C4 86 - idle problem

[thom]

Need technical assistance with my 86, recently it has developed an unstable idle, vary 300+/- rpm while in any gear postion. otherwise the engine performs perfectly, no driveability problems.


To fix this problem, I removed the throttle body, cleaned it thoroughtly, replaced the idle air control valve,with a new one, and visually check vacuum lines/hose.... still have same idle problem.


I'll greatly appreciate any help, and want to thank you in advance for your assistance.

Thom

 

[The Car Whisperer]

Did you have this problem before you did repairs or after? Did you take the throttle body off to clean it? Retrace your steps and make sure you don't have any air leaks between the mass air flow sensor and the engine. Any air leaking in will cause the ECM to constantly adjust the fule flow to compensate. When you replaced the idle air control valve did you adjust it according to specs? Did you check the voltage at the TPS, it may need to be adjusted.

 

[vader86]

Remove and clean the IAC valve and Throttle body, reset the TPS voltage and get a reading on it, its possible the TPS is bad. Also check for vaccuum leaks.

Other things would be MAF and its relays, and the ignition system.

 

[scorp508]

When you replaced the IAC motor did you go through the idle reset procedure? If not that can cause that sort of idle surge. Let us know....

 

[Blue 88]

Newer Vette owner-

I have about the same situation. My Vette idles at 700, then to 600 RPM, then 700, then 600....do I have the same thing going on here?

 

[Schrade]

Vacuum leak.

Test:

Quote:
(ME) Lopin' idle is an air issue; gotta' be a vacuum leak somewhere.

Block the TB linkage, brick on pedal, anything to get a STEADY idle, then start sprayin' startin' fluid EVERYWHERE, a little at a time, to find where it's gettin' sucked in, and causin' a brief rev increase.

Vac leak will make a engine lope idle - 20 year old lawn mower, or a new car on the lot...

Find a leak in a tight spot? Try a book of matches. Light the whole book, blow 'em out quick while the match heads are burnin', and see if the smoke feeds into some spot...

 

[Blue 88]

Vacuum leak.​

Test:​

Great ideas! Thank you very much. I like the starter fluid trick. Have a great week end and thanks for the quick response!

 

[c4c5specialist]

HI there,
A faulty injector will also cause this condition.
Check them all.
Allthebest, c4c5