Hi
I have an 86 Corvette and the idle seems to bounce all over the place.
I havent driven it this summer, but this seems to be an ongoing problem.
Here is what happens.....
I start the car with no problems, it goes to high idle and as it warms, the idle starts to come down. A few times between 146F and 174F the idle bounces a bit up and down.
When it get to 190F, I rev the car up to about 2k and let off right away to bring it down to its idle. From here is where it bounces between 48 and 84 on the tach. Up and down...48, 84, 55, 76, 52, 80, 65, 78, etc, etc. It eventually smoothes out...sometimes faster than other times.
One thing I noticed is that I can immediately stop the bouncing idle by putting the car into gear. It doesnt fluctuate in gear. And when I put it in park, it is still smooth....until I rev it up again.
The tps is new an the voltage is set.
Any ideas?
Thanks

 

Ensure that your throttle cable is not binding internally and not letting the TB snap back all the way.....rev the car up and get it to stay at it's higher idle you describe then see if you can push the throttle arm back toward the stop...if it moves then your cable is binding....

 

Clean the IAC and TB.

 

I think I fixed it. All I did was unplug the IAC and plug it back in while it was running. It hasnt fluctuated since....cold or hot.
How the heck is that a fix?

 

Maybe it was not plugged in correctly? I was going to say a vacum leak but atleast you got it corrected. Thats good news and it did not cost you any money or waste alot of time on trying to fix a problem that was not there.

 

Have you replaced the original multec injectors?

 

The plug was clipped on good. I had to work the seal off and keep the clip held up. Maybe a bad wire in the plug end?
As far as i know, original injectors....why? Problematic?
At least I know where to look....I dont think I have a permanent fix...time will tell!

 

If you have multecs, they will no doubt fail eventually. They are non-servicable. Just saw your year, you don't have multecs. You can ohm your current injectors to see if they are working as well. You can also test them to see if they are leaking or not spraying normally.

 

You wiped the oxidation off of the contacts by separating and re-plugging the connector.

 

Argh....problem is still here!!

I havent driven the car simce i thought I fixed the problem a few months back.

Here is how it went today.....
It is about -15C (5F) and i went to start the car. It fired up and ran perfect on high idle. As it started to warm up the idle came down, but still ran 100%. As it coolant hit 168F on the dash, the car started to fluctuate or hunt the idle. It bounces from 88 to 48, 97 to 62, 78 to 37, etc, etc in about 2 second intervals. It did stall afew times when the idle would drop off in the high 20's, low 30's on the tach.
I had to drive the car into my garage with the TPS unplugged. When I unplugged the TPS, the idle was smooth at 14 on the tach. Even at 220 on the temp gauge, i plugged the TPS back in and it still fluctuated.

I have replaced the TPS sensor with new and re-set it about 10x's! I have removed and replaced the IAC with new and cleaned the throttle body. I have chacked and set the timing. I have checked and fixed the slightest vacuum leaks.

What is happening at that temperature? Is it a temp issue or just because the RPM is dropping below 800 and it cant find its base RPM?

Could it be an ECM? When I thought I had it fixed a few months ago, I re-checked the operation about 4 or 5 times by starting the car and letting it run to operating temp. It didnt hunt or fluctuate the idle then.

Throw some ideas my way.....Im getting frustrated trying to fix the same problem for months! I want to be able to drive the car this spring and i have a few other items i would like to repair before i drive it too. This is the major problem though.

Thanks for the help!!

 

You may want to check for Service bulletins and recalls. I know a LOT of the mid 80's Chevy's had issues with the PROM being over sensitive to MAP changes which caused this. It is the system compensating for the MAP reading and raising the idle. When that happens it compensates for the high MAP reading and the idle drops. This becomes somewhat of a paradox for the system and it goes cyclic. high, low, high, low, on and on.

 

Any codes? - if its smooth until it warms up may be problem associated with the closed loop operation. If you data log you can watch the IAC steps and see if they are moving along with the idle.

 

Well with most of these the IAC is the victim. It is going to be changing to change the idle. The problem is finding why it is being commanded to do so.


I forgot to mention earlier you can remove the MAP sensor, plug the manifold and put a hand vacuum pump on it. Pump it up to about 12-14 psi and see if the car stops hunting. If so you can have a leaking MAP or the PROM issue stated above. I am just not sure if the TSB's apply to your car or not.

 

i don't think the 86 has a MAP sensor since it is not speed density. Remove the IAC and clean the TB. make sure the IAC is working and the pintle is free, Also set it to a max of 28mm from flange to end. Also check to MAF burnoff relay for operation and clean the MAF

 

hmm, well not that it is 100% accurate but my manual shows that they used them as well. This was in the feeling it out years, so I guess anything is possible

 

Have you re-set the minimum idle speed ?? Sounds like it is not happy with it's current setting.
TPS needs to be re-set after any minimum idle speed adjustment.

 

Hi
Sorry for the delay.....havent been able to work on the Vette until now.
Can someone explain the MAF and when it is used?
How or when do I check the operation of the relay?

Thanks

 

Im going to start back at the basics tonight.....
What should the compression test results be?
What should the plug wires ohm at?
The timing should be 6 ('85) but what is optimum setting?
What should vacuum be at warm idle in park?

Still want to know function of burn off relays
Thanks

 

Compression is 165-172 psi
I didnt ohm the wires because I remebered they were new as is the cap, rotor, plugs, iac, tps
Timing?
The vacuum was 22" when I fired it up on fast idle. It dropped to 19.5" @ 560-600 rpm and slightly fluctuated .5". I accounted this to the fluctuating base idle of 560-620 rpm.

I went and verified the idle/tps setting again. 450 rpm w/no iac. set tps to .56 volt.

I wonder if I have a motor mount or tranny mount problem. I disconnected the belt and eliminated anything driven off the front. I wonder if it could be a bad or unbalanced torque. The best way to describe it is it seems like an unbalanced wheel. Above or below a certain speed the tire balances it self out. Same with this engine. Sitting in 1 spot the 900-1000 rpm range vibrates the whole car alot. after 1100 it seems to smooth right out. I know from owning another Vette it should run smoother. Should I just get used to this? argh!

 

Not the problem but you are not supposed to remove IAC plug while engine running.
http://www.corvetteactioncenter.com/...and+Idle+Speed

I have a problem with the fact that it idles OK in gear.
The ECM gets a signal to say that it is in gear and adjusts the IAC accordingly.That would seem to indicate a ECM problem or at least the warm, closed loop , out of gear settings