I'm trying to track down an issue with my '04 Cooper that I've had since I got it. It's a very noticeable jolt when decelerating in gear. I'll be in third gear, 3k RPMs, then take my foot completely off the accelerator, start breaking, and when the RPMs hit around 1500, the car jolts like there's some play in the drivetrain.

I thought this might be a faulty dual mass flywheel, but I just replaced the clutch and flywheel, and the problem has not gone away.

I just noticed now when jacking the car up on one side to change tires, if I grab the front wheel, I can rotate it back and forth with maybe 1/2" of play, despite the transmission being in gear and shouldn't rotate.

Don't believe it is the drive axles going out, as the play is the same on both sides and I can visually see they're rotating at the transmission end as well.

So that leaves me to believe something in the transmission is going out... Maybe the differential? It's the 6speed, which I've read is pretty bulletproof, but things can always break. Anyone experience anything like this? Or have some tips on how I can narrow down what's causing the issue? Thanks!

 

I doubt it's the transmission.

I would first check be the motor mounts and then the suspension. If you haven't changed the lower control arm bushings yet they are probably bad.

When you moved the tires were you turning it clockwise or pushing it side to side? There's always going to be some play if your are just trying to spin it.

If there is play while applying pressure that would indicate a bad wheel bearing or a ball joint/tie rod issue. Usually if there is a problem you can see which part is bad.

 

Definitely not ball joints or control arm bushings or wheel bearings, I replaced all of those while I was replacing the clutch. It's not side to side play, that would be something in the suspension, instead it's when I rotate the wheel clockwise/counter clockwise. I realize there's supposed to be some play, but I believe this is excessive.

I'm suspicious that it's the motor mounts, as I inspected those when I was doing the clutch and they didn't seem bad at all. Especially the lower motor mount, which seems it would have the most impact on rocking back and forth, it seemed to be in good condition. Maybe I'll put a go pro underneath there and drive it for a bit and to give me a good idea what's happening.

The reason I suspect the transmission or differential is that there is a distinct gear noise at low speeds when the drivetrain rocks back and forth.

 

So, I did some detective work and stuck a GoPro underneath the hood and went for a drive to see what was going on when I feel the jolt on deceleration. There was some pretty significant movement of the engine.

So looks like it's motor mounts! Hopefully they're all easy to replace, would've been nice to do it when I had the whole car apart for the clutch change.

 

Well, I finally got to replacing all the motor mounts, including a much stiffer race busing for the transmission mount. Shifting feels great now!

Unfortunately, this jolt on deceleration is still there. I'm convince it must be in the transmission somewhere, but at this point I'm just going to keep living with it, already replaced enough things in this car.