My 1990 F150 5L is still idling high after cleaning the throttle bores and changing the idle by pass valve. Cleaning it never helped.
My question is this- has anyone ever had a high idle due to a gasket leak in the upper intake manifold? I could find no fault codes in the computer

 

Do some serious vacuum leak checking. Also, make sure the "butterflys" in the throttle body are closing all of the way, and that there's no carbon buildup causing them to not fully close.

 

According to "Ford Fuel Injection and Electronic Engine Control" by Charles Probst, improper idle speed can also be caused by: Engine too cool or too hot, A/C input, throttle sticking, linkage binding, and Speed Control linkage. The book also goes on to say that the Oxygen Sensor can be faulty or the ignition timing be off. Good luck. P.S. It also talks about vacuum leaks, contaminated throttle bores, etc. but others have already addressed that.

 

Had the same problem with my '90 Continental. Turned out to be the Throttle Position Sensor connection was corroded. Removed the connection and put some dielctric grease on the pins and pushed the connector off and on a few times to clean it up. Solved the problem. Did I underatand you did a propane check and the idle speed increased or decreased? Increased means you definitely have a vacuum leak below the Throttle Body. Decreased means no vacuum leak. However, you could have both a vacuum leak and the bad TPS without setting a code.

Regards

John