Strange issue... I can start the car and it’ll hit the mark, ~800-900 on the tach and read the same on my laptop through EEHack. It’s target is 850 and it’s cammed so it hunts a bit in that 100 rpm window - that part seems normal to me.

Now, just sometimes when I come to full stop the idle hangs high, around ~1000 or more, for no apparent reason. TPS reads 0%, it’s targeting 800-850, and the laptop confirms it is 1000+ rpm. IAC counts seem normal, around 30. It’ll remain at that fast idle until I drive more, then in most cases at the next stop light it’ll be back down to 800.

Top end gaskets are new, as are most vac lines, I can’t seem to find any obvious vacuum leak, nor do I really suspect one since this is an intermittent issue.

One stray thought is that the throttle body is getting tired and maybe the butterfly’s are catching in their bores. It’s coming up on 120k miles. I did have the TB mostly apart this winter for cleaning and it felt perfectly smooth, no catches. If they do catch, would that typically also show a TPS above 0? Only reason I think it might be the TB is that it never does it until after I’ve driven the car, it will idle indefinitely from a start at the targeted rpm.

Any other thoughts as to the cause? It’s just an annoyance, might do it once in a 40 minute drive, might not do it at all.

 

Exact same issue on my supercharged 93

 

If the throttle cable is properly adjusted so that 0% is indicated when your foot is completely off and 100% is indicated when your foot is to the floor, then yes, your throttle plate hanging would show as greater than 0% TPS in EEhack.

I seem to recall idle fluctuation issues in modified LT1s having something to do with the "spider" in the intake manifold. Something about the way the bypass air system works causing issues. I only have a stock engine so I'm afraid I can't be much help in that respect.

 

Mine has done this for years and i’ve found the throttle blades get caught up. Yearly I take the TB off and lubricate the shafts which seems to help.
One way to confirm - Get out of the car when the idle is high. Open the hood and manually push the throttle back on its stop. If the idle comes down you’ll confirm the blade was stuck open.

 

Haven’t been able to correct the issue by pushing on the throttle cam.

It’s just very odd that this is a sometimes only problem. Drove the car for an hour a few nights ago and it idled perfectly normal.

Throwing a new IAC at it, just to rule it out. I’m seeing ~30 counts at 0% tps which seems dead nuts on but hey, the IAC is from 94, it may have had its day.

I’ve read a few threads on setting up a new IAC, from wiggling the pintle out to match the old IAC to a start/stop procedure. Neither is instilling by a bunch of confidence as being the right way. Haven’t consulted the FSM just yet. Can I just reset it through EEHack?

 

Have a new IAC, haven’t had the time to install it just yet but with some time off work in the coming week will get to it. Did locate a tiny vacuum leak. I had replaced my brake booster about 10 years ago and the vacuum fitting on the replacement booster had extra ports that my car did not use. It came with rubber caps to seal them off.

I pulled this tiny cap off and it looked just fine, but give it a pinch and you can see the rubber is cracked. I was able to suck air through it. It’s a pretty small leak but a leak nonetheless. I replaced it with a silicone type cap, the little red cap in the background, it’s what I had on hand that fit. Seems to have made a change for the better though not yet perfect as the pcm continues to report an idle higher than it is targeting. Point being, this cap as installed appeared just fine, it didn’t reveal it’s crack until I removed and inspected.

IAC is next as well as a thorough inspection of the cruise control vac lines as they are the only remaining underwood vac lines I have not replaced. The cruise works normally so I mostly expect they are ok.

 

Replaced IAC this morning and it is now idling perfectly at the commanded rpm. Counts are in the low 30s. It got a new TB gasket too, seemed far easier to remove the TB to do the job than not.

The original IAC, though reporting normal counts must still have been misbehaving. It did have a little carbon/oil soot on the pintle, more so discoloration than anything I could feel/remove. I did notice that the same coating went down the shaft as well, it’s probably just gummed up.

 

Congrats on getting it fixed! Glad it was something simple.

 

thanks for the tip

i have a similar issue occurring