too high idle

Hello to all. I recently purchased a 1994 GMC 3/4t with a 7.4 TBI and 130,000 miles. It has dual fuel (propane and gas) and on either fuel it has an excessively high idle.

Sometimes it idles at about 12-1300 rpm, but usually around 1000-1100 in park or neutral and 8-900 in gear. The only trouble code I had was the oxygen sensor (which I replaced), but it didn't help the idle. It was running quite rich, but haven't noticed it since the 02 sensor replacement.

Another funny symptom is when I'm going down a long hill coasting in gear at about 60 mph, the idle will go up to about 2500 rpm and the truck will accelerate, then drop down to about 1500 rpm and deccelerate, then back up to 25. It continues to do this until I either apply gas or slow down to under 50 or so.

I've checked to idle air control valve, but haven't cleaned it yet. The EGR valve looks fine, and the throttle body doesn't have a vaccuum leak.

Also, forgot to mention I noticed my fuel injector connectors were quite badly roasted and brittle. The wires got so hot they are actually cracked in places and bare wires are showing. I've got the new connectors and wiring to replace them, but haven't had the time to do it yet. Don't know what happened, but the propane adaptor on the TB wasn't installed correctly when I bought it, so they may have had a backfire or something.

Any ideas/suggestions?

Well, while cleaning the idle air control today it came apart in my hands. I needed to use the truck and they didn't have one in stock where I buy my parts so I extended it all the way by hand and re-installed it without plugging it in. The idle came down nicely, but each time I started it, it seemed to change a little bit (800 the 1000 then 900 etc). So the question is, was the IAC faulty, or is the computer sending it the wrong voltage? The saga continues.....

Your not alone ...I have a 1997 GMC Sierra with an idling prob With over 110K miles I changed two sluggish O2 sensors ...before and after the cat ...cleaned the TPS...replaced the IAC (mine came apart too)....no help there ...my IAC will pull the idle to norm after the vehicle has stopped...if I unplug it the truck runs fine ...but the "Service Engine Soon" light comes on after a while