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its strange, you can pull upto a light and stop, idle hits 8/900, then hits 3000 and will not come down, and i have looked when this happens under the hood, and the throdle is all the way back, can't get any lower?
check the iac like ken00 said, on my 89 bronco II it was doing the same thing and it was the engine coolant temp switch. the one that relays info to the computer not to the gauge. it is located to the left side of the thermostat housing as you look at the engine. its worth a try.
The ECT is a basic thermistor (resistance decreases as temperature increases) any good manual will have a table. Here's one I took out of a Chilton's manual http://www.xmission.com/~vondm/fordtempsensor.txt Just make sure you're checking the ECT and not the dash gauge sender -- they are different.
A bad ECt might also trigger a fault code. Have you pulled any codes out of the computer? Ken00 has posted instructions (only requires a jumper wire and a voltmeter) in his tech info post.
Are you certain you don't have a vacuum leak? That would be a major cause of high idle, especially on a speed density system.
Also, just make sure he hasn't bought any of those little "performance modules" of of Ebay, they plug into the the.. just forgot the word, but it's right behind the throttle body.. anyways but they raise RPM's when at a stop for like 2 seconds or more, I tried one out just to see, and all it did was raise my RPM's up until I killed my ignition, or until I started to go. Also made exhaust rich. But also check for vacuum leaks.