* Idle Air Control (IAC) Valve: This is the most common culprit. The IAC valve regulates the amount of air entering the engine at idle. If it's dirty, sticking, or failing, it can't maintain the proper idle speed. Cleaning it (often with carburetor cleaner) is a relatively easy and inexpensive first step. If cleaning doesn't work, replacement is likely needed.
* Throttle Position Sensor (TPS): The TPS tells the computer the position of the throttle. A faulty TPS can provide inaccurate information, leading to poor idle and potentially other drivability issues. A malfunctioning TPS often throws a diagnostic trouble code (DTC).
* Mass Airflow Sensor (MAF) Sensor: The MAF sensor measures the amount of air entering the engine. A dirty or failing MAF sensor can cause a lean condition, leading to rough idling. Cleaning the MAF sensor (with MAF sensor cleaner ONLY – avoid other cleaners) is worth a try.
* Vacuum Leaks: A vacuum leak can disrupt the engine's air/fuel mixture, especially at idle. Inspect all vacuum lines for cracks, holes, or loose connections. Pay close attention to the lines connected to the intake manifold and the brake booster.
* Fuel Pressure Regulator: A failing fuel pressure regulator could cause inconsistent fuel delivery, leading to poor idling. This is less likely if the car runs fine under acceleration.
* Dirty or clogged fuel injectors: While less likely if it runs well while driving, clogged injectors can sometimes cause poor idling. Fuel injector cleaner might help, but a professional cleaning or replacement may be necessary.
Troubleshooting Steps:
1. Check for Diagnostic Trouble Codes (DTCs): Use an OBD-II scanner to check for any stored trouble codes. This can provide valuable clues about the problem. Auto parts stores often offer free code reading services.
2. Clean the IAC Valve: This is the easiest and cheapest thing to try first. Locate the IAC valve (usually on the throttle body), disconnect it, and clean it thoroughly with carburetor cleaner. Reconnect and see if the problem is resolved.
3. Check for Vacuum Leaks: Visually inspect all vacuum lines and connections for any leaks. You can use a propane torch (carefully!) to check for leaks near the intake manifold – the RPMs will increase if there's a leak in that area.
4. Clean the MAF Sensor: Carefully clean the MAF sensor using MAF sensor cleaner. Do not touch the sensor element with your fingers.
5. Check the TPS: This requires a multimeter to test the voltage output. You can find instructions online for testing the TPS for your specific vehicle.
If you're not comfortable performing these tasks yourself, take your car to a trusted mechanic for diagnosis and repair. Trying to fix complex automotive issues without the proper knowledge and tools can be dangerous and potentially lead to further damage.
