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
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!