Hi, welcome to Just Answer.
Hmmm, I can't think of anything else at the moment, possibly letting the engine bay dry out with the hood open, may do the trick. But let me 'opt out' of the question so if any other GM Experts here have ran into this before, they can offer their advice as well...hang in there other Experts will look through the question throughout the rest of the day. If I come up with anything else, I'll post back here.
Dry out the underhood fuse box and check fuses ENG 1, ECM 1 and ECM B.
Had this happen to my car and ENG 1 fuse blew.
Disconnecting the IAC valve will typically not set the check engine lamp. It's very unusual that after washing the engine you could actually short an electrical circuit out, so the problem must still lie in water intrusion or a blown fuse as the other Expert mentioned; unless this condition is entirely coincidental and the ECU has failed...which would be too coincidental. Unfortunalty at this point the IAC circuit needs to be tested with factory equipment, unless the problem can rectify itself. I can offer you the factory procedures for testing below to see if anything can further help you issolate this. Than I can opt back out of the question and other Experts will continue to look through this and see if there is any that have run accross this. In my experience a high idle is commonly due to a binding throtle, a vacuum leak, a bad IAC, or a bad ECU...possibly a vacuum leak was produced when the enigne was cleaned??
CIRCUIT DESCRIPTION The control module controls engine idle speed with the IAC valve. To increase idle speed, the control module retracts the IAC valve pintle away from it's seat, allowing more air to pass by the throttle bore. To decrease idle speed, it extends the IAC valve pintle towards it's seat, reducing bypass air flow. A Tech 1 scan tool will read the control module commands to the IAC valve in counts. Higher the counts indicate more air bypass (higher idle). The lower the counts indicate less air is allowed to bypass (lower idle).CHART TEST DESCRIPTION Number(s) below refer to circled number(s) on the diagnostic chart.
IAC VALVE RESET PROCEDURE
DIAGNOSTIC AIDS A slow, unstable, or fast idle may be caused by a non-IAC system problem that cannot be overcome by the IAC valve. Out of control range, IAC Tech 1 scan tool counts will be above 60 if idle is too low, and zero counts if idle is too high. The following checks should be made to repair a non-IAC system problem.
Hmmm, that sure does sound internal to the ECU, but it's hard to say without proof. Just go ahead and keep the post open, we have a great staff of GM Experts and another may have run across a similar problem.
Sorry, sometimes it's difficult to interpret what going on without having my hands in the car, unless I've experienced an identical occurrence.