Our 93 E150 5.8 occasionally idles very high at operating temperature. It behaves the same way with a new idle air valve. What should I try next?

Sixto
93 E150 Chateau 5.8 194K miles

 

Air leaks at the intake are common causes for high idle. Cracks in the intake tube to the throttle body if it is MAF equipped, also.

Pull codes to see if there is anything listed, might be a sensor problem. jim

 

Agree with a intake manifold leak, or a bad, broken vacuum hose would cause a high idle. Any intake manifold leak would cause a high idle.

Do you get a very high idle after shutting off, and restart??

If you hit the gas pedal hard, does the high idle go away???

If this is your problem, it could be the temperature sensor, that reads directly to the ECU, and not the dash gauge sensor. The one mounted just above the thermostat housing, on the Tee for the heater hose.

 

Probably related to the "uh-oooga" horn, or possibly the 8 track player/AM radio.
If those are not bad: check the drum brakes, manual window cranks, and then the hand crank engine starter.

Seriously though: probably a vacuum leak somewhere like mentioned above.

External starter lol

 

Good call on the ECT sensor, Wildman. I forgot that one. A quick(?) check is to pull the connector on the IAC to see if the idle speed drops. I say quick because that is after all the intake hoses and plumbing are removed! haha
jim