RPM's stay high after stopping but drop to normal after a while

The problem was the new IAC I had replaced. Apparently this car is picky about the IAC valve and insists on OEM. Off the market from parts store do not work as mentioned in many threads on the net. I replaced it with the original Motorocraft and everything was fixed.

RPM's too high (1,500) when the car is in drive and AC on, when AC off runs perfect. I have tried two new IACV, two TPS, MAF sensor, new PCV valve. Cylinder head done at machine shop. No vacuum leaks. Tested with a Snap On scanner and every thing looks fine. Do not know where else to look or test.

Four cylinder front wheel drive manual.

My car has had this problem for about a year. Here is what I know: heat is a factor the car has to be warmed up about an hours drive time before it acts up. Outside temperature also plays into to it. If it is hotter outside then it speeds up the problem, all winter no problem. The car starts to rev when you are trying to stop you can have it out of gear clutch in or out it still will do it. You can knock it down sometimes by shifting to a higher gear it will rev between 2,000 and 4,000 rpm's but has gotten as high as 5,000 once. We have replaced the computer no luck their, clutch seems to be fine my mechanic is stumped. I have no dependable car. Can you offer any suggestions?

Seeing this was the first time I knew someone else shared my pain! So a bit of a backstory bought the car advertised as "good car for a mechanic" I am a new mechanic*A&P*, but I know my way around the engine and have a strong mechanically inclined brain so I took on the project for my son's first car. $350.00 for a 2000 Ford Focus Wagon SE, 2.0, DOHC, the interior still looks pretty decent. Turns out the engine threw a rod. Bought a $100.00 eBay special and got everything installed, fumbled through learning vacuum lines and testing sensors. The bonus is that the engine with the thrown rod had several new parts that the previous owner had thrown at it, so it was like having parts exchange for anything I deemed suspect. But once all the obvious easy things had been fixed or adjusted I still had a big problem. Press the gas and the car continued acceleration after I let off the gas pedal. At first, it only did it while the A/C was on then later it did it anytime. I researched and watched hours of videos to no avail, I was ready to pay the price of taking to the dealership for diagnostic. Stopped in to see them and understood right away they wanted to throw parts at it until it subsided, and I was not inclined to oblige. Determined to figure it out or watch the car go up in flames (joke). I started rechecking everything dealing with air flow through the intake and was quite settled somehow some way it was the Intake Air Control that was the culprit. So I removed it from the extremely hard to reach position it is in for the third time, becoming very efficient at it I might add, and examined it with disgust, wondering why it has caused me so much aggravation. To my surprise, I found the ticket to the promise land! Nowhere in anything I have read or watched has it mentioned the IAC having an adjustment screw! All that was left was adjusting it to make everything hunky dory. Now my sixteen year old can take possession of the car that will last for years to come, or until his first accident (the reason for buying a cheap first car).