[ImperialCrown]

Big vacuum leak somewhere.
A spray bottle of water to test seam and joint integrity is useful. The sound and engine speed should change when the leak sucks water.

 

[ImperialCrown]

Make sure that the blue rubber seal hasn't fallen off the EGR tube as it was being installed.

 

[ImperialCrown]

The IAC can't be tested with an ohmmeter. Any readings would be inconclusive.
Plug the electrical connector back into the IAC and hold it in your hand. Have a helper turn the ign key from off to run.
The pintle should cycle out, then in.

A light layer of carbon is normal. If the pintle tip and its seat is heavily coked up, the IAC position has to 'adapt' to its new position to keep RPM within bounds.

 

[ImperialCrown]

Chiltons and Haynes leave a lot to be desired. Used factory manuals are the best deal.

 

[ImperialCrown]

The only way I have ever done an IAC/PCM diagnosis is with a capable scan tool. There are tests and commands that can be used to see if the PCM can control and the IAC can react.
If this fast idle just began after the intake manifold/fuel rail/injector service, I'm pretty sure you are dealing with a vacuum leak.
I would pull things back apart for a look.

 

[ImperialCrown]

The Rockauto website shows 1999 and 2000 3.5L use the same IAC part number.