idle stuck at 900 rpm? wtf?

Have you tried to replace the IAC?

The IAC is a common problem with fords. Even though there might not be a code or any dirt in the IAC, many times as they get older they react slowly. Because they are working, no codes, test good, but because they are slow they cannot adjust fast enough. Idle goes high, so the IAC closes and the idle starts to drop, when desired idle is achieved, it starts to open but opens too slow so it dropps down too low, opens all the way, idle raised to desired level, startes to close, closes to slow so the idle goes way too high, and the pattern continues. I would try by replacing the IAC and seeing if this fixes your problem

High idle. These are often caused by vacuum leaks. Spray solvent around all areas of the induction system and if the idle changes, you found the area of your leak. This also could be the cause of a bad IAC.