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 soemtimes 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***

 

First thought is a bad CTS (coolant temp sensor), CTS connector, or wire. You have classic symptoms.
Second thought is a worn throttle shaft bore. When at idle do not turn the shaft but wiggle it up and down, side to side to see if the idle changes.

 

Thanks for your reply, Talfry_yn!

A couple clarifying questions:
1.) CTS - you're referring to the one that feeds the ECM, correct? I ask because the cooling fan kicks on at 225 based on the dash reading. My understanding is that there are 2 coolant temp sensors... one for ECM exclusively and one for the dash reading. If the fan kicks at 225 - they must be in sync, no?

2.) What is the throttle shaft bore? Not familiar with that term? Is that there the cables connect? Could a 42,000 care have excessive wear to give play? I want to check it - just not sure where or what to wiggle.

Thanks again - much appreciated!!!

 

Yes the CTS feeds info to the ECM. As far as correlation to the dash gauge, the dash gauges are deemed unreliable so both sensors could be reading spot on at 225°F or it could just be a coincidence. On my '95 the dash gauge reads about 20°F lower than the CTS.
Have you done the base idle speed procedure from the FSM? It's a great place to start.
At 42K miles I wouldn't worry about the throttle shaft bores.
Do you have the ability to scan for IAC counts?