Hi guys,

I am not one who usually asks for help, but I am frustrated beyond belief and need some perspective. My 86 has an idle issue that I don't like. The idle hunts around a bit, especially when the car is in Neutral or Park. I'm used to a rock steady idle in all of my other cars, so maybe I am spoiled. The idle goes up and down over a range of about 150 RPM. The idle is stable at first, then the hunting gets worse the longer the vehicle idling. It's like the computer is overcorrecting the IAC. Is this normal?
Another issue that has come up recently is that the car has been running rough when the engine is cold.
This is the current status of my car:
Fresh engine rebuild, about 9,000 miles on it. I bumped up the compression to 10 to 1 on it, basically stock otherwise except for CompCams cam that is intended for TPI engines with stock computer.
I've chased down and fixed all the vacuum leaks I could find.
All new or refurbished ignition components (cap, rotor, coil, wires, plugs)
The plugs all look good and clean, every cylinder looks like it is firing OK.
Clean throttle body, new Delco IAC (tried aftermarket one also, same problem).
Refurbed ECM with latest and greatest GM chip.
New throttle position sensor, adjusted correctly per manual.
Base timing is 6 deg. per the book.
Hooked the car up to WinALDL, nothing jumps out at me in the numbers. When I am hooked up, the cable puts the car in the 10K mode, so the idle is bumped up to between 900-1150 range. Idle still hunts here, the IAC counts are in the 30 to 45 range, going back and forth.

Any input would be greatly appreciated.

 

My 86 acts similar to yours and I've done all the basic stuff as you have. My next step is to repair the throttle blade shaft as there is some play in it that may be causing the problem. I've read on here that this can cause an idle problem and it makes alot of sense. I'm not saying this is your problem but it might be a thing to check. Good luck.

 

Is the car staying in closed loop at idle????

 

That's one of my targets too. There's not much play in mine, and I am trying to find a way to confirm that the part's bad before I change it. Let me know if it works out for you.

 

It's hunting around even before it has a chance to go into closed loop. If it's going in and out of closed loop at idle, what would cause that and what effect would it have?

 

I have the same problem. But when I go closed loop it stays there. I'm thinking worn throttle body shaft bearing. There is a lot of forward and back play in the shaft on the drivers side. I'm thinking new throttle body. There are a couple of places that will recondition the TB - new bearing sleaves and bore out the body to 52mm. But they cost almost as much as a stock TB. A new TB from GM was about 245.00 a couple of months ago.


[Modified by Mike263, 6:53 PM 4/30/2003]

 

Anybody have any links or contact info for TB overhaul.

 

ive got the same problem on my 88 but this is what ive done

-Swapped out the TB for a larger one, installed new IAC and the TPS is set at .64dcv
-i have a hypertech chip, but problems started long after i installed it
-checked for vaccum leaks all over the place, none found

What else could be causing this?? im thinking an EGR problem? Im gonna try putting in the factory chip to see if that changes anything.

Also my idle goes up to 1000 right after i start it, and stays there until the engine gets over 150 degrees oil temp (no specific point that ive noticed, but always over 150.) The cold start injector wire is broken, but as i understand thats a crank only circuit, so it shouldnt be affecting anything, would it?

Any thoughts?

Thanks! Milan
:cheers:

 

Sorry ToddAT we are killing your original post, so if anybody has any other ideas for him post them up please. I'll start another post on the TB bushings.

 

Have you tried resetting the IAC?

 

I haven't seen any info on it. What's the procedure?

 

There is a tech tip covering this procedure. Go to "Tools" at the top of the page and click on tech tips. Easy procedure to do.

 

Thank you! I've already been through this procedure, will go through it again to make sure that I got it right. I've have seen mention of an "idle learn" procedure that the ECM goes through, but haven't seen anything regarding this on the L98, just the LT-1s.

There's another thread on the idle subject that just came up. Someone had gone through several throttle bodies and an intake manifold before he got his running right. Lots more for me to check I guess.

 

I think ski down it answered it best in another post. Here it is!!

You said more there than you realize most likely and you are 100% correct. When tuning a car you want the IAC to have as little control over the desired idle as possible. Let me expain further. If you have the blades shut as vetracer pointed out, the idle air much come from the IAC alone. With that you will get surging etc, because the car is constantly trying to adjust, so it will windmill, then stall. If you open the blade, as you open them more and more the less the IAC is required and in control. You do want to let it have a little control, about 20 IAC steps is what I look for, but upwards of 30 is OK. With those parameters you should have a nice idle.

Here is why. In short form. If you close the blades completely, the IAC will start to windmill, as it raises in idle it will get further from the desired idle speed and it will suddely adjust by cutting air flow, if you have the blades completely closed it will cut all the air off to the engine, not realizing it. By the time it figures out that its gonna stall, it opens up fully again. By leaving the baldes open a little, even if the IAC shuts completely off, you still have enough air to maintain a min idle, hence the reason why when you set the IAC/TPS you go through the min idle setup, with the IAC closed..Make more sense now?

:cheers:

 

Hey thanks agaent 86, i was just gonna start typing...I find myself repeating myself more and more here :lol: I need to start cutting and pasting more. :D

What I described above or that agant 86 posted for me, was assuming that there are no intake leaks etc. By the looks of the IAC readings your getting you don't have any or the IAC values would be 0, in otherwords the IAC would be completely shut off, trying to slow the idle, but because of the leak it can't. HINT to people that suspect an air leak, and have access to a scanner.

I just thought of something, maybe I could read the voltage from the IAC to see what a reading of zero is, I have tons of people email that do not have scanners and complain of hunting idle, this would allow them to test it and see if the voltage corresponds to the 0 position of the IAC.........hmmmm have to add that to my list of TO-DOs. Like its not big enough already :rolleyes:

Anyways, where was I, Yes. You need to make sure everything else, PCV etc is all working properly, replace if not yet done. Then you want to go through the PROPER way to set min idle, then adjust your TPS, you can tweak the blades out a little at a time to minimize the IACs control. I try to get it down around 20-30, that way the majority of the air is coming from the TB, and not the IAC. If the reverse is occuring you will have idle problems for sure.

Hope you get it. :cheers:

 

Thanks a bunch everyone. I have a lot to look into this weekend, I'll let you know how it turns out. :grouphug:

 

OK, I think I have it all fixed now. This is what I did:

New connector on throttle position sensor, one of the pins was bad. I am pretty sure this was why the car was "cold-blooded", which just started recently.

New connector on IAC. Didn't make a difference, but the old one was kind of worn, so it couldn't hurt.

Still had the "hunting idle", so I adjusted the minimum idle speed screw in until the hunting went away. This is not the specified way of setting it, but it worked. Somebody suggested it here on the forum and it makes sense to take some of the load away from the IAC. Checking it on the computer, the IAC is staying pretty much in one place, once it sets the speed. None of the jacking back and forth it was doing.

Took it out for a spin :steering:, it ran great and still idled the way I like it. :hurray:

I hope this helps somebody else out there too. :seeya

 

Thats good news!! I have to redo mine today. I hope this corrects mine also. :cheers: