* IAC (Idle Air Control) Valve: This is the most likely suspect. The IAC valve controls the amount of air bypassing the throttle plate at idle. When cold, the engine needs more air, and a faulty IAC valve might not be supplying enough, or might be sticking. Cleaning it (with carb cleaner) is often a simple first step, but replacement might be necessary.
* MAF (Mass Air Flow) Sensor: This sensor measures the amount of air entering the engine. A dirty or faulty MAF sensor will send incorrect data to the computer, leading to poor idle, especially when cold. Cleaning it (with MAF sensor cleaner - *never* carb cleaner) is worth trying, but replacement may be needed.
* Throttle Position Sensor (TPS): This sensor tells the computer the throttle's position. An inaccurate reading, especially when cold, will cause the computer to adjust the fuel mixture incorrectly, resulting in a rough or nonexistent idle.
* PCV (Positive Crankcase Ventilation) Valve: A clogged PCV valve can cause excessive crankcase pressure, affecting the engine's breathing and leading to poor idle, particularly noticeable when cold. Replacing it is relatively inexpensive.
* Fuel System Issues: A problem with fuel delivery, such as a weak fuel pump, clogged fuel filter, or faulty fuel injectors, can prevent the engine from getting the fuel it needs to idle properly when cold. This is less likely to be the *only* cause of a cold-idle problem, but could contribute.
* Coolant Temperature Sensor (CTS): This sensor tells the computer the engine's temperature. An inaccurate reading will lead to incorrect fuel mixture adjustments. A faulty CTS is more likely to cause problems with starting and running *generally*, not just at cold idle.
* Vacuum Leaks: Leaks in the intake manifold, vacuum hoses, or other parts of the vacuum system can disrupt the air/fuel mixture and cause idling problems, especially at cold temperatures.
Troubleshooting Steps:
1. Check for obvious things: Make sure the battery is fully charged and the battery terminals are clean and tight. A weak battery can affect the idle.
2. Clean the IAC and MAF sensors: This is the cheapest and easiest thing to try first. Use the correct cleaner for each sensor.
3. Inspect vacuum hoses: Look for cracks, splits, or loose connections.
4. Check the PCV valve: It's relatively easy and inexpensive to replace.
5. Use a code reader: A diagnostic code reader (OBD-II scanner) can reveal any diagnostic trouble codes (DTCs) stored by the car's computer. These codes can point to the specific problem area.
6. Professional Diagnosis: If the above steps don't resolve the issue, it's best to take your Mustang to a qualified mechanic for a proper diagnosis. They have the tools and expertise to pinpoint the problem more effectively.
Remember to always consult your owner's manual for specific information about your car's systems. Good luck!
