Ford Repair: Hiigh RPMs When Cold, intake plenum, metal surfaces

Question
I have a 1992 F150 5.0L. When my truck is cold and I am driving the RPM seem to rev high before shifting but as the truck warms up the high RPM's seem to fade to normal. I have replaced the EVP sensor but the problem is still there. I have no check engine light's. Could this be the result of a bad IAC? Thank you

Answer
There are several things that could cause these problems.
Have you checked for codes? Even with no CEL on in your truck, there could be codes stored that might help find the problem.

Common problems include the gasket between the throttle boy and intake manifold; leaking EGR valve; leaking intake plenum gasket; faulty thermostat; faulty temperature sensor; faulty IAC valve.

The gaskets between metal surfaces will sometimes leak cold and not hot because as the metals expand from heat  the gap may close up. I suggest you see if there are any codes stored to help with this problem.