I have a 1990 T/A 5.0 TPI T5. I have a serious love hate relationship with this car. I've owned the car for six years now and have had the same idle issue all along. When I bought the car it was idling high all the time and was overheating. I was able to verify a dead spot in the TPS sensor and a carboned up IAC. Both components were replaced and the idle reset procedure was done. The vehicle started and idled right where it was supposed to! Awesome, fixed, Right? Nope! Drive the car down the road and after a few stop and starts @ stop signs, the idle started coming back up and settled around 1600-1800 rpm and would not come back down. Got back home and performed the reset procedure again. Starts and idles fine, again. Drove it around for a while and no problems. Stopped @ the store, shut it off. Got back in, started it up, idling high again. I have gotten frustrated with this issue and, for a while, had the ECM condemned in my head, thinking that the ECM must be telling the IAC to open intermittantly. It wasn't until recently that I was brainstorming with a friend and we were kicking around the idea that the MAP sensor could be providing incorrect vacuum readings to the ECM and causing high idle. I can't find any descriptions of the input/output functions of the ECM to see if my thought process is correct. I do own a scanner (actually multiple) but the one thing I never thought to look @ was the MAP readings. The car is off the road for the winter and the engine has been removed for mods. I would like some input from others about the MAP sensor possibly causing my issue.

 

Check and see what your IAC step count is. I believe it should be in the 30-40 range when the engine is warm, but don't hold me to that number. If it's at 0, then you probably have a major vacuum leak somewhere to fix. Map values might be useful too, they should be fairly constant while the engine is idling in PARK and should return to the same value after goosing the engine.

 

Well, it's true what they say, technicians overlook the most things on their own vehicles. I am replying to this post out of pure humility! I am in the process of painting my engine bay and have started taking out a lot of parts, under the hood, that I've never had out before. It has come to my attention that there is a vacuum line that runs along the firewall and then out in between the inner and outer fenders, going to the vacuum storage reservoir. I never saw that one before. Much to my surprise, the vacuum "T" near the reservoir, has an end cap on the unused side of the "T" and it is dryrotted badly. After kicking myself in the a** and swearing to myself for a few minutes, I plugged off the vacuum line and the vehicle runs and idles normally. Boy do I feel stupid!! I've owned this car for 8 years and just found this issue recently. So, to all those techs out there that over think their own vehicles, K.I.S.S.!!

 

One end of that "T" goes to the vacuum reservoir, the other to cruise control on fender.
I also chased a vacuum leak and found the same thing!

 

Yeah, my car doesn't have cruise control, so the end of the "T" that would go to the cruise, was capped off. The cap was dry rotted and leaking vacuum. Very frustrating and satisfying find. I'm just happy I found it (even though it took me six years).