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View Full Version : Car wants to sometimes stall at idle, runs fine under throttle.



The_Ghost
07-12-2006, 02:53 AM
title says it.

Otherwise healthy 3.8SC. I make full boost, and when it's not doing the weird shudder/stall thing it likes to idle at 600-700, which is perfect.
5spd Trans is in good shape, but shifter will need to be rebuilt soon. I dont think this is the problem at all.

Here's what it does. I'll be at a stop light in neutral idling just fine at 600-700 between 18-22in/hg of vac and then it sputters once and then the idle starts dipping down, down down until it stalls unless I catch it with a quick blip of the throttle and will stay running as long as I play with the throttle a little bit to bring the rpms up to 850-1000. Apparently the car has relatively new plugs (less than 5k ago) and was probably rebuilt at some point. I also cleaned the MAF a week or so ago with MAF cleaner, and got all the little filaments of the MAF clean. It is not behaving like it's missing cylinders, all the power is still right there as soon as I let out the clutch and start driving, it's just fine. It's only at idle that it does this. I also have had the computer try to relearn idle repeatedly I think, since I've had the battery disconnected as well as go dead several times.

What could the causes be?

Thus far my theories are:
DIS is dirty and needs new dielectric grease and a good cleaning?
Idle Air Control valve is failing or stuck? (not a common problem, but hey)
I improperly taught the PCM idle? What is the correct procedure to relearn idle?

It's a very weird problem, but it comes and goes sporadically.

Thanks for your input!
-Ghost

Quik95SC
07-12-2006, 03:30 PM
These things have been no to go out with no warning. Do you know how old the IAC is? It may very well not be completely gone yet, but working its way there. I have learned that this silly little valve is the root to a lot of rough, non, high idle problems considering everything else is normal.

Just a thought, don't rule it out until you know for sure.

Smitty

Sweet90SC
07-12-2006, 03:39 PM
I had a similar problem with my 89 after I took the DIS off once....I cleaned it up good and regreased it then reinstalled it and the problem went away....mine did it only part of the time too...it would just go idle low then stall. I have a 5 spd and that fixed it...might be worth a try..

Rick

shadow357x2
07-12-2006, 03:42 PM
I agree with Smitty on that one, I would start with the IAC and a good throttle body cleaning.
I had a simular problem with one of mine awhile back and it ended up being the bmap sensor (guess I called it right) the little rectangular thingy on the passenger side of the fire will mounted to the wiper cowl. That was my cure BUT I cant say thats yours.



S

LJGriggs
07-12-2006, 03:43 PM
This is most likely caused by bad engine mounts. Unless you have changed the stock units out with solid mounts, the rubber cracks and the fluid leaks out and they collapse. For some reason, don't ask me why, this will cause the engine to stumble while idling (at traffic lights). I've heard it has something to do with vibrations affecting the knock sensor, but I can't swear to that.

The_Ghost
07-12-2006, 11:37 PM
well, thought I was getting full boost but apparently not.
The collar nuts on the IC pipes were loose. It now idles normally and just got a whole lot quicker :eek:

Quik95SC
07-13-2006, 01:52 PM
Like I said in my previous post "if all other things are normal" Obviously that was not the case. Glad you found it with only minor delays and no money output.

Smitty