I've a '97 Civic in which the engine fluctuates between fast idle and 800rpm after the engine reaches operating temperature. The engine speed does not fluctuate under steady driving speed. Furthermore, idle speed does not fluctuate when the AC compressor (and evaporator fan) is engaged.

I previously had an OBD code of P0117 but cleared that after I replaced the ECT sensor and thermostat. No other codes have appeared.

What could possible be failing?

 

So did you bleed and bump the cooling system after refilling it?

 

Yes.

 

I fixed this by taking the IAC valve off and cleaning it. The valve was stuck from years of grime buildup.