1. Vacuum Leaks: This is the most likely culprit after replacing the IAC. A vacuum leak can cause a variety of problems, including a fast idle. Check these areas carefully:
* Intake Manifold Gaskets: These are prone to failure and can create significant vacuum leaks. Inspect them visually for cracks or damage.
* Vacuum Hoses: Inspect all vacuum hoses connected to the intake manifold, IAC, and other components of the idle control system. Look for cracks, splits, or loose connections. Pay special attention to hoses that are brittle or showing signs of age. Replace any questionable hoses.
* PCV Valve: A faulty PCV valve can cause a vacuum leak, and it's relatively inexpensive to replace.
* Throttle Body Gasket: Make sure the throttle body is properly sealed to the intake manifold.
2. Throttle Position Sensor (TPS): A malfunctioning TPS can provide incorrect signals to the engine's computer, leading to a fast idle. This can be tested with a multimeter or a scan tool.
3. Mass Airflow Sensor (MAF): A dirty or faulty MAF sensor can also cause problems with the idle. Clean it carefully with MAF sensor cleaner (following the instructions precisely) or replace it if necessary. A bad MAF sensor will often throw a code.
4. Engine Coolant Temperature (ECT) Sensor: An inaccurate ECT sensor can cause the computer to adjust the idle speed incorrectly, especially when the engine is cold.
5. Idle Air Control (IAC) Motor itself (Even if new): While you replaced it, there's a tiny chance the new IAC is faulty. It's less likely but worth considering. Check the part number against the one listed in your repair manual and verify you got the right part for your Ranger.
6. Computer Issues (ECM/PCM): In rare cases, a problem with the engine control module (ECM or PCM) can cause unusual idle behavior. This is less likely but possible. A scan tool check is essential here.
Troubleshooting Steps:
1. Visual Inspection: Carefully examine all vacuum lines and connections.
2. Vacuum Leak Test: Use a vacuum gauge or your ear to listen for hissing sounds near the intake manifold and vacuum lines while the engine is running.
3. Scan Tool Diagnosis: This is crucial. A scan tool can read diagnostic trouble codes (DTCs) which will pinpoint the problem more accurately. Many auto parts stores offer free code reading services.
4. Test Sensors: Test the TPS, MAF, and ECT sensors with a multimeter according to the specifications in a repair manual for your specific vehicle.
5. Clean the Throttle Body: Even if it's seemingly clean, cleaning the throttle body with throttle body cleaner can often resolve minor issues.
If you've checked all of these and the problem persists, it's time to consult a qualified mechanic. They have the tools and experience to diagnose more complex issues. Providing the codes retrieved from a scan tool will greatly assist the mechanic.
