High Idle Issue

1992 F150 302 fuel injected. To start truck, accelerator needs to go to the floor. Idle goes up to 2500 rpm when engine reaches running temperature and stays there. The only way to bring it down is to disconnect the IAC. Have already replaced ECT (engine temp sensor), IAC (idle air control valve) and IAT (intake air temp sensor). Engine will idle at normal rpm with the IAC disconnected. No obvious vacuum leaks. Tested with new MAP sensor but symptoms did not change. Disconnected battery to reset error codes, no change in symptoms.

My father had a '90 Bronco w/ 5.0 and his idled high after he had it for a while, the longer he owned it the higher the idle RPM would be. He took it to Ford and they found that the upper and lower intake plenums gasket was partially torn allowing the extra volume of air causing the increased idle RPM. It may not be your cause, just something to check out. I hope this helps, Todd