High-Probability Causes:
* Dirty or Failing Throttle Position Sensor (TPS): This sensor tells the computer the throttle's position. A dirty or faulty TPS sends inaccurate signals, leading to a rough idle. Cleaning it is often a cheap and easy first step.
* Failing Mass Airflow Sensor (MAF): The MAF measures the amount of air entering the engine. A dirty or faulty MAF will cause incorrect fuel delivery, resulting in a rough idle, poor performance, and potentially increased fuel consumption. Cleaning it (carefully!) is worth a try, but replacement might be necessary.
* Vacuum Leaks: Cracks or holes in vacuum hoses can disrupt the engine's air/fuel mixture, causing a rough idle. Inspect all vacuum lines for cracks, looseness, or damage. Pay close attention to the lines connected to the brake booster and the intake manifold.
* Dirty or Faulty Idle Air Control (IAC) Valve: This valve regulates airflow at idle. A dirty IAC valve can stick open or closed, causing rough idling. Cleaning it is often effective, but replacement may be needed.
* Ignition System Problems: Worn spark plugs, faulty plug wires, a failing distributor cap or rotor (if equipped), or a weak ignition coil can all contribute to a rough idle. Inspect and test these components.
* Fuel Delivery Issues: A clogged fuel filter or low fuel pressure can restrict fuel flow, causing a rough idle. Check the fuel pressure and consider replacing the fuel filter.
Less-Likely, But Still Possible Causes:
* Camshaft Position Sensor (CMP) Problems: A faulty CMP sensor can disrupt the engine's timing, leading to a rough idle.
* Crankshaft Position Sensor (CKP) Problems: Similar to the CMP sensor, a faulty CKP can cause timing issues.
* Low Compression: Worn piston rings or valve problems can lead to low compression in one or more cylinders, causing a rough idle. This usually requires more extensive diagnostics.
* Internal Engine Problems: More serious issues like worn bearings or other internal engine damage can also cause a rough idle, but these are less likely to be the initial cause unless there are other accompanying symptoms (e.g., knocking noises).
Troubleshooting Steps:
1. Visual Inspection: Start by visually inspecting the engine bay for obvious signs of problems like loose vacuum hoses, damaged wires, or leaks.
2. Check Engine Light: If the check engine light is on, get the codes read at an auto parts store (often free) to identify potential issues.
3. Clean/Replace Sensors: Clean the MAF and TPS sensors (carefully follow instructions for each sensor). If cleaning doesn't resolve the issue, consider replacing them.
4. Check Vacuum Lines: Inspect all vacuum lines for cracks, leaks, or loose connections. Replace any damaged hoses.
5. Inspect Ignition System: Check the spark plugs, wires, distributor cap (if applicable), and rotor for wear or damage. Replace as needed.
6. Fuel System Check: Consider checking the fuel pressure and replacing the fuel filter.
If you're not comfortable working on your car yourself, take it to a mechanic for diagnosis and repair. A rough idle can be a symptom of several different problems, and proper diagnosis is important.
