Thread: 2000 Grand Prix GT Idle and Hesitation Issues

So my 00 Grand Prix has had small idle and hesitation issues for longest time. I loaded the parts cannon many times, no change. I finally got my hands on a serious scan tool. Messing around with the scan tool and the car I have discovered the following.

Idle adjusts constantly, it small amounts but noticeable on the tachometer, like it can't figure out how to get where it wants. Pulling out you get a slight hesitation when getting on the gas.

When you press the accelerator very lightly (idle or in gear), it still shows 0 as throttle position on the scan tool. The car will rev up a little like it should, and then the idle will adjust down some (like the computer see's the idle increase and tries to bring it back down). If I let off the gas while doing this, the RPM's will drop way down, and it will sputter, almost stall and catch itself (it did stall once or twice while doing this).

When the car is off key on, throttle position on scanner shows 0% to 100% evenly and accurately, even when I barely touch the pedal. Engine on, it doesn't register until I get in the pedal a little more.

On the scanner during the same night (after I had shut it off once) I also had a time where it began reading 1% and 2% throttle position with no input. I went out and made sure to push the throttle closed when it read out these results, but that made no change on throttle postion, neither if i revved the car, and the idle was a little higher then usual. I turned it off again, restarted and back at 0%, and still not seeing my light throttle inputs
Car smelled a little like gas when I was testing the throttle by feathering it and letting off.
All other live data I got seemed normal from what knowledge I have on that stuff.

The following had been done over the years I've had it, without change. Throttle Position Sensor, LIM UIM Gaskets and UIM manifold, MAF sensor, PCV Valve, IAT sensor, Throttle body gasket and cleaning, All fuses checked, battery terminals cleaned.
It has a high pitch turbo like noise it makes after revving over 3000 RPM, but I checked and checked and checked for Vac leaks, and even smoke tested it, and found nothing.

Not sure what to do anymore! Should I go get a whole throttle body and swap it over? Any idea's or area's to look at would be appreciated. Maybe a top swap would fix this...

I had the same high pitched sound and it turned out to be the hose coming from the booster to the back of the UIM. The fitting on the UIM was leaking ever so slightly and throwing everything off. Replaced the tree fitting and orings on it and threw a light coat of gasket sealer around it before install and once it snapped into place solidly on the UIM the issue went away. Seemed to be worse due to the aftermarket UIM as I never had that issue on the OEM one.

Also, did you use a stock MAF or aftermarket? These cars are known to be picky with aftermarket stuff. I usually just grab one from a JY if need be. Plenty of cars came with the 3800 as long as you get the right part# which should be stamped on the top of it.

Another thought is to check the wiring harness to the MAF and TPS. They get cooked by engine heat over the years and get brittle.

Thanks for the reply. MAF sensor was a JY unit, and made no change when I switched it.
The issues were here before AND after I did the whole intake gasket job. I used metal felpro for the LIM gaskets, and a ATP brand UIM gasket kit with Upper Manifold. I'll check the tree out, maybe something is messed up with it. Wiring looks good, but I could trace it farther to make sure.

Update. The vacuum tree did seem kinda wobbley. Replaced the seal with a slightly thinker one. It seems more solid, and it seems to be sealed, unless I wiggle it pretty hard then it I can hear it leak. This issue was here with the old intake and vaccum tree, so I doubt this is the problem. Tonight's fixing made no difference in the way the car runs.

I forgot to mention in original post, a little bit a ago, I purposely unhooked a small vacuum line and the car seemed to run slightly better, which is strange.

Which small vac line did you disconnect? Can you post a pic of where it connected?

It was from the purge solenoid to the TB vaccum port. I tried taking off the solenoid and blowing through it, can't blow through it, so I think it is good/not stuck open.

Have you checked the fuel pressure regulator? Pull the line off of it and see if any fuel leaks from it. Might also want to check fuel pressure as well. Could be a bad reg.

I actually shotgun replaced the fuel regulator already lol, when i did intake gaskets. Fuel pressure is good, if I recall it is around 45-49 PSI? Fuel filter was replaced also.

Worked on it tonight, checked all spark plugs, they all look good.

I also found out the car has a air pump in the front drivers wheel well. Is that California emissions? I took the hose off and plugged it just to see what would happen. No change.

I don't man, this still sounds like a vac leak to me. When you replaced the PCV valve did you install BOTH o-rings in it? Maybe the MAP sensor is borked? I know they like to break when removing them to do the PCV valve sometimes.

All looked good when I put back together, I checked again tonight, Both o-rings on, map sensor looks good, pcv valve good. The only thing I can think of anymore is it is some sort of computer issue. I can't find any other thing wrong. It's a 50k mile car.

Have you replaced the ICM or any of the coils? When my ICM started to die it would fluctuate the RPM's. As it got worse it would rev up and then idle down and almost die while trying to idle. Took me forever to find it. Found that I had a cracked coil that took the ICM with it. 2nd time I've had that happen. FYI, if you go the ICM or coil route get a factory one from a JY. Aftermarket on these is junk. I've been running JY ones with no issues on both my 3800's. Factory coils on these are best. No need for the red or yellow ones.

Mileage may not matter it's the age at this point. My GP just turned 490K today and it's on at least it's 2nd ICM/coil pack setup.

I have not replaced any coils or ICM before.

I will check the ICM and coils. I have a JY unit from a chevy venture laying around. I replaced one on a chevy venture that had issues, but the ICM was never the problem, so then I kept the extra. Rock auto says same part number. I'll take a look and switch em, can't hurt. I guess I should inspect coil packs for anything suspicious looking.

I cleaned battery terminals, they were fairly clean when I did that too.

Here are some links to explain how to check the coil packs with a multimeter. The first time I had this issue it was obvious as when I clean off the body of all the coils there was a crack in one of them. 2nd time no cracks on them but one of them tested bad.

Testing coils & spark plug wires - GM Forum - Buick, Cadillac, Olds, GMC & Pontiac chat


Part 1 -How to Test Ignition Coil Pack -Misfire Troubleshooting Tests (GM 3.8L)

I visually checked them 2 days ago, ICM and all coils look fine, The venture ICM is different I realized. It would have been the same if this was a 3.1 GP. Terminal number five had corrosion/rust on it. Cleaned up good, put all back together, no change.
Oddly enough, my 00 Regal Supercharged I did spark plugs and wires on later that same day, ALSO had rust and corrosion on only terminal number 5. Thats a bit strange. That car runs and drives flawlessly.
I will have to do the more in depth test and see if I find anything.

Wait, you have another 3800 powered car?? The ICM/coils are the same between the N/A cars and the S/C cars. You could swap the ICM/coils from the Regal to the GP and see if it makes any difference before throwing more money at it.

And FYI, my ICM "looked fine" even after the coil messed it up. There was something internal that got jacked up on it.

Tonight I pulled the fuel rail and checked for leaky injectors. They're fine. I noticed when I had the key on, the IAC motor was making noise for some reason. I robbed one off the buick, no noise. So i started it and yet, no change in the way it runs. I unplugged the IAC motor completely and it ran better, drove around no hesitation, and when I lightly press the gas and let off, no stalling. Idle is generally higher, and still not steady with unplugged. Not really what I call progress. Let me know if this means anything to you.

I'll swap the ICM and Coils tomorrow or friday and see what happens there.

Swapped all coil and ICM from regal to pontiac. Unfortunately, no change in the pontiac, no change in the regal.

Well, at least we know both are good, so that's a plus. Going back to your original post, have you scanned the Buick to see how the throttle position reads on it? I'm beginning to wonder if the TB itself is screwed up somehow. When you cleaned it up, did you clean out the hole for the IAC and whatnot or just the butterfly?

Also, when you did the LIM/UIM did you clean up the injector holes in the LIM and replace all the injector o-rings (rail and LIM side both)? I know you said you smoke tested it but thought I'd ask.

Great idea. I'll check the buick and see how it reads. When I cleaned, I don't think I took of the IAC. When i swapped the IAC from the buick, I looked in where it goes and it looked super clean. I did clean up the injector holes, and oiled the o-rings some, did not replace, seemed like they were in good shape.