I have that exact problem on my cougar. I'm running an 88 speed density five speed computer and I'm almost convinced that is the problem. I got the computer really cheap off of ebay and the guy said nothing was wrong with it, but I've changed TPS, IA bypass valve and motor, cleaned the TB, checked codes, I get no codes even when the damn thing is idling high. The weird thing is when it's idling high and I test the codes, as soon as the scanner tells it to start test, it idles exactly where the computer wants it to for the test, then when the test is over, it goes low for a few minutes, but after driving it it's back up again. I'm almost positive it's the computer so I'm gonna convert to mass air and see if the new computer helps. Only other thing I can think of is a ground problem I've got. My ground strap from my computer to the chassis is kinda ghetto and worn from removing the computer so much. Anyway, if you find out anything be sure and post. I'll do the same.
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Michael Black
Quantum Motorsports
Norman, Oklahoma
1988 Merc Cougar 5.0 HO, P&Ped heads, 2.25" custom mandrel bent dual pipes, T5 five speed tranny
15.43 @ 91.08mph (not shabby for a 3600 pound car)
60' 2.453 Street tires suck!!!
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