1999 Grand Prix GT shifts funny after long drive periods

I have a 1999 Grand Prix GT which I really like a lot. I only have one problem, and it kind of bothers me. For the first hour or so of continuos driving, the car shifts just fine. After that initial hour, when I accelerate through the gears, each time the car shifts into a gear it will shudder or rumble or vibrate. Is there something wrong with the transmission? The car only has 50k miles on it. I hope you can help.

There is a chance you have a trans problem but you could also have a ignition problem or fuel problem. Sometimes a engine miss will show up during a shift. Take it to the shop and have it check out.

I have a 1999 Grand Prix GT which I really like a lot. I only have one problem, and it kind of bothers me. For the first hour or so of continuos driving, the car shifts just fine. After that initial hour, when I accelerate through the gears, each time the car shifts into a gear it will shudder or rumble or vibrate. Is there something wrong with the transmission? The car only has 50k miles on it. I hope you can help.


I have a 2000 grand prix gt, which I think has the same transmission. I am finally getting it fixed this week. I had three different places look at it. All three came up with the same code p1811. I am getting a valve body rebuild. My car is doing the same thing as yours. I called GM which was no help. I filed a complaint with nhtsa.gov. Gm has updated the parts for this trans since it came out. I know about four people who have had the same problem. One place said I would need a complete rebuild. After all my research I think a valve body repair is all I need. Which is gonna be about $1000.00. I hope you have better luck.

From personal memory I don't think you need a valve body rebuild but you do need the PCS soleniod replaced.

GM does have service bulletins about this problem - have your mechanic read them before doing any work. Flatrater is correct about it being an issue with the PCS. The repair also includes installing a stainless steel sleeve into the PCS piston's cylinder.

I have a 99 GT. After about an hour of city driving my trans out of nowhere will shift really hard. It seems to clunk into gear, and shake the whole car, like I ran over a speed bump. The strange thing is that I can turn the car off for a minute and start driving again and the problem goes away for another hour. Any advice? Aamco said it's probably the clutch plates and if I drive the car any longer I can burn the tranny. Another guy offered to pour stabilizer stuff into it and said the problem may go away. Will the stabilizer do more harm than good if i have to re-build it later? Thanks

I also have a 99 Grand Prix and the same things are happening with the Trac Off light and the stalling. I changed the fuel filter and no change. Later took it to a dealer and they checked the computer, it said the crank went out of sync twice?? He said to keep an eye on it, but not to do anything right now. So far my car only does this about once a week.

Most likely your tranny is gettin hot. Get it looked at ASAP if not you could lose 1st gear or maybe reverse.

also, how often are you checking your tranny fluid?

I agree with pawn. If your tranny is around 50k. Replace oil, filter and pan gasket. Install a tranny oil cooler(external, cause it already has one inside the radiator). Also check your fan motor and flush radiator, since this is a city problem looks like your cooling system may starting to fail. If you solve the cooling system problem soon enough your tranny and engine will last a lot longer...
Overheating is the main cause if costly reapairs like blown gaskets, failing trannies and big stuff like this

I've heard alot of people having alot of problems with the 99-01 Grand Prix Tranny. It kind of suprises me because i have no problem with mine and i run the hell out of it.

I bought my 99 gt with 26,000 miles on it. Now it has 78,000 miles on it and has had two rebuilds and a brand new one straight out of the crate. Yeah I'd say I've had some problems with it.

I bought my 99 gt with 26,000 miles on it. Now it has 78,000 miles on it and has had two rebuilds and a brand new one straight out of the crate. Yeah I'd say I've had some problems with it.

How did it blow up in the first place?

Well the first one was from me racing it and using the the auto stick instead of just leaving it in drive. So they rebuilt it, and about two months later it started shifting rough all the time but only out of first gear. So they tore it apart again and replaced all the clutches and about a or two later it would shift out of first gear, so they replaced it with one from the factory. And it all cost me only $50.00, gotta love aftermarket warranties.

Well the first one was from me racing it and using the the auto stick instead of just leaving it in drive.

How long did it take for it to go when you were doing that? I do it all the time to mine and have no problems yet. My Grand Prix has 72,000 miles on it.

it took about 10,000 miles or so, but I was practically driving with it instead of the in drive

so you probably noticed it slipping soon.