* Idle Air Control (IAC) Valve: This is the most likely suspect. The IAC valve controls the amount of air bypassing the throttle plate at idle, especially crucial when the engine is cold. A dirty or malfunctioning IAC valve can cause erratic idle speeds, especially during warm-up. Cleaning it (with throttle body cleaner) is often the first step, but replacement might be necessary.
* Mass Airflow Sensor (MAF) Sensor: The MAF sensor measures the amount of air entering the engine. A dirty or failing MAF sensor can provide inaccurate readings, leading to an incorrect fuel mixture and irregular idle, particularly when the engine is cold and the airflow is more sensitive. Cleaning it (carefully, with MAF sensor cleaner) is worth trying.
* Coolant Temperature Sensor (CTS): The engine computer relies heavily on the CTS to determine the engine's temperature. An inaccurate CTS reading can cause the computer to adjust the fuel mixture and idle speed incorrectly, leading to a high idle when cold. A faulty CTS will often give a false reading causing the engine to run rich or lean, altering the idle.
* Vacuum Leaks: Even a small vacuum leak can significantly affect idle, especially when the engine is cold and the vacuum system is under more stress. Carefully inspect all vacuum lines and connections for cracks, breaks, or loose fittings.
* Throttle Body: While you've replaced the TPS, the throttle body itself might be dirty or have sticking components. Cleaning the throttle body thoroughly might help.
Troubleshooting Steps:
1. Clean the IAC valve: This is the easiest and cheapest thing to try first. Remove it, clean it with a suitable cleaner (brake cleaner is often used, but a dedicated throttle body cleaner is preferred), and reinstall it.
2. Clean the MAF sensor: Use a MAF sensor cleaner and follow the instructions carefully. Do NOT touch the sensor element with your fingers.
3. Check for vacuum leaks: Use a vacuum gauge or listen for hissing sounds around vacuum lines.
4. Clean the throttle body: Use throttle body cleaner to thoroughly clean the throttle body and its passages.
5. Check the coolant temperature sensor (CTS): This can usually be tested with a multimeter to see if it's giving appropriate resistance readings based on its temperature. Your vehicle's repair manual should have specifications for testing the sensor.
6. Check engine codes: Use an OBD-II scanner to read any diagnostic trouble codes (DTCs) stored in the engine computer's memory. These codes can pinpoint the problem more precisely.
If cleaning and inspecting these components doesn't resolve the issue, you may need to replace the faulty component(s). A professional mechanic with a scan tool can help diagnose the problem more definitively if home troubleshooting fails. Remember to always disconnect the negative battery terminal before working on any electrical components.
