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1985 Corvette Idle Problem

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drcorvette

Guest
Here is my dellema.
I own a 1985 Corvette automatic with 55000 miles. I have had an idle problem that no one seems to be able to fix. Everything is stock except for the normal stuff ... open air box, K&N Filter, Air Foil, new MAF sensor, etc.
No problem with cold or warm starts. On a cold start car will idle at around 1000 RPM. Once at normal operating temp on a good day car will idle in gear at a stop light around 530 to 560 RPM but most of the time car will surge. It seems to be hunting from 490 to 640 RPM constantly. It does not stall out.
Last major tuneup was approx 10000 miles ago. New exhaust front to back, new O2 sensor. Thinking my IAC was changed to but not sure ...
In all your brilliant minds ... can anyone tell me why the surging as its driving me nuts!!!
I have Rapid Fire Plugs and new wires I am about to install but I don't think this will solve my problems.
I could start changing all my sensors one by one, my IAC and my TPS but would like to know if someone out there in vette cyber land has ever experienced a similiar problem. Thinking I even feel some sort of surge at around 50 MPH but this could be all in my head!!!
Where the hell do I start???
Any suggestions would be most welcome!!!

Gary
 
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c4ever

Guest
Wish I had the answer for a fellow 85 owner but I haven't experienced an idle problem. I'll take a look at some diagnostics in my manual tonight and post. It would be nice to get it down to the proper list of suspects. I will say that all sensors can be tested for operation prior to wasting your money and replacing what you don't need.
Carlo
 
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ErnieN85

Guest
Hi, Some things to try. Clean the throttle body & idle air passages. You'll have to remove the throttle body & Idle air motor to do a good job. Use carb cleaner and brushes to get the thing squeaky clean. re install and set up the Minimum air and TPS voltage as speced in the manual. BTW use a higher min air there is a fac on the forum as to how to do it. at the same time check for broken vacuum fittings and or lines, and hoses. also it's a good idea to replace the PCV valve. this has cured the problem for me each time it's occured. 157,000 mile and still going!
 
D

drcorvette

Guest
Idling Con't

Hey all ... Thanks for your replies.
Question ... How often to you all clean your throttle bodies???
 
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ErnieN85

Guest
Depends on how much blowby you have with my old engine (worn out) about every 5000 mile. With the new Engine 30,000 and it wasn't near as messy
 
D

drcorvette

Guest
ECM

Could this idling problem be my ECM????
 
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tom_85_vet

Guest
I have the same problem exactly. I have tried most of the suggestions. I believe what is happening involves the 9th injector, once it reaches operating temp, the 9th injector shuts off causing the engine to run lean. The computer then goes into a search mode, causing the rpm to go up and down until setting at a low default value (about 560). You should notice that on a cold start, once the car reaches 160 it goes into this mode.

Things that I need to try are verifying the IAC is adjusted correctly (the setting is in the manual). I also have a procedure to setting the computer idle at operating temp.

I think the best guess on fixing the problem would be the procedure on setting the computer idle. I will try to locate the procedure, but basically it involves resetting the computer (disconnecting the battery) at operating temp.

Check all the obvious stuff: codes, vacuum leaks, dirty intakes, and fuel pressure, but you may have the same problem as me.

I will keep you posted if I can fix my problem and I hope you do the same.
 
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c4ever

Guest
Gary,
I would go with the list I mailed. I would think the ECM an unlikely suspect.
 
D

drcorvette

Guest
Idling

Thanks Carlo ... I will follow your email to me and Thanks Tom ... I will look into that cold start injector.

Question ... are your '85's auto or 4+3. If auto what is your RPM stopped at a traffic light in gear at normal operating temp???

I am going to fix this problem no matter what it takes!!!!
 
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c4ever

Guest
I'm running an auto in the 85. Idles in gear at about 520 to 550 under normal temps.
Good luck with the fix. Always turns out to be something pretty basic.
 
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tom_85_vet

Guest
The same, "I'm running an auto in the 85. Idles in gear at about 520 to 550 under normal temps."
Please keep us informed.
 
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tom_85_vet

Guest
I did some searching on other threads at this site and there are a couple of threads with simular syptoms. Do a search on rough idle. It sounds like the IAC could be the cause of the problem. I heard that the after market IAC could cause problems, which is what I have. I will try adjusting/cleaning and setting idle at operating temp, but if no luck maybe I will have to buy the GM IAC.
 

jsinga

Well-known member
Joined
Nov 23, 2001
Messages
280
Location
Savannah, GA, USA
Corvette
87 Silver Coupe Way over 200k miles
try unplugging those sensors one at a time. The IAC can be unplugged and the car will run at min air idle. try tapping on the TPS. You can also unplug it and the MAF sensor. I'm not sure it will run very well with the TPS unplugged but I know it will run with the MAF unplugged. Look at the fuel pressure real close a pump will get hot and weaken. cold start injector doesn't work unless the car is pretty cold, like outside air below 60 degrees F.
you might also look at all your engine grounds. vaccuum leak is a high possibility.
let us know what you find.

JS
 
D

drcorvette

Guest
Hey all ... thank you for all your replies.
By the sounds of it I guess my idle is normal because it too is in the 520 - 550 RPM range but my problem is I just have to smooth it out so it is more consistently in that range.
Thinking tomorrow I will pull the IAC and try cleaning the spindle or maybe just going to GM and replacing it.
That's a start anyways.
Don't stop the replies ... if you find out anything let me know and if I do fix my problem I will pass it on.

Gary
 
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tom_85_vet

Guest
Try cleaning it and adjusting the length before spending any money. I have some time on my hands (mother-in-law is in town). I will do the same. I hate to spend any money, but my problem might be the after market IAC. One thread indicated that after market IAC could cause problems.
 

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