[Snufkin]

I am trying to figure out what could be causing what I can best describe as a bad vibration under acceleration. 

 

It is most prevalent between 60 to 100 km/h (40-60mph) and when accelerating hard in 2^nd or 3^rd gear. Acceleration starts smooth at lower speeds then the vibration starts and it feels like the car is fighting itself. This is definitely something I can feel through my seat and pedal rather than a noise. In fact, I can’t hear any suspicious noises related to the problem at all.

 

The worse case of it was just yesterday, when I drove over a mountain range. Climbing up the mountain with twisty roads really aggravated the issue. This time even at lower speeds there was a lot of vibration. And I discovered that when I put my hand on the centre console and accelerated, I could feel a thunking sound from somewhere under the car, and the speed of this thunking changed depending on how hard I accelerated. I couldn’t replicate this thunking later in the day on a hill that was less steep.

 

Possible culprits:

 

Torque tube? I have read a little bit about the issues with these and it seem I should be able to hear the problem rather than feel it?

 

Old tyres? Tyres are about 10 years old and was my initial suspicion, but the thunking noise makes me think it might be something else

 

The engine mounts are new and I very recently had a wheel alignment and tyres balanced, so I’m pretty confident I can rule these out.

 

Any suggestions or ideas are greatly appreciated!

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[Bulti]

I suppose you could check if the weels are all tightened up correctly, as a start.

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[Snufkin]

hmm yes good point! Will definitely double check that tomorrow. A bit scary if they aren't

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[968Syncro]

The rear axles have rubber boots over the CV joints (4 of them).  If one of these rubber boots breaks, the leftover rubber material can become out-of-round and cause vibrations.     You'd probably have to jack the car up a bit to do a good inspection.  Its best to fix them, else the grease will go out, the dirt will come in, and then the repair expense will include the CV joint in addition to the boot.

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[94SilverCab]

10 year old tyres? That's not good no matter what.

 

DM Flywheel?

Torque Tube Bearings?

Wrong Engine mounts installed?

Something in front suspension not correctly re-assembled after engine mounts installed?

 

JMHO,

 

Jay

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[968Syncro]

Jay makes a good point about the engine mounts.  They are different for the manual trans vs the tiptronic.  My 968 (Tiptronic) had a vibration when I first test drove it -- the seller reported new engine mounts.  I told him no deal unless this vibration is resolved -- he went back to the shop and they installed the correct ones for the Tiptronic and the problem was gone.

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[az968]

10 yr old tires could be out of round if sitting too long or cracking. Could still be a balance issue, maybe a wheel weight fell off.. Check shock mountings, could be loose.  But, replace those tires immediately, you are asking for trouble getting frisky on the twisties.

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[flash]

not likely to be related to tires or wheels or anything like that.  acceleration is the key here.  the engine rotates under load.  i would be looking for something making contact during that time.

 

failing that, i would be looking at items like the torque tube bearings or the dmf if it is a manual, and the flex damper if it is a tip.  input shaft bearings are also suspect.

 

torque tube bearings do make noise, but it is a buzzy noise, not a clunk.

 

the dmf can definitely create a vibration, but doesn't usually clunk

 

the flex damper in a tip will absolutely clunk and vibrate

 

CV joints can do all of the above

 

do not count out the motor mounts.  if they put in aftermarket ones,  they are known to fail in a very short time

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[ds968]

No clue about any of the other possibilities, but even if the motor mounts are the incorrect ones , the vibration would not manifest itself only in the situation posted above, in fact that's when it would be less noticeable ..

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[Snufkin]

Thank you for all the helpful replies and suggestions. Sorry I should have mentioned that it is a tiptronic not manual. 

 

I'm going to put the car up on jacks this afternoon and inspect the CV joints, plus anything else I can look for myself. Fingers crossed it isn't the flex damper.

 

I've moved cities and have a new mechanic since I had the engine mounts replaced, which was only 3,000km ago but I'll get my new guy to check them.

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[flash]

i am betting on flex damper

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[Snufkin]

CV boots are all in good condition, not splits or cracking or leaking fluid. Only thing out of place was a missing screw from the ATF reservoir, so at least I can stop that slow leak.

 

I remember reading that it is possible to check the flex damper through an inspection hole or something similar. Can anyone provide any more detail on how to do this? I can't find wherever it was I originally read this.

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[968Syncro]

Here is a snapshot from the PET.  Number 18 is the cap for the inspection hole on the bell housing. 

 

   

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[flash]

you may need to rotate it around to see it.  it is common for them to crack, and not show completely broken.  you would be looking at the rubber part in there

 

as preparation, this is a ridiculously expensive part, and the job is a pain in the butt.  last time i checked it was over $2k to do the job.

 

advice:  do not install a used part.  it will be no better than the one you have in there now, and the labor to do it a second time will kill you

 

second advice:  upon discovering the part has failed, STOP DRIVING.  continued driving will destroy the bearings in your transmission, as well as beat the hell out of the back end of the crank fittings

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[Snufkin]

Yep it's the flex damper. Took a look through the inspection hole and can see a crack running around the middle of the rubber. It's too big a job for me to do so will get a quote from my mechanic.

 

Is there anything else I should ask him to look at while he's doing it? Damage or wear to other parts that may have been caused by the damaged flex damper. I've read the torque tube needs to come out so should I look at doing anything related to that?

 

Thanks again for pointing me in the right direction. 

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[flash]

the torque tube and transmission have to come out.  at that point, i would be looking at exhaust hangers (usually pretty dried and cracked by now), rear main seal (look for leaks), service the trans (oil and filter), and because you would have easy access, change the fuel filter and check valve.

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[Snufkin]

Thanks Flash.

 

I have timing belt/balance shaft belt and water pump work due as well so it will be an expensive visit!

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[flash]

oof!  that's gonna leave a mark

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[Bulti]

Better to have it fixed, so the car runs like it should. Although expensive, everybody who wants to drive a healthy 968 faces about the same problems. It's either bite the bullet or sell the car I'm afraid.


Once you've thrown a bunch of cash at it, it's a very reliable car. If maintained properly, but that speaks for itself :-)


And as advised: check if the motor mounts are OE-ones.

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[oppositelock]

Just a suggestion if the budget allows; pull the engine out and leave the tube and trans in. The belts and
pump are easier on the stand. The engine mounts will be super easy. Also you can reseal the right/lower
balance shaft housing that gets cooked by the exhaust. If you are near or past 100K do the cam chain and
rub pads now. This is all a lot of "while your in there", but you'll be good to go for the next 100K
(except belt and water pump changes). This is all easy work once the engine is out.

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