Posted by Hacksaw (aka Andrew) on January 01, 2005 at 11:52:33:

All, I have searched a lot of messages to try to solve my disco's idle problem. With all of your help I finally got it cured. I did notice however that a lot of people don't ever 'close the loop' and write down how the fix went. Here we go then for others to use. My '95 Disco (3.9) suffered from a hanging idle. When driving the truck then slowing down for a light or a corner, the idle would hang at around 1000 rpm (by dash tach) until the vehicle was stopped forcing the driver to brake against the motor. Once stopped within about 3 seconds the idle would slowly walk down to the proper idle speed (600?).
After reading many posts, I went to work. Being rather particular about the cleanliness of my truck I was disappointed to find a mess of goo in the PVC system (this has been cleaned out many times before but...). The 'T' reducing pipe to the manifold was plugged. I'm sure this didn't help. A road test showing a great improvement to the smoothness of the engine but the hanging idle still persisted. To my surprise I found a vacuum leak in the diaphram of the vacuum advance unit on the carb! A visit to the dealer (ordered $200CAN part) and a web tip (use a heat gun to remove stuck distributor rotor, and the vacuum leak was gone. This reallyb helped the throttle response but didn't cure the hanging idle. Along the way I found a small vacuum leak from the Idle air control valve (stepper motor) to the fuel regular, and replaced the vacuum hose. After cleaning out the IACV I still had the same hanging idle problem, though at a much lower rpm now. It still wasn't right but at least I have MUCH greater throttle response and will no doubt get better mileage though I'm writing this before I do another mileage check. After some searching for an aftermarked PN for the IACV, my local NAPA shop had new valve on the shelf! I used PN 2-1738. It was a perfect match (thanks everyone for posting that PN). Another $100CAN though. I didn't get to price this out at the dealer so I can only assume I got a deal. This is my first non rover part on the truck so I was a little worried. I was determined to find the problem with the truck rather than resort to altering the base idle speed. Something had to be dirty or dead. Though the stepper motor seemed to work (cycled the idle down properly given enough time). It didn't get revived with any amount of cleaning or WD. After checking through my factory shop manuals (both mechanical and electrical trouble shooting) I couldn't find any tests for the stepper motor. I went and purchased a replacement.
The end result though....Hurray, not only do I have a proper startup (truck used to rev quite high on initial startup...1200rpm) then settle into a proper idle depending on if it was cold outside, or until warmed up. Now it starts up at around 1000 rpm and settles into a nice smooth idle (way better than before). I have owned this truck since it was 2yrs old and have put 120,000km on it. I'm sure now the vacuum advance was beat long ago. I didn't know my truck would snap to attention that quick. I am a firm DIYer and without all of your help from this forum I would have spent many more hours solving this problem. I don't know how I can contribute back to the forum but will do my best to help out. When you lay it all out on paper, it doesn't look that tough a fix. Clean up everything, locate/fix all vacuum leaks, replace stepper motor. It's like having a new truck! Now if I could just get my short wife to stop falling out of the truck when she gets out, I would be in heaven. Thanks again all.