* Ignition Coil(s): A failing ignition coil (or coils, as these cars often have multiple) is a common cause of misfires. They can provide insufficient voltage to the spark plugs, leading to weak or inconsistent sparking. A coil test is essential.
* Distributor Cap and Rotor (if equipped): Older vehicles like the '91 Cavalier may still use a distributor. Cracks or carbon buildup on the cap and rotor can prevent proper spark distribution. Inspect these closely for any damage or wear.
* Ignition Control Module (ICM): This module controls the ignition system's timing and spark delivery. A faulty ICM can cause erratic misfires.
* Worn or damaged crankshaft position sensor (CKP): The CKP sensor tells the computer the engine's rotational position, allowing for precise spark timing. A failing sensor can lead to misfires.
* Worn or damaged camshaft position sensor (CMP): Similar to the CKP, a failing CMP sensor can disrupt the ignition timing.
* Fuel delivery issues: While less likely immediately after a spark plug and wire change, problems with the fuel pump, fuel filter, fuel injectors, or fuel pressure regulator can also cause misfires. A weak spark can exacerbate these issues.
* Vacuum leaks: Air leaks in the intake system can disrupt the air/fuel mixture, leading to misfires.
* Engine computer (ECM) problems: Though less common, a faulty ECM can cause a variety of issues, including misfires. This is usually diagnosed last, as it's often difficult to test.
Troubleshooting Steps:
1. Verify proper plug gap: Even new plugs can have improper gaps. Check the gap with a feeler gauge and adjust if necessary.
2. Check for spark: Use an inline spark tester to verify each cylinder is getting a strong spark.
3. Inspect distributor cap and rotor (if applicable): Look for cracks, carbon tracking, or other signs of wear.
4. Check ignition coil(s): Visual inspection for cracks or damage, and potentially a coil test with a multimeter.
5. Check for vacuum leaks: Listen carefully for hissing sounds around the intake manifold and vacuum lines.
6. Scan for diagnostic trouble codes (DTCs): Use an OBD-I scanner (if your car has that capability) to check for stored codes that might indicate the problem area.
If you are not comfortable working on your vehicle's ignition system, take it to a qualified mechanic for diagnosis and repair. Improper work in this area can be dangerous.
