3.8 very high idle

I looked in the trouble shooting section of my haynes repair manual. They recommend looking at (1) sticking throttle linkage (2) vacuum leaks at intake manifold and (3) fuel injection or engine control system malfunction.

Numbers one and two are easy to check. You could pull the rubber intake hose off the TB and see if there is a build up of carbon, or a mechanical problem with the throttle blade not closing. And if you have access to a vacuum gauge you could check the vacuum. if you don't have a vacuum gauge you can get one a sears for less than $30.

At idle your car should between 17-21hg. Make sure the car is warmed up before you do the test.

If the vacuum is low you have another leak and you need to track it down. You can isolate the leak by clamping off different vacuum lines while watching the gauge. When the vacuum returns to normal you know that the leak is in that circuit.

If numbers one and two check out normal you have problems elsewhere and you may want to take it to a ford dealership to have them diag. the rest.

You replaced the IAC? Where did you buy the iac at? If you got it from autozone,chnces are you have a defective part.Return that part for another elsewhere .Your idle shouldn't jump like that after replacing an IAC