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Hey guys, 05 GXP is shuddering terribly when shifting from 2nd to 3rd gear. I mean really bad! I have had this problem before but not as severe. Replies to an earlier post by me, long ago, suggested a power problem. The car has been repaired so many times, I honestly don't remember what fixed it, but I am sure that you guys were correct. The check engine light is not on. I am not sure about any codes but the last time it was in the shop, recently, it had no codes except for a vacum line that was loose. The problem is getting exceedingly worse, so I am surprised that I don't have a dummy light flashing! I am going to change the plugs myself and was wondering if I can have the coil packs tested anywhere while they are off of the car. Since I am changing the plugs already, I would like to just pull the coil packs and have them tested somewhere. I am not too cheap for a good mechanic, just too broke for one! I am no mechanic, but I know how to use a wrench. I would rank my capabilities somewhere between "intermediate" and "very annoying to those who know more". Any suggestions would be much appreciated.

Have your checked your front transmission mount? They often cause very perceptible shifting as the powertrain rocks around, and are also a common failure point for all 2000+ Bonnevilles.

Thanks for replying! The trans mount was replaced a few months back but it wouldn't be a stretch to suppose that it may have broken again. I'll check it out. Merry Christmas!

Dealer mount or aftermarket?

I broke an aftermarket (Anchor) in 2 months.

mblaze wrote:Hey guys, 05 GXP is shuddering terribly when shifting from 2nd to 3rd gear. I mean really bad! I have had this problem before but not as severe. Replies to an earlier post by me, long ago, suggested a power problem. The car has been repaired so many times, I honestly don't remember what fixed it, but I am sure that you guys were correct. The check engine light is not on. I am not sure about any codes but the last time it was in the shop, recently, it had no codes except for a vacum line that was loose. The problem is getting exceedingly worse, so I am surprised that I don't have a dummy light flashing! I am going to change the plugs myself and was wondering if I can have the coil packs tested anywhere while they are off of the car. Since I am changing the plugs already, I would like to just pull the coil packs and have them tested somewhere. I am not too cheap for a good mechanic, just too broke for one! I am no mechanic, but I know how to use a wrench. I would rank my capabilities somewhere between "intermediate" and "very annoying to those who know more". Any suggestions would be much appreciated.

Anything with the coil packs would be constant and consistent and happening all the time. Idle, drive, reverse, and any gear in drive. It is based on "timing" and commanded continually by the PCM to do it's job. If it's a hard shift, especially between 2 - 3, I'd look into the trans mount as previously suggested.

The ignition coils in this car, while not fallable, are pretty foolproof. They're a combination C-N-P design (coil near plug), and act as an ignition module being all in one coil and path to the plug. Half of the unit is mounting safe and secure inside the head and protected from the elements. The only electrical connection to it, is the PCM feed to the individual coil module.

Thanks to you both. How many engine mounts, and transmission mounts are there each? 2 engine, one transmission? I need to do my homework before I take it in again. It's been snapping motor mounts on a pretty regular basis. Makes me wonder if a broken trans mount would contribute to this? It's been to the local dealer a million times and they are just piss poor...Pay them $$, get a few miles down the road, turn right around and go back, leave it for a day or 2 more, pay more $$, repeat. No exaggeration. This is not a major city, and I really think that mine is the only GXP that they ever see! I tried an indy, he means well but... I just need to point somebody in the right direction.

The one being referred to here is only one, down in front, mounted to the front crossmember of the subframe.

If you look up the hockey puck mod for non-GXP Bonneville's, you will be able to find pictures of what we're referring to. The GXP doesn't used the exact same mount as the V6 cars, but it's still in the same location, same concept. This mount always breaks and causes the hard thunking/clunking/roughness experienced during gear shifts as the engine torques under load and shifts as the transmission itself shifts.

Here's the one in question:

Thanks! I will let you know how it turns out.

Hi did you ever figure out that vibration? Mine does the same.

Put in new spark plugs, engine mount, and tranny mount (both mounts were broken again.) It helped but the shudder was still there. Turned out that it was the TCU. The bad news is that the tranny has to be partially dropped to access it, so the labor cost ain't cheap. Shifts nice and smooth now.

Thanks for the reply, your discription is almost like you were driving mine, is the TCU internal to the transmission? And if you dont mind me asking how much did the repair cost. Thanks for your time!