Possible Causes:
* Faulty Idle Air Control (IAC) Valve: The IAC valve regulates air flow at idle. If it's stuck open, it'll cause the high RPM reading even with the engine off. The PCM (Powertrain Control Module) is likely trying to compensate, but the engine won't start because it's flooded or the fuel system is confused.
* Large Vacuum Leak: A significant vacuum leak can also cause the engine to rev up erratically when the ignition is on. This creates a false air signal that throws off the fuel/air mixture.
* Throttle Position Sensor (TPS) Problem: A faulty TPS can send incorrect signals to the PCM, leading to erratic idle and starting issues.
* Mass Airflow Sensor (MAF) Problem: Though less likely to cause the high RPM with the key on, a faulty MAF sensor can cause starting and running problems. A dirty MAF sensor can also affect it.
* PCM Issues (less likely): While less probable, a problem with the PCM itself could cause this behavior. This is generally a last resort diagnosis after other potential issues are eliminated.
Troubleshooting Steps:
1. Disconnect the IAC Valve: Disconnect the IAC valve's electrical connector. With the key ON, does the tachometer still read 2000-4000 RPM? If the RPM drops significantly or goes to zero, the IAC valve is almost certainly faulty. If the problem remains, move to the next step.
2. Check for Vacuum Leaks: Carefully inspect all vacuum lines and hoses for cracks, holes, or loose connections. Pay close attention to the lines connected to the intake manifold, brake booster, and other vacuum-operated components. Use a vacuum gauge (if available) to test for leaks. A hissing sound is a good indicator.
3. Inspect the TPS: The TPS is usually located on the throttle body. Check for any physical damage or unusual movement. A multimeter can be used to test the sensor's voltage output according to the factory specifications (consult a repair manual).
4. Check the MAF Sensor: Inspect the MAF sensor for any visible debris or damage. Clean it carefully with MAF sensor cleaner (following the cleaner's instructions precisely).
Important Considerations:
* Safety First: Always disconnect the negative battery terminal before working on any electrical components.
* Repair Manual: A factory repair manual for your 1994 Mustang 3.8L is highly recommended. It will provide specific diagnostic procedures, wiring diagrams, and component specifications.
* Professional Help: If you're not comfortable performing these checks yourself, take your Mustang to a qualified mechanic. The problem could be more complex than initially suspected.
The high idle with the key ON is a strong clue, pointing toward the IAC valve or a substantial vacuum leak as the most probable culprits. Start with the IAC valve disconnection test—it's the easiest and quickest check to eliminate a common cause.
