable): If you have fuel injection, measure fuel pressure to check the fuel pump.
4. Perform a compression test: This is crucial for diagnosing engine mechanical problems.
5. Inspect vacuum lines: Look for cracks, leaks, or loose connections.
6. Check the air filter: A clogged filter restricts airflow and can cause rough running.
Because the rough running occurs at *all* engine speeds, the problem is likely a fundamental one affecting fuel delivery or ignition across the rev range. Start with the ignition system components, then move on to fuel system and finally the mechanical aspects. If you're not comfortable doing this work yourself, take it to a mechanic familiar with older vehicles. Mentioning the symptoms clearly will help them diagnose the problem more efficiently.
