What could it be? Driveline clunk on/off throttle

[Torxuvin]

Been a tough week for the zhp. I've done a bunch of highway trips. (passed 105k) I've noticed since I replaced the rear differential bushing that I still have a slight thunk/hesitation when I first press or fully disengage the throttle. (noticed at highway speeds 70-80, but I think it's there all the time) Very occasionally I'll hear the noise and it will feel like something was out of alignment and then comes back together through the gas pedal. I lifted up the car to check everything out earlier.

Things it's not:
Rear or front differential bushings
Subframe bushings
Subframe
Guibo
Center support bearing

Things it could be:
Transmission bushings?
motor mounts?
RSMs?
Rtabs?
Something engine related?

I'm kind of at a loss here guys.

Also the temp gauge dipped at highway speeds off throttle coming back through the mountains. (which seems to be a t-stat/temp sensor issue) So the car is basically parked until I do the cooling system refresh.

[Avetiso]

Could be engine mounts. I remember when I had a Jetta, if I ever hit the gas hard or suddenly let it go, I'd get a rough hit into the cab of the car. But, the VW had its engine sideways, which made it worse. It had bad mounts.

On a BMW, ehh, I would think it's something else, though. BMW engines are smooth, and considering that it's straight, not sideways, I doubt it.

[Torxuvin]

Could be engine mounts. I remember when I had a Jetta, if I ever hit the gas hard or suddenly let it go, I'd get a rough hit into the cab of the car. But, the VW had its engine sideways, which made it worse. It had bad mounts.

On a BMW, ehh, I would think it's something else, though. BMW engines are smooth, and considering that it's straight, not sideways, I doubt it.

Yeah. I don't think it's RSMs either, bc there are no noises over bumps or road imperfections.

[kayger12]

My first thought reading that was engine mounts...


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[Hornung418]

At 105k miles I would suspect engine and transmounts need to be replaced. Suggest AKG poly or OEM M3 mounts.

From a GS3, this was sent.

[LTCMontana]

It sounds strange but I was chasing the CES light for a long time. I had the same 'thump' mainly while on cruise control. We finally diagnosed it as a bad MAF. Replaced the MAF, no more thump. It is actually pretty logical. The MAF was out of tolerance and it woul sense the thottle change and shut fuel off instantly vs slowly; causing a minor thump feel in the driveline.

Recommend getting a scan tool that can read your MAF and see if it is in tolarance.

[BavarianZHP]

Hmmm. I have a clunk too (not loud, but not quiet either) but it's at low speeds - 1st or second gear. Sometimes it occurs as I switch to 2nd, but most of the time it's when I release the throttle in either of these two gears. Admittedly, I thought it was normal since this is my first manual I'm not yet completely familiar with them. Also sometimes at a stop it's difficult to shift into first, it requires more effort and sometimes feels rough - not sure if that's normal or not.

[Hornung418]

The thump you are describing, Bav, is the infamous rear differential bushing...ole no. 9. That will wear out and produce a nice thud when starting from a stop or shifting into 2nd.

Get all three differential bushings replaced when you can. No real damage...just an annoyance. Although some say that it will damage the subfloor, that's only the case if the subframe bushings are shot.

[johnrando]

Good info Justin.

[LivesNearCostco]

I've had two kinds of thumps. First it was a thump when letting out the clutch pedal in 1st or 2nd gear. That was a busted motor mount. Felt sort of like it was the forward middle of the car, say around the shifter area, though it was actually forward of my feet.

The other was a vibration when accelerating at certain speeds, not while shifting or clutching. It was a bad guibo plus bad CSB, but I understand Torxuvin already replaced those. That was felt through the seat and sounded like it was coming from the middle of the car, around the rear seat footwell driveshaft hump.

If you suspect motor mounts, there are a few ways to diagnose it.
1) Raise car safely and look at top of mount with mirror and light, see if oil leaked out.
2) Raise car safely and then gently raise motor with jack and block of wood under oil pan. See if motor mount bracket lifts out of mount.
3) Remove air filter housing and examine driver's side motor mount from top. Should be able to see top of passenger side mount without removing anything, though removing SAP might make that easier.

[Torxuvin]

It sounds strange but I was chasing the CES light for a long time. I had the same 'thump' mainly while on cruise control. We finally diagnosed it as a bad MAF. Replaced the MAF, no more thump. It is actually pretty logical. The MAF was out of tolerance and it woul sense the thottle change and shut fuel off instantly vs slowly; causing a minor thump feel in the driveline.

Recommend getting a scan tool that can read your MAF and see if it is in tolarance.

Hmmm. I've got it all apart right now. I had a huge vac leak, so I'm hoping that will solve the problem. I will clean the maf while I'm at it though.


I've had two kinds of thumps. First it was a thump when letting out the clutch pedal in 1st or 2nd gear. That was a busted motor mount. Felt sort of like it was the forward middle of the car, say around the shifter area, though it was actually forward of my feet.

The other was a vibration when accelerating at certain speeds, not while shifting or clutching. It was a bad guibo plus bad CSB, but I understand Torxuvin already replaced those. That was felt through the seat and sounded like it was coming from the middle of the car, around the rear seat footwell driveshaft hump.

If you suspect motor mounts, there are a few ways to diagnose it.
1) Raise car safely and look at top of mount with mirror and light, see if oil leaked out.
2) Raise car safely and then gently raise motor with jack and block of wood under oil pan. See if motor mount bracket lifts out of mount.
3) Remove air filter housing and examine driver's side motor mount from top. Should be able to see top of passenger side mount without removing anything, though removing SAP might make that easier.

Thanks for the tip. I have the air housing and most of the front of the engine apart right now for the cooling system refresh. I'll check those out while I'm at it.