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Question: 1986 High Idle When Warm/Closed Loop


New member
Jan 3, 2023
Downingtown, PA
1986 Corvette
Hi folks!

I'm in desperate need of some help. Over the past few months, I've been working to get my father's 1986 Corvette (L98) back on the road. She's inspected now and running very well with the exception of the idle. When cold and initially started, it'll crank for about 5 seconds or so and fire up. Once started, the car will idle at about 950/1,000 RPMs. As soon as it gets warm, the idles will slowly tick up to about 1,600 RPMs and stay there.

What's odd about this scenario is two-fold. First, it starts and idles exactly where she should for a cold Pennsylvania day but there have been to two occasions now, where she'll idle at 700/750 RPMs in closed loop -- both occurrences were when the outside temperature was below 30 degrees and she had sat for a few days without being run. When this happened, both times, it only lasted for about a minute and the idle shot up to 1,600 RPM again. I feel that, this proves there are no vacuum leaks otherwise, that 700 RPM idle wouldn't be achieveable/possible.

I've done a ton of research and the IAC has been replaced, the TPS voltage is correct @ 0.54 volts, and I even did a smoke test for leaks just to be sure. I've also read a bit about the temp sensor for the ECM being a possibility but I know its working correctly because the fans kick when my cluster read-out is 225 degrees which is sent via a separate temperature sensor.

One final thought... for years, my father always disconnected the battery after driving the car. Could those repeated battery disconnect/re-connects have done something to the ECM? Is an intermittant vacuum leak possibility? Also, wouldn't a vacuum leak cause an idle issue at start-up not when once warm? I feel, but do not know, that a vacuum leak would wreck havoc on the idle at *all* times - not just when warm.

Also, my instant MPG and average read-out on the cluster is intermittent. Sometimes it works, other times its dead at 0 MPG. Not sure if its related but trying to be as detailed as possible.

Lastly, not sure if relevant, but when I do the paper clip into the ALDL connector, sometimes it'll trip and sometimes it'll take a few tries for the SES light & cooling fans to come on.

Any help or suggestions is most appreciated. Thank you in advance!

***Note: You may see this cross-posted in other C4 forums as I really want to get to the bottom of this issue***
I had a similar problem about 5 yrs ago. I spent about $100 and had the transmission checked - no issue. I then tool it to a shop that had a mechanic that worked on older corvette ( another $100) and he couldn’t find any issue. I then did some thinking about the problem. It only is there with the engine hot and the hood down, hood up no problem. I them suspected it could be an issue with the throttle body. I got an email ad from BBK about their high performance throttle body for corvettes. I gave them a call and talked about the difference in the 52MM & 58MM version. I ordered the 58MM & did the machining to increase both plenum opening to 58MM and installed the throttle body. That corrected the problem. I think that there was a problem with the butterfly warping as the engine heated up and with the hood trapping the heat in the engine compartment- that was the issue. I think there are places that you could send your original throttle body, for a rebuild if you want to keep it original.

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