* Idle Air Control (IAC) Valve: This is the most common culprit. The IAC valve regulates airflow at idle. If it's dirty, sticking, or malfunctioning, it can cause erratic idle speeds. Cleaning it (with appropriate cleaner) is often the first step in troubleshooting. If cleaning doesn't work, replacement is likely needed.
* Throttle Position Sensor (TPS): The TPS tells the computer the throttle's position. A faulty TPS can send incorrect signals, leading to erratic idling. A faulty TPS can also cause poor drivability issues, so checking this is important even if gear operation seems fine.
* Mass Airflow Sensor (MAF) Sensor: While less likely to cause *only* idle issues, a dirty or faulty MAF sensor can provide inaccurate air readings, affecting the fuel mixture and causing idle problems. Cleaning the MAF sensor (carefully, with the correct cleaner) is worth trying.
* Vacuum Leaks: Leaks in the vacuum system can disrupt the engine's air/fuel mixture at idle, causing the RPM fluctuations. Inspect all vacuum hoses and connections for cracks, breaks, or loose fittings.
* Dirty Throttle Body: A dirty throttle body can restrict airflow, especially at idle, contributing to the inconsistent RPMs. Cleaning the throttle body (again, with the appropriate cleaner) is a good maintenance step.
* PCV Valve: A clogged Positive Crankcase Ventilation valve can cause a buildup of pressure, affecting the idle. Replacing this is relatively inexpensive.
* Engine Coolant Temperature Sensor (ECT): A faulty ECT sensor provides inaccurate temperature readings to the engine control unit (ECU), causing it to adjust the fuel mixture incorrectly. This can lead to erratic idle, especially when the engine is cold.
Troubleshooting Steps:
1. Check for error codes: Use an OBD-II scanner to read any diagnostic trouble codes (DTCs) stored in the car's computer. This will often pinpoint the problem directly.
2. Clean the IAC valve, MAF sensor, and throttle body: This is the easiest and cheapest place to start. Use the correct cleaner for each component (do *not* use brake cleaner on the MAF sensor).
3. Inspect vacuum lines: Look for any visible cracks or loose connections.
4. Check the PCV valve: Replace if necessary.
5. If the problem persists after these steps: The problem may lie with the TPS, ECT sensor, or a more serious underlying mechanical issue. Professional diagnosis may be needed at this point.
Remember to disconnect the negative battery terminal before working on any electrical components. If you're not comfortable performing these repairs yourself, take your Alero to a qualified mechanic. The erratic idle, though not affecting driving, is still a symptom of a problem that should be addressed.
