Did you check for vaccum leaks ? Spray some carburettor cleaner at the intake seals, fuel injector seals and other places where rubber vaccum parts (hoses) are. If there is any leak, i.e. unmetered air getting in, the rpms would change. If you replaced the coolant temp sensor and if the wiring harness is o.k., it is more likely to be unmetered air getting in from somewhere. Have you checked the oxygen sensor also ? It must be producing a changing voltage, if you see a constant voltage coming out from the oxygen sensor, either the feedback system is broken due to a broken cable or the sensor itself is bad.

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Saumil S. Patel