Thread: 2008 GXP Grand Prix Transmission Failure

Yesterday, while driving about 40 mph, when I tried to accelerate, the trans acted like it was in neutral--no drive, no reverse. There was no noise--it acted like I shifted to neutral. Car has 166,000 miles, but I've only had it two years, so don't know the previous history, but suspect the trans was rebuilt at least once.

Had the car towed to our local transmission shop. Tech says $2,300 for rebuild with a two year, 24,000 mile warranty. He has done others like this and suspects the converter failed. In his opinion, the small torque converter is the weak point with these GXP transmissions. They have an Impala there right now with the identical drive train and similar failure.

I plan to go ahead with the rebuild, but would like to hear if any of you have info that would help prevent this from recurring.

Originally Posted by bocoogto

Yesterday, while driving about 40 mph, when I tried to accelerate, the trans acted like it was in neutral--no drive, no reverse. There was no noise--it acted like I shifted to neutral. Car has 166,000 miles, but I've only had it two years, so don't know the previous history, but suspect the trans was rebuilt at least once.

Had the car towed to our local transmission shop. Tech says $2,300 for rebuild with a two year, 24,000 mile warranty. He has done others like this and suspects the converter failed. In his opinion, the small torque converter is the weak point with these GXP transmissions. They have an Impala there right now with the identical drive train and similar failure.

I plan to go ahead with the rebuild, but would like to hear if any of you have info that would help prevent this from recurring.

Get that quote in writing, that seems very low. Also ask about the components they will be using for the rebuild, as it was already stated there are some parts that would be a good upgrade to use. Any good reputable shop knows all about the shortcomings of these transmissions, make sure you ask about any options before they start. Good luck, post back your outcome.

As I stated, this shop has done quite a few of these transmissions on Impalas and Pontiac Grand Prix's. It's a local reputable shop, so I have no qualms about the quoted price or warranty. Their estimate on completion is two weeks out due to how busy they are.

Technician says the weak point is the torque converter and a larger one is not possible because of the bell housing design. The converter has a lock-up clutch, which may be what failed.

I'm looking for specific suggestions on what components are available that would improve reliability. I'm also hoping this rebuild gets rid of the nasty shudder that shakes the car every time I stop in "Drive."

That shudder may be a motor mount, and should be changed if accessible when getting the transmission rebuilt. Sonnax , Superior Transmission Parts, or TransGo all make upgraded components. The shop will already know about these but its up to you to ask for any options.

Talked to technician today. He ordered the kit from TransGo. I called TransGo and they said they have only one kit for this transmission. Technician also said he ordered a premium converter. His experience with these transmissions is that failure to use premium rebuild components results in too many failures, which cost the shop money honoring the warranty.

Technician said he ordered the converter and all other parts from TransGo, including their shift kit. Will be interesting.

Since I won't be drag racing, road racing, or pulling a trailer, why a transmission cooler? I've never seen the transmission temperature above 190 degrees.

The LS4 cars suffer from engine compartment heat soak as everything is so tight. Different geographic location may see more favorable transmission temps if you live in areas that don't see high heat ranges,but it is good money spent as these cars should have had these installed factory installed and for sure heat has played a big part in the overwhelming transmission failures in these cars. It really should be first thing added , and installation is very straight forward if you decide to do it yourself. I really never thought mine ran hot either, usually right around 190 until I got stuck in a traffic jam and watched it climb to 240+ . Consider it an investment in your new transmission.

Transmission rebuild was finished this past Monday. Failure was caused by a sheared pump drive shaft through the center of the converter. Repair cost was $2,300 with a 2 year, 24,000 mile warranty. The transmission had been replaced in the past with a GM rebuild--the sticker was still on the torque converter.

The shudder I previously posted was caused by a broken lower motor mount, which was replaced while the trans was out. This eliminated the downshift shudder AND an annoying clumking whenever a bump in the road was hit. I though the struts were bad, but since the motor mount replacement, the car rides like a new one. 166,000 miles on original struts that still work well is amazing!

Good deal, and a nice price. That's a sharp looking car, don't see a lot of red in my area. Good luck with it.

Changing the fluid evert 25k. Helps as well.


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I appreciate the comments about fluid temperature and changing fluid. But, note that my trans failed due to a sheared pump drive shaft--no relation to fluikd condition or temperature. My trans did have fresh synthetic ATF that was not discolored nor did it smell burned.

Thanks for the suggestions.