Rear end thud clunk under hard acceleration from a dead stop

[M3ltdown]

Hey guys,

I’ve noticed an issue recently, from time to time (I’m always in sports mode) when I try to give it acceleration from a stop or taking off from a stop light (small roll out, then accel)

I feel and hear a thud from the rear end

it feels like I have something heavy in the trunk and it slides/smacks to the back of the trunk when accelerating, if anyone ever driven a trailer it’s very close to that feeling.

I’m not sure if it’s just normal behaviour or what not, drivetrain slop.

27k kms
Jb4 + CAI
285/35/19 rears
No lights/codes

any idea what it could be? Anything common that should be upgraded with after market parts?

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[Funkjaw]

Can you tell us more about the car? Is it 8AT or 6MT?

Lot of 6MT users including myself have noticed drivetrain slop when in sport mode with auto-rev match disabled that makes a bit of a thud between gears; more noticeable with music off and hard shifting. If you leave auto-rev match on and keep it in normal mode it isn't noticeable especially if you have music playing.

My 2 cents. If its an 8AT I don't have experience with that and other users will need to chime in.

 

[dustwhyn]

Put the car up on a lift with the trans in neutral and the parking brake engaged. Rock the guibo on the diff side back and forth. If it seems like there’s an excessive amount of movement (It should be very minimal, like less than 1/2 an inch of movement from side to side), it’s most likely that your diff was set up incorrectly when it was assembled and has allowed itself to loosen up over time and mess up the backlash. In this case, what you’re feeling is the excessive gap when the teeth on the pinion gear engage with the differential teeth, and the diff will need to be replaced by the dealer if it’s still under warranty. It’s not something that can be fixed without a removal and rebuild of the diff. Not saying that this is what your situation is, no way of knowing without the car being in front of me, but this has happened to me personally and it was almost exactly what you’re describing. If it goes from one clunk to multiple clunks over time and starts to do it at highway speeds when you load and unload the drivetrain, then it is definitely this. I chased this issue for months on my last car, replacing the driveshaft, guibos, bushings, axles, everything. Finally found out it was the diff itself after all of that.

 

[itzTang]

Sounds dumb af but if you have license plate frame....

Dam your license plate made of concrete or what? Lol

 

[Rensuhlo]

Sounds dumb af but if you have license plate frame....

I don't think my ///M frame would cause a clunk. It's lightweight.

 

[kyle9]

Put the car up on a lift with the trans in neutral and the parking brake engaged. Rock the guibo on the diff side back and forth. If it seems like there’s an excessive amount of movement (It should be very minimal, like less than 1/2 an inch of movement from side to side), it’s most likely that your diff was set up incorrectly when it was assembled and has allowed itself to loosen up over time and mess up the backlash. In this case, what you’re feeling is the excessive gap when the teeth on the pinion gear engage with the differential teeth, and the diff will need to be replaced by the dealer if it’s still under warranty. It’s not something that can be fixed without a removal and rebuild of the diff. Not saying that this is what your situation is, no way of knowing without the car being in front of me, but this has happened to me personally and it was almost exactly what you’re describing. If it goes from one clunk to multiple clunks over time and starts to do it at highway speeds when you load and unload the drivetrain, then it is definitely this. I chased this issue for months on my last car, replacing the driveshaft, guibos, bushings, axles, everything. Finally found out it was the diff itself after all of that.

I was going to say “sounds like the diff” but you said more words

 

[itzTang]

I bought some cheap ass ebay plate frame. There are only 2 top screw to secure. The bottom part of the plate frame sometimes gets stuck between the plate and the bumper. Whenever you go over a bum or road imperfections, I would hear a clunk.

Uhhh you got to check your mounting point then. Thats way too loose. Mine is also only held on by the top 2 screw. Doesn’t even budge. The thread is made of plastic so taking the plate or frame off and on wears down the thread.

 

[supra_rsa]

Have you managed to bottom out on the issue? I’m experiencing a very similar thing if not the same

 

[FLtrackdays]

I feel and hear a thud from the rear end

it feels like I have something heavy in the trunk and it slides/smacks to the back of the trunk when accelerating, if anyone ever driven a trailer it’s very close to that feeling.

I bought some cheap ass ebay plate frame. There are only 2 top screw to secure. The bottom part of the plate frame sometimes gets stuck between the plate and the bumper. Whenever you go over a bum or road imperfections, I would hear a clunk.

I took it he meant hard thud. Like something heavy in the trunk

Any chance Peetskee that you have an aggressive transmission tune? I knocked mine down a notch and it went away. But after Dustyn’s juicy explanation, I wanna check all that now!!

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