* Idle Air Control (IAC) Valve: This is the most probable cause. The IAC valve regulates air entering the engine at idle. When warm, a malfunctioning IAC valve might be sticking, providing inconsistent air amounts, leading to the RPM fluctuations. Cleaning the valve (with appropriate cleaner – *not* brake cleaner) is the first step. If cleaning doesn't solve it, replacement is likely needed.
* Throttle Position Sensor (TPS): A faulty TPS provides inaccurate readings to the engine's computer (ECM), affecting fuel delivery and idle control. A bad TPS can cause erratic idling, especially when warm. Testing the TPS voltage output is crucial for diagnosis. A multimeter is needed for this.
* Mass Air Flow (MAF) Sensor: While less likely to cause surging *only* at warm idle, a dirty or failing MAF sensor can still contribute to inconsistent fuel delivery. A dirty MAF sensor should be carefully cleaned (specific MAF sensor cleaner is required).
* Vacuum Leaks: Small vacuum leaks can cause erratic idle. These are harder to pinpoint, but inspect all vacuum hoses and connections for cracks, looseness, or damage. Pay close attention to the area around the intake manifold.
* Fuel Pressure Regulator: A faulty fuel pressure regulator can cause fluctuating fuel pressure, particularly when the engine is warm. This can lead to inconsistent fuel delivery and idle surging.
* Engine Coolant Temperature (ECT) Sensor: While less likely the primary cause, a faulty ECT sensor could provide incorrect temperature readings to the ECM, leading to improper fuel mixture adjustments at idle.
Troubleshooting Steps:
1. Check for vacuum leaks: Carefully inspect all vacuum lines and connections. A simple test is to spray carb cleaner (or starting fluid) around suspected leak points while the engine is idling. A change in RPM indicates a leak.
2. Clean the IAC valve: Remove the IAC valve, clean it thoroughly with a dedicated IAC cleaner, and reinstall it.
3. Clean the MAF sensor: Use a MAF sensor cleaner and follow the instructions carefully. Do *not* touch the sensor element with your fingers.
4. Check the TPS voltage: Use a multimeter to check the TPS voltage output according to the specifications in your repair manual.
5. Advanced diagnostics: If the above steps don't resolve the issue, you'll likely need to use a scan tool to check for diagnostic trouble codes (DTCs) from the ECM. This can pinpoint the faulty component more precisely.
Important Note: Before attempting any repairs, consult a repair manual specific to your 1993 Mazda Probe GT. This will provide detailed diagrams, specifications, and procedures. Improper repair attempts can cause further damage. If you're not comfortable working on your car, take it to a qualified mechanic.
