1992 Park Ave. Ultra unsteady idle

I thought I had taken care of all my engine problems in my 1992 Park Avenue Ultra; it felt great to have its performance restored. But lately there's been an issue with the idle on this car that I can't seem to trace down.

At times the car has this kind of hunting idle where it will bounce between maybe 750-950 rpm. It's just not smooth; it doesn't seem like such a big deal, but the bounce rate is rather quick, maybe every 1/2 second. It sort of irritating to sit and watch the tach bounce up and down while listening to the engine sort of chug-chug-chug.

Here a few more preliminaries, I did have troubles with performance before this, engine stumble under hard excelleration, and at that time replaced fuel filter, spark plugs and cables, coil packs, O2 sensor all favoring Delco products. Now engine performance and accleleration is very good except for the idle.

In troubleshooting this idle issue I first looked, again, at fuel pressure and vacuum which are both good. Then I latched onto the idea of it being the Idle Air Control unit and went through the process of pulling the Throttle Body for a good cleaning along with trying a different IAC from my local U-Pull-It. So far no improvement.

I wound up replacing the Throttle Position Sensor which tested with a flat spot near WOT, and after adjusting the new sensor for the right closed-throttle value, .4 volts, things have been a bit better. As you can imagine, I've pulled the battery cable a few times and reset the PCM and it goes through the relearning process with idle. When I turn on the AC the idle goes up but comes down to around 1k.

Next, recently I'd been noticing my temperature gauge reading low probably because I replaced the radiator due to a leak in the plastic caps. I replaced the thermostat and took a look at the temperature sender for the PCM (under the throttle body). I was going to replace it, but an ohm test and the fact I had picked up the wrong replacement at Advance Discount on a Sunday afternoon lead me to just cleaning the existing sensor up and reinstalling it. Now my temperature gauge reads 180 on the highway.

So, when I first start up the car idle goes up to about 1500 rpm then drops to around 900, but after driving a short distance it sometimes drops way down to 500-600 almost stalling out. Then it picks up and may start fluttering between 750-950 rpm. This is all without me touching the gas pedal.

After warming up, the idle can be fine in traffic. But at other times this strobing idle comes back.

Whew, I think I covered it all. I don't have a scanner and there are no codes set. Ideas? BTW, any coments on the Actron SuperAutoScanner (CP9150?)....can it do all the Tech1 can? Oh yes, I posted a similar thread on another Buick forum because this one was doing database management yesterday.

Thanks!!!

Imbedded links were removed for you, as well as your duplicate post!

:wtf:

Weird! Why do you have several links to another auto forum in your post?

You're welcome to post on AF although you have one at the other forum but please remove or edit out the links.

WTF is Right! I'm sorry, but this is what I got when I previewed my post. It must be a "feature" of the forum. I did not imbed links to other forums. Earlier I tried to edit out the links, but it got even more messed up. Ugh, what a PITA! I just tried to edit the original post and got no where. I will strip the junk they stuck on top of my text and repost. Hopefully turning off "automatically parse links in text" will stop the link. Again, sorry!

Not a problem and I understand. I've seen this happen before. I noticed the original post at the other auto forum screwed with or parsed the links over there for some reason and if you do a copy and then paste here in a post all the embedded stuff carries over. To cut down on confusion when our members look at the first post of the thread I edited your Post #1 to match your corrected post as close as possible. The corrected/updated post has been deleted.

A couple of things here..

First these cars are intensely picky about the MAF filaments... I will suggest a can or CRC MAF Cleaner... Also if you have not done so you may want to check for some vac leaks at the top of the plenum... At times those lines can degrade and leak.. This can cause some idle issues..

It may also be a good idea to yank the Throttle Body and clean it, paying close attention to the IAC Channel..

Hello Jrs3800,
I did use CRC cleaner on the MAF filaments when I was troubleshooting the acceleration problem I had a couple of months ago. At that time I also found out you just can't swap out the MAF with one that just looks the same. And yes, the filaments are clean. Maybe I need to lightly go over them with a Q-Tip saturated in the cleaner.

A week or two ago, I did pull the Throttle Body and used just about a can of CRC TB cleaner on it cleaning it up and tried to get the whole thing as well as the IAC channel as clean a possible as you suggested.

I wish I had a scan tool. The one I've considered getting is the Actron SuperAutoScanner, CP9150? But, I would like to know if it is capable of the same functions as the mighty TECH1. Any suggestions?

It's probably worth going over these areas again. About the only stuff I haven't checked are the fuel injectors. But the car really moves when you mash on the accelerator, and you know, I have to check it afer each fill-up.

You mentioned the plenum, would that be the vacuum manifold sitting on top of the supercharger? I'll check them a little closer. I read a post about using, I think, a propane torch to hunt vac leaks. I can't quite figure that one out. Mabye I got that mixed up with some other procedure? I have replaced some of the vac tubing up there; I'll look closer. But I had a vac gauge connected at various points and it's very steady even when it's doing this chugging idle.

Thanks again!!

I read a post about using, I think, a propane torch to hunt vac leaks. I can't quite figure that one out. Mabye I got that mixed up with some other procedure? The propane test is a very reliable method for testing for vacuum leaks. Use the torch (unlit, of course) and open the valve to allow propane vapors to escape. With the engine idling, point the torch head around all areas that could develop a vacuum leak. If the engine should ingest propane through a vacuum leak, you will notice an instant increase in idle speed. I have been using this method for over 40 years and it is much safer than spraying flammable liquids all over the exterior of a hot engine!

UN-LIT.....(the light goes on).....That's a great idea and I can't wait to try it. Thanks!

Remember that phrase, "a giant sucking sound?" That maybe a clue as to what's been going on with my idle troubles.
I went on a trip this weekend and stopped for gas at Sam's Club (OMG....$3.92/g for high test!)

Well, I did not hear that in-rush of air when I cracked open the gas cap. And, come to think of it, I haven't noticed it for a while now. Something clicked; I remembered my cousin has a Dakota pickup that will raise a code if you don't have the cap screwed down all the way.

So, I zipped down to Advance/Discount and got a replacement cap and now I'm cautiously optimistic. It maybe just the placebo effect, but the idle does seem better, almost steady.

I'm hoping the daily drive tomorrow will prove that I have found the problem, finally. I still am going to try that propane trick....that is SO cool!

I'm just wrapping this one up, finally. It did go off into a couple of other threads. But this is where it started.

It turned out I had an air leak in the gasket between the supercharger and the lower intake manifold. I did happen to stumble on to it not using propane, but with carb cleaner. In a desperate effort to locate the vacuum leaks I started spraying carb cleaner around some points on the engine and hit the spot just under the nose cone where the leak was located. The rough engine idle straightened up like magic!

I then searched around for supercharger gasket troubles and found a few responses on a Bonneville forum that said it's a rather common problem for the gasket to fail, but they didn't go into the unsteady/rough idle condition that gasket failure will cause.

I'm not sure if I helped cause the trouble by high-pressure washing the engine at some point. But, when I did pull the SC the gasket was pretty messed up.

The idle is now great and I can't wait to get it out on the Interstate tomorrow!