Common Causes:
* Dirty or Faulty Mass Airflow Sensor (MAF): This sensor measures the amount of air entering the engine. A dirty sensor will provide inaccurate readings, causing the engine to stumble and idle unevenly. A faulty sensor will do the same. Cleaning it (with MAF sensor cleaner, *not* carb cleaner) is often a good first step.
* Dirty Throttle Body: A buildup of grime and carbon deposits on the throttle body can restrict airflow and affect the idle. Cleaning the throttle body is often a relatively easy fix.
* Vacuum Leaks: Small cracks or holes in vacuum hoses can cause erratic idling. Inspect all vacuum lines for damage, especially those near the intake manifold and brake booster.
* Faulty Idle Air Control (IAC) Valve: This valve regulates air flow at idle. A dirty or malfunctioning IAC valve will cause the idle to fluctuate. Cleaning it might help, but replacement is often necessary.
* Faulty PCV Valve: A clogged or malfunctioning Positive Crankcase Ventilation (PCV) valve can cause excessive pressure in the crankcase, affecting the engine's breathing and idle.
* Dirty Fuel Injectors: Clogged or partially clogged fuel injectors can lead to an inconsistent fuel supply, resulting in rough idling. Cleaning them or replacement might be necessary.
* Spark Plugs and Wires: Worn spark plugs or damaged spark plug wires can cause misfires, leading to uneven idling.
Less Common, But Still Possible Causes:
* Faulty Crankshaft Position Sensor (CKP): This sensor tells the computer the position of the crankshaft. A faulty CKP can cause a variety of problems, including rough idling.
* Faulty Oxygen Sensor (O2 Sensor): While more likely to affect fuel economy and emissions, a faulty O2 sensor can also contribute to erratic idling.
* Problem with the Engine Control Module (ECM): While less likely, a problem with the computer itself could cause erratic idling. This is a more serious and expensive issue to diagnose and repair.
* Low Engine Compression: Worn piston rings or other internal engine issues can lead to low compression, affecting the idle.
Troubleshooting Steps:
1. Check for trouble codes: Use an OBD-II scanner to read any diagnostic trouble codes (DTCs) stored in the ECM. This can pinpoint the likely source of the problem.
2. Inspect visually: Look for any obvious signs of damage to vacuum hoses, wiring, or other components.
3. Clean the MAF sensor and throttle body: This is a relatively easy and inexpensive first step that often solves the problem.
4. Check the PCV valve: This is also relatively easy to check and replace.
If these steps don't resolve the issue, you'll likely need to take the vehicle to a qualified mechanic for further diagnosis and repair. They'll have the tools and expertise to diagnose more complex problems. Remember to always use the correct parts and procedures when working on your vehicle.
