Fast Idle on 91 ZR-1

[Anonymous]

Would appreciate opinions on what to do/where to look for this problem:My 91 ZR-1 has been idling a bit high lately. The real annoying part is when you're slowing down and push the clutch in and the idle goes to 1500 rpm. When sitting for at least 15 seconds, the idle goes down to about 1000 rpm but it takes awhile. The car wants to run at 1500 rpm all the time once in motion, coming to a stop and when pushing the clutch in. Additionally, my gas mileage has gone to h*ll
.Other than the idle, the car runs fine. Any ideas on where to look or what to do to fix the problem? I couldn't find anything in the Chevy service manual about adjusting the idle on a ZR-1. On a tuned port or LT-1, you can start the car, disconnect the IAC plug-in, and then adjust the throttle/idle screw. I tried that on the ZR-1 and backed the throttle screw out all the way. That got me from 1200 rpm down to 1000 rpm but it's still idling too fast and wanting to operate at a higher rpm once moving.

[Anonymous]

Greg:Try cleaning the trottle body's butterflies with Gumout and see if they go back all the way and are not binding ( worn shaft bushing)?

[Anonymous]

I agree with Joseph, also check and clean the IAC valve.Also take a look at this: http://zr1.net/ZR1_issues.html#2

[Anonymous]

Thanks for the replies. I'm sure the throttle body butterflies are not binding. They are operating very smoothly. I have a lot of experience with that problem on a Lingenfelter 383 car I have. Also, I have already pulled the IAC and gave it a good cleaning even though it looked like new without a spec of anything on it. Is there any kind of adjustment on it like on the TPI/LT1 IAC where the pintle needs to stick out 1 1/8" ?I don't think it's a computer chip/memcal problem since the car has a DRM chip and it's only just recently started having this problem. It went for 2 years without the problem on the exact same setup. I've also tried clearing the computer a couple times, no change.Any other ideas? I was wondering about vacuum leaks from a hose that may have blown off but from the best I can tell, they are all still attached(at least the ones I can see).
bans25|Fast Idle on 91 ZR-1|63.127.199.195|yes|yes|1042208443|You can check for vacum leaks by turing the key on (but do not start the car) and listen to the vacum pump run, if it keeps cycling on and off, or keep running you have a leak. Mine usualy goes about 70 seconds beteen cycles.

[Anonymous]

Rather tham the gumout, I would use 3M throttlebody cleaner. I was told that some of these throttle pltes may have certain coating. Also, some of these products may foul up O2 sensor. I just cleaned primary plate, secondary plates and IAC solenoid and after a squirt cleaned out IAC hole w/ Q-tips. This eliminated high and fluctuating rpm problem.Why does it show L-98 nunder my name? In my profile I ve checked ZR-1? Anyone?(Edited by Byong Yun at 12:15 am on Jan. 14, 2003)

[Anonymous]

Byong Yun..You move up with posts I think after 50 you make LT5..I"ve been having the same idle issues.. I will try cleaning out the throttlebody and IAC this weekend. Would have done it sooner but every weekend the sun has been out and I just end up going for a drive.....

[Anonymous]

Hey, wegone.Is that how its done? Thanks.

[Anonymous]

secondchance15K miles in the last 9 months and she keeps running better.

[Anonymous]

Hi,New to this board, but old to ZR1's. My '91 exhibited the same problem at about 18,000 miles. Problem appeared one day, and steadily got more consistent. I learned a lot about the LT5 then, and members of other forums (and the knowledge base on this site)helped a lot, and I went thru everything, including IAC, TPI, PCV's, cruise control, new chip, removal of the plenum and re doing the vacuum. Still no fix. I discussed it with the service manager of a local dealer, and he (contrary to my expectations) took an interest because it was a ZR1. He got the car and called GM. They sent him some software that he put on a laptop and plugged into the car while he drove it. Conclusion after a couple days and many calls to GM. An upgraded computer was needed. The original on the 91 had been there since the beginning, and this was an issue handled by later software improvements (different from the similar improvements in the "chip", which had already been upgraded). So he ordered a new computer, put my already new chip in it, and all was fine. Cost... about $200.That was two years ago, and the car is nearing 40,000 miles now without a hiccup.Hope this helps. It took me about 6 weeks of going thru different issues before I gave up and got the dealer involved. The problem was really devilish. After each fix, it seemed fixed... until the engine warmed up thoroughly. Best of luck.Regards(Edited by ZR1991 at 9:01 pm on Jan. 16, 2003)

[Anonymous]

Great info ZR1991.