High idle
Thread Starter
Le Mans Master
Joined: May 2004
Posts: 5,866
Likes: 60
From: Hellinois
Finalist 2020 C4 of the Year - Modified
High idle
My 85 has a high idle that I can't seem to fix, the chip is set for an idle of 850 rpm and no matter where I set the base idle the engine idles at 950. I have set the base idle at varying rpm's from the recommended 450 all the way to 700 and it still idles at 950 once everything is set and hooked up. I have a new IAC in a new BBK throttle body. The IAC is getting signal, I checked that with a noid light. It's not a vacuum leak because when setting base idle I can change it to whatever I want just fine. Could this be a faulty tps?
Team Owner
Joined: Feb 2006
Posts: 24,460
Likes: 7
From: sw Ohio
my '85, the rpms will drop to 300 and sound like its gonna die if I dont give it gas. It does this when the engine is warmed up and i'm sitting in traffic. I've replaced the spark plugs and vacuum lines, still does this.
Instructor
Joined: Jun 2005
Posts: 219
Likes: 0
From: Davison Michigan
Well here is another one. I know you were looking for advice but....I have the same problem. Just put in new gaskets from the intake manifold up. Cleaned the thottle body and IAC valve, set minimum idle and adjusted the TPS to .54 volts. This site was great for getting me through all that. But, my car is idleing about 900 - 950 on the tack. I know the tack might not be the most accurate way to check RPM but the engine is idleing fast. Hope someone out there has suggestions.
Safety Car
Joined: Mar 2005
Posts: 4,723
Likes: 11
From: Winnipeg Manitoba, Canada
Originally Posted by Midnight 85
Well, whoever solves this will make at least 5 people happy. TTT
Racer
Joined: Sep 2005
Posts: 385
Likes: 8
From: north branford ct
What do your cars idle with the IAC disconnected. I would think the only way it could idle faster is if the IAC is letting in to much air, but being a rooky there is probably a lot of things that could tell the Iac to open prematurly
Corvette Stories
The Best of Corvette for Corvette Enthusiasts
Top 10 Most Expensive Corvettes Ever Sold on Bring A Trailer
Brett Foote
10 Things Every Corvette Owner Needs (2026 Edition)
Michael S. Palmer
8 Most "Only Corvette Owners Understand" Quirks and Problems
Pouria Savadkouei
10 Reasons the C6 Z06 is Still A Performance Benchmark After 20 Years
Joe Kucinski
How Much Horsepower Every Corvette Engine "LOST" in 1972
Joe Kucinski
Top 10 DOs and DON'Ts for Protecting Your Convertible Top!
Michael S. Palmer
Top 10 Most Explosive Corvettes Ever Made: Power-to-Weight Ratio Ranked!
Joe Kucinski
150 hp to 1,250 hp: Every Corvette Generation Compared by the Specs That Matter
Joe Kucinski
8 Coolest Corvette Pace Cars (and Replicas) of All Time
Verdad Gallardo
Safety Car
Joined: Mar 2005
Posts: 4,723
Likes: 11
From: Winnipeg Manitoba, Canada
Originally Posted by mudslide
What do your cars idle with the IAC disconnected. I would think the only way it could idle faster is if the IAC is letting in to much air, but being a rooky there is probably a lot of things that could tell the Iac to open prematurly
Thread Starter
Le Mans Master
Joined: May 2004
Posts: 5,866
Likes: 60
From: Hellinois
Finalist 2020 C4 of the Year - Modified
Originally Posted by jimmers
I posted about that alil while ago. With my IAC disconneted my car idles great! As soon as its plugged back in my idle shoots up about 700rpm more. The throttle body is completly cleaned with a new IAC. Very weird imo.
Burning Brakes
Joined: Jun 2002
Posts: 1,001
Likes: 1
From: sd
I too had this problem with my new 383 but I feel that I have it solved now. I removed the IAC mount from the BBK throttle body and completely scraped, wire brushed, and cleaned that out. I started looking at my IAC a little more and thought that the plunger was moving a little stiff so I played with moving it up and down for a while and that seemed to help. (Now everyone at this point get your mind out of the gutter
) I then put everything back together, reset the TPS and IAC and it idles fairly well between 850-1000 in park and 800-900 in drive. I feel the difference in rpm's is directly related to the cam so I'm not too worried about it. I also feel that the learning function of the ECM takes a while to get used to the engine although I could be completely full of **it too. I realize this might not be true but on Insider Hints ol' Myron Cottrell seems to think that it could take up to 500 miles for the ECM to get used to the engine changes. I hope this gives some of you guys a little hope and help. Good luck!
) I then put everything back together, reset the TPS and IAC and it idles fairly well between 850-1000 in park and 800-900 in drive. I feel the difference in rpm's is directly related to the cam so I'm not too worried about it. I also feel that the learning function of the ECM takes a while to get used to the engine although I could be completely full of **it too. I realize this might not be true but on Insider Hints ol' Myron Cottrell seems to think that it could take up to 500 miles for the ECM to get used to the engine changes. I hope this gives some of you guys a little hope and help. Good luck!
Team Owner
Joined: Oct 2000
Posts: 96,443
Likes: 9
From: Fort Knox, KY
Cruise-In IV Veteran
Cruise-In VI Veteran
Cruise-In VII Veteran
Cruise-In VIII Veteran
Hmmm. That is weird. One possible thing to check might by the little gasket on the IAC itself. When I cleaned my TB and IAC last weekend, the gasket had either cracked or rotted away, allowing air to enter at the base where it screws in and resulting in a vacuum leak.
It's worth investigating.
It's worth investigating.
Pro
Joined: Apr 2005
Posts: 590
Likes: 0
From: Boynton Beach, FL
Same but different
Interesting thread. Add my name to the list. I've been fighting an uncertain idle problem on my 93 for a year. Mine is a little different though. Sometimes it will idle high, sometimes low. It can vary from 500 rpm to 1200 rpm. It varies. When I start the engine whatever idle it decides to idle at will be set. Once started the engine will idle at whatever speed it picked until I shut off the engine, going closed loop doesn't change anything. If I restart it, it might idle at the old rpm or a new one. Car runs fine except for idle. Its driving me nuts. No codes are set. For various reasons I've installed a new opti, new ICM, new IAC, new TPS, cleaned throttle body, checked for vacuum leaks, etc.
I'm stumped. Only two things are left, the ECM or the coolant temp sensor. As I understand it, the input from the coolant temp sensor is used by the ECM to control the idle speed. Anyone have any other ideas.
Larry
I'm stumped. Only two things are left, the ECM or the coolant temp sensor. As I understand it, the input from the coolant temp sensor is used by the ECM to control the idle speed. Anyone have any other ideas.
Larry
Thread Starter
Le Mans Master
Joined: May 2004
Posts: 5,866
Likes: 60
From: Hellinois
Finalist 2020 C4 of the Year - Modified
vetracr, I can pretty much say it's NOT the coolant temp sensor, I replaced that too. I sure hope it's not the ECM, mine is only a couple years old but you raised an interesting question there. I guess since we're not getting any ideas here I might call Gordon Killebrew, a call to him would be cheaper than experimenting with a new ECM.
Thread Starter
Le Mans Master
Joined: May 2004
Posts: 5,866
Likes: 60
From: Hellinois
Finalist 2020 C4 of the Year - Modified
I just got off the phone with Gordon Killebrew and he says it's the PROM, that can't be because I just checked you guy's profiles and most of you are bone stock or don't have any mods listed. Muffin and I have quite a few mods but the problem is the same stock or modded.
GK says it's not the ECM and I hope he's right, I called my buddy Jay at Vette2Vette to see if he had a good ECM I could borrow but the only one he had was in a fire. I'm not about to buy another ECM until I know for sure it will fix the problem. I'm going to try again this afternoon and do a couple things different, if I have any results I'll let you know tonight.
GK says it's not the ECM and I hope he's right, I called my buddy Jay at Vette2Vette to see if he had a good ECM I could borrow but the only one he had was in a fire. I'm not about to buy another ECM until I know for sure it will fix the problem. I'm going to try again this afternoon and do a couple things different, if I have any results I'll let you know tonight.
Thread Starter
Le Mans Master
Joined: May 2004
Posts: 5,866
Likes: 60
From: Hellinois
Finalist 2020 C4 of the Year - Modified
Okay guys, here's what I did today, I went to the tech section and used the idle setting method outlined by Lars Grimrud. The only difference is that his method is to set the idle at 400rpm with the car in drive and the e brake set. Of course if you have a manual then it's still 450 rpm. My engine is modded just enough that it won't stay running below 450 so I SLOWLY adjusted the idle down until it died then went just a bit further. After reconnecting the IAC and setting the tps for about the 200th time I ended up with an idle of around 925-950 in neutral with a solid 900 in gear. ski_dwn_it has been in touch with me all along on this because it's his chip in the car and he and I both seem to think that my new IAC is not working correctly. I can accept the idle I have now since it is only a bit more than the chip calls for but you stock guys are not going to be helped at all. I think I can truthfully say that our problem HAS to be either a vacuum leak, (this does not apply to my car since I know there are no leaks) or a defective IAC. This is assuming that you all have done the minimum air set procedure correctly. Go to tech and follow Lars directions and see what you come up with. I am going to put my old IAC back in Monday so I'll get back to you all then.
Le Mans Master
Joined: Dec 2000
Posts: 7,839
Likes: 22
From: San Diego, Ca
If you really want to look into it, you're going to need to scan it. Specs are 50 +/- RPMS from Targeted in Drive; +/- 100 RPMS in Park/Neutral - You don't know what it's targeting until you scan it nor can you tell how the IAC is being pulsed to meet that target. Of course there's always a possibility that everything is working correctly and that the throttle body is simply worn out. Otherwise, lot's of things can effect the correct idle speed. Rich injector(s) - is it rich or lean?; a skewed Coolant Temp Sensor; TPS above .62 (at least check it at the ECM along with checking it at the sensor); faulty air pump circuit - clamp off the header hoses and see what happens; faulty fuel vapor solenoid - clamp or plug the throttle body inlet; a Bum Park/Neutral Switch - you can check the signal at the ECM Pin ; Faulty MAF or MAP - on MAF cars disconnect it and see what happens on a MAP look for an idle voltage of about 1.2 volts; and loss of the a/c request signal or an a/c signal with the a/c off - again you can check it at the ECM Pin, but all of these things (and others) are far easier to look at if you invest 150 Bucks in a Scanner. If all the sensors are working and it won't maintain spec idle, it's either the throttle body or the ECM.
Thread Starter
Le Mans Master
Joined: May 2004
Posts: 5,866
Likes: 60
From: Hellinois
Finalist 2020 C4 of the Year - Modified
SunCr, thanks for your response, I have mine figured out now. I put the old IAC back in and checked everywhere for a vacuum leak. I found no leaks and now the car idles at a tad over 900 in neutral and almost right on 850 in drive. The chip is tuned for an idle of 850 so I am happy it is as close as it is. The other members who were having the same trouble can benefit greatly from your advice, thanks.
I want to thank ski_dwn_it again for giving me all the help he did and for making me a fantastic chip. The car is running super strong now thanks to Jesse.
I want to thank ski_dwn_it again for giving me all the help he did and for making me a fantastic chip. The car is running super strong now thanks to Jesse.
Le Mans Master
Joined: Dec 2000
Posts: 7,839
Likes: 22
From: San Diego, Ca
Glad you found it - the only other item that might make the idle unstable is low voltage at the ECM. Again, a scan shows what that voltage is, but the lower the voltage sensed by the ECM, the longer the injectors are pulsed. A fault on the sensing circuit or with the ECM leads to too much fuel. I think Ski Down It actually captured and posted some logs showing this condition. Not something I would normally suspect, but when everything else looks ok, another thing to check.
Le Mans Master
Joined: Oct 2003
Posts: 6,130
Likes: 2
From: seattle WA
Sounds like SunCr hit the nail on the head, one thing I'd like to add is our C4s have so damn many vac. lines, and I've checked with Propane and WD40-more than once, finallllllllly found one. Wish they'd come up with the Smoke Test in a can, some shops use a smoke test and I can see the benefit, my understanding is you can actually see the smoke get sucked into the leaking area.