* Faulty 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 malfunctioning, it can cause erratic idle speeds. Cleaning it (with appropriate cleaner) is often a first step, but replacement might be necessary.
* Dirty or Faulty Throttle Position Sensor (TPS): The TPS tells the computer the throttle's position. A dirty or failing TPS can send incorrect signals, leading to fluctuating idle. Cleaning it (carefully, with electronics cleaner) is worth trying.
* Vacuum Leaks: Small vacuum leaks in hoses or gaskets can disrupt the air/fuel mixture, causing idle instability. Inspect all vacuum lines for cracks, holes, or loose connections. Pay close attention to the lines connected to the IAC valve and the brake booster.
* Mass Airflow Sensor (MAF) Issues: While less likely to cause a *specific* jump between two RPMs, a dirty or failing MAF sensor can affect the air/fuel mixture and contribute to idle problems. Cleaning it (carefully, with MAF sensor cleaner) is worth a try.
* EGR Valve Problems: The Exhaust Gas Recirculation (EGR) valve can also affect idle. A sticking or faulty EGR valve can lead to erratic idling.
* Computer/PCM Issues: While less common, a problem with the engine control module (PCM) can cause erratic idle. This is usually diagnosed through advanced scan tools.
* Faulty or Weak Battery: A weak battery can cause voltage fluctuations which can affect the idle.
Troubleshooting Steps:
1. Check for Vacuum Leaks: Visually inspect all vacuum lines and connections. Listen carefully near the engine for hissing sounds which might indicate a leak.
2. Clean the IAC Valve: Locate the IAC valve (usually on the throttle body) and carefully clean it with an appropriate electronic parts cleaner. Make sure to disconnect the battery negative terminal before doing this.
3. Clean the Throttle Body: While you're cleaning the IAC valve, clean the entire throttle body. A buildup of carbon deposits can hinder its operation.
4. Clean the TPS: Carefully clean the TPS sensor with electronic parts cleaner. Be gentle and avoid touching the sensing element.
5. Check the MAF Sensor: Clean the MAF sensor carefully, using a MAF sensor cleaner (DO NOT use anything else).
6. Check the Battery and Alternator: Make sure the battery is charged properly and that the alternator is charging the battery correctly. A weak battery can cause voltage fluctuations.
7. Advanced Diagnostics: If the above steps don't solve the issue, you'll likely need to use a scan tool to retrieve diagnostic trouble codes (DTCs) from the PCM. This will help pinpoint the problem more accurately.
Important Note: Always disconnect the negative battery terminal before performing any work on the electrical system of your car. If you're not comfortable working on your car yourself, take it to a qualified mechanic for diagnosis and repair.
