High-Probability Issues:
* IAC (Idle Air Control) Valve: This valve regulates air flow at idle. A dirty or malfunctioning IAC valve is a very common cause of rough idling, especially in older cars. Cleaning it (with carb cleaner, being careful not to damage the delicate internal components) is often a cheap and effective first step. If cleaning doesn't work, replacement is relatively inexpensive.
* Throttle Position Sensor (TPS): This sensor tells the computer the throttle's position. A faulty TPS can cause erratic idle and hesitation during acceleration. A bad TPS can lead to incorrect fuel delivery and ignition timing. Testing this is crucial; often a simple multimeter test can reveal a problem.
* Mass Air Flow Sensor (MAF) Sensor: This sensor measures the amount of air entering the engine. A dirty or faulty MAF sensor can cause a lean or rich fuel mixture, leading to a rough idle and hesitation. Cleaning it (carefully, with MAF sensor cleaner) is worth trying, but replacement might be necessary.
* Vacuum Leaks: Small vacuum leaks can disrupt the engine's idle and cause rough running. Inspect all vacuum lines and hoses for cracks, holes, or loose connections. Pay close attention to the lines connected to the brake booster, intake manifold, and PCV valve.
Medium-Probability Issues:
* Distributor Cap and Rotor: While you have new plugs, worn distributor cap and rotor contacts can cause misfires, leading to rough idle. Inspect them for cracks, corrosion, or excessive wear.
* Ignition Coil: A weak or failing ignition coil can result in weak sparks, causing misfires and rough idling.
* Fuel Injectors: Clogged or faulty fuel injectors can cause inconsistent fuel delivery, leading to a rough idle. This is harder to diagnose at home and often requires specialized tools or a professional assessment.
* PCV Valve: A clogged PCV valve can disrupt crankcase ventilation and negatively impact engine performance, including idle. Replacing it is relatively cheap and easy.
Low-Probability Issues (but still possible):
* Engine Coolant Temperature Sensor: A faulty sensor can provide incorrect readings to the engine control unit (ECU), leading to improper fuel mixture and rough idle.
* Oxygen Sensor: While less likely to directly cause a rough idle, a malfunctioning oxygen sensor can contribute to poor fuel mixture over time.
Troubleshooting Steps:
1. Start with the easy checks: Inspect vacuum lines, clean the IAC valve, and visually inspect the distributor cap and rotor.
2. Check for diagnostic trouble codes (DTCs): Your 1991 Cavalier likely has a diagnostic port. A code reader (available cheaply from auto parts stores) can provide valuable clues.
3. Test the TPS and MAF sensor: These require a multimeter and some knowledge of automotive electrical systems, or you can have a professional mechanic check them.
4. Consider a fuel pressure test: This requires specialized tools, best left to a mechanic.
Important Note: Since the car teases to stall on initial acceleration, the problem likely involves the fuel delivery system or ignition timing under load, making the TPS and MAF sensor prime suspects.
It's best to systematically check these components, starting with the most likely and easiest to address. If you're not comfortable working on your car yourself, take it to a qualified mechanic for diagnosis and repair. Providing them with the details about the rough idle and hesitation during acceleration will help them pinpoint the problem more quickly.
