Thread: Engine rev's up and down at stop/idle when warm. (Long Desc)

Hello everyone!
I'm new here, not even sure i'm posting in the correct section for this but hey, gotta start somewhere i guess!
So, the problem i'm having with my car (1999 Grand Prix SE 3.8L V6 "NA" 233,xxxmiles) is a bit of an odd one and i need help narrowing down causes.
This is going to be quite a long explanation with as much detail as i can provide.
So if you're not up for the long read, feel free to move along.

When i purchased this car back in February of 2016, it had what sounded like a loud asf rod knock.
But i wasn't COMPLETELY convinced thats what it was. So i decided to purchase the car for $500 knowing
that a (New used) motor is relatively cheap for these things.
I loaded it on a trailer and brought it home.
Short story is that while preparing to pull the engine out i was in the front passenger wheel well and noticed the
Harmonic Balancer was fubar, AKA the rubber was crack, split and dry rotted asf on it.
So i went and picked up a new one, installed it and PRESTO, the "Rod Knock" sound was completely gone!
So, later that day i was preparing to take the car on its maiden voyage (Test Drive) and so i was letting the engine warm up for a bit first, and once about 10mins rolled around, it started pluming smoke out of the tail pipe.
So i decided to delay her maiden voyage awhile longer.
After much research and talking to mechanics, i decided it was the Lower Intake Manifold Gasket that was the issue.
And boy was i right, i replaced the gasket with a new way better quality Metal Gasket and once again PRESTO, problem solved!
All seemed well at first, but the issue in the title started right away on her first drive.
But at the time i figured maybe it had something to do with the cars Drive Cycle after being unplugged for so long.
So i drove the car around 250miles back home, as i was at my dads in another state, fixing the car.
The 3hr drive went smooth as butter. But the very NEXT day after getting home, a new problem arose.
Anytime i'd let off the gas to slow to a stop, the engine would stall. But it would always start right back up no problem
whatsoever.
After a little while of throwing parts at the car to try to solve it (Fuel Pump, Spark Plugs, IAC, Brand New Upper Intake Plenum) i finally hit the nail on the head with replacing the Throttle Position Sensor and once again PRESTO, problem solved!
So now i have a pretty solid running and driving car. Except for one pretty pressing issue which has progressively gotten worse. The Idle/Stopping issue mentioned in the title.
When i first start the car, its business as usual. The car starts instantly the split second i hit the ignition. Car starts a little high in the revs and slowly lowers (Business as usual) but once the car idles for around 5mins or drives long enough to get warm/hot, it starts revving up and down like crazy, up to i've seen around 2500rpms and down so low that it stalls the engine. When the revving starts, its gradual. At first you barely can notice the revving, then as the next minute or so goes by the revs get higher, then lower, higher, then lower, eventually causing it to stall.
Now, this happens no matter what gear i'm in (P,R,N,D,3,2,1) it literally doesn't matter.
BUT once i put it in gear and actually start moving, the problem stops, until i stop again.
Also theres no Check Engine Light!

Aaand there you have it!
Thats the problem i'm struggling so much to diagnose!
ANY help is MUCH appreciated!
Thank you for your time!

Originally Posted by Scottydoggs

spray the top of the engine with carb cleaner, if the idle jumps up while spraying you found the leak area. im wiling to bet you broke a vac line during the lim job. large vac leak will cause a 500 rpm surge and stalling.

What about the CPS? Or would that be a constant problem, not intermittent like he's getting?

Check the large vac line that goes from the booster into the UIM. The vac tree that attaches to the UIM can leak and cause this issue. I ended up rtv'ing my tree on my old UIM to fix it.

Ya thats why i replaced the UIM before. It was slightly warped and causing a vacuum leak around the bottom.
I've done some rudimentary Carb spraying for vac leaks, but ill be more meticulous next time i check.
How can i check if the O2 Sensor is bad? Don't i need special wrenches to remove it? And can i clean it?

Ok so, that Y off the throttle body was fubar on my car when i got it. So my dad and i got creative and fashioned our own. Its well sealed and no leaks, but its not the same hard plastic/rubber the original was, idk if thats causing the issue but i personally don't think it is.
Is there a specific reason i should use AC Delco?

Alriiight, thank you all so much for your help! When i return home ill be running through all these checks!
I'm not home to check it atm. Having to live with my dad for awhile because he had surgery and i'm here to take care of him till he recovers.
When i do check or have my bro check for me, ill update you all on the issue!

So, i have a couple video clips for you guys.
One is a nice view of the bad old paper, Lower Intake Manifold Gaskets. (I figured some of you might be interested in seeing them)
The other is a clip of the idle problem we're trying to solve.
Clip One: https://drive.google.com/open?id=0B4...lM3LS0wZEJsYW8

The idle problem video is from when it wasn't nearly as bad. When it first started doing it, it was intermittent and wouldn't always do it. Now it does it literally every time once the engine starts to warm up a bit and won't stop until i either start moving, or until the idle dips enough to stall the engine.
Clip Two: https://drive.google.com/open?id=0B4...DlRSEp0QjZnQ1E

Oh jeez i hope thats not what it is... that gets expensiiive.

I actually have the same issue with my car, except it is an 04 Comp G, but it does this very randomly, mostly when in Drive. Also get a low voltage light on DIC. Guesss il try the carb cleaner trick and go from there, any other suggestions?

Have you cleaned the throttle body yet ? And if you want to check the MAF sensor, tap on it with a hammer handle and watch to see if the square wave jumps around on the scope. You can get a brand new Delphi from Amazon way cheaper than the local parts store's off-brands.

I had this intermittent stalling/idle problem with my 2000 GT, and it wound up being my MAF sensor. When I put a Snap On scanner on it, it was putting out literally 0.00 Hz. You can try maybe unplugging it, and letting the car run on Speed Density, and see what happens. (Note, this WILL cause a check engine light, but it is just a test, and you can reset the light.) Just an experience I had. Mine would stall, and start right back up everytime just yours, and my fuel pump tested good, and my FPR.

Since my '03 is doing this, I went and got some carb cleaner like was previously suggested. Sprayed everywhere, nothing. I also picked up some CRC Sensor Kleen to clean the MAF, except for some reason, I have some weird torx screws holding my MAF in place. They have a pin looking thing in the middle of the screw head, so a normal T20 torx doesn't fit. . What kinda tool do I need for this?

Also, about how many lb/min should my MAF be reading at idle? The crappy scan tool I have has limited params for this platform, so I can't see a hz reading for the MAF, just lb/min. Mine is reading between 0.5 and 0.7 at idle.