fbgh2o

I have a driveline vibration that appears at speed. I only feel it over 60 mph, but a mechanic who owns a 996TT and 997TT says he can feel it at idle and low speed. I don't.

I have eliminated the tire balance as the source and steering wheel is rock steady at speed.

There may be a bit of warp in a disc brakes. Would this show up as driveline vibration?

However the mechanic thinks it is the dual mass flywheel and before I start down the path, I would appreciate any thoughts.

Fishey

what tire sizes are you running? Also does it get worse in the rain?

Dock

Wheel bearing(s)?

Kevinmacd

Just a thought front drive viscous clutch?

TT Surgeon

Wheel weights?

993GT

what shape is the transaxle mount in?

fbgh2o

Thanks for the ideas guys.

225/40 and 295/30

Hmmm... will investigate

Another source to investigate

Don't think so as they have been rebalanced

no overt signs of wear

Macster

Brake rotor warp - actually deformation of the rotor -- is rare. More often any pulsing of the brakes when braking is caused by uneven pad material to rotor surface deposition caused by hard braking followed by leaving the brakes applied when the pads/rotors have not been properly bedded in, or by my experience hard braking and leaving the brakes applied with the rotors rusty from leaving the car parked after washing. (This happened to my VW Golf TDi, not my Porsches, fortunately.)

You have to be sure the tire balance is good. Too often in these cases tire balance is eventually found to be at the root cause because the tire shop was unable or simply didn't balance the tires right.

Know too that often these tires new start out balanced but as they wear can go out of balance. The out of balance condition can be mild but present and noticeable.

Another cause of vibration at speed is improper tire inflation pressures. I have found at least in my Boxster I can induce vibration very much like a mild tire imbalance by over inflating the front tires by a few PSI.

Alignment can also contribute to this feeling of vibration.

However, if the vibration is present at engine idle with the car I assume stationary that is not tire imbalance or driveline imbalance but something else.

Vibration at idle is not a failing dual mass flywheel symptom I'm aware of. 'course, this doesn't mean it can't be the DMF.

Generally the flywheel clunks when moving the car off from a stop or at other times when shifting or when in gear you lift off then reapply the throttle.

At idle does pushing in the clutch affect the vibration? If so then this could be the DMF.

Dock

I experienced a similar vibration when at speed and it was the result of the front wheel bearings being bad. The vibration and noise felt like the drive shaft to the front wheels was out of balance.

CincyScott

When do you feel it? Does it get worse as speed increases? Does it get better? Only when cold? Only when hot... More data may help. If it happens all the time when driving, it's not likely a tire issue.

AWD cars are very sensitive to road force balance though. Did the tire guys give you those figures?

speed21

Front diff will cause vibrations through steering wheel at speed.

"02996ttx50

not the dmfw. tire pressure or uneven road surface concurrently.

fbgh2o

FYI - just changed the engine mounts from RSS to stock with no appreciable change. But the car does seem to have a no stock transmission mount... aftermarket perhaps?

Macster thank you for the thoughtful reply. There is no difference with the clutch pushed in.

Definitely speed related. Most apparent on the highway over 70 mph. but it is a seat of pant vibration and not evident in the steering wheel.

Will pull the wheels and check. Might even look for run out.

Macster

Problems with vibration can be a real challenge to sort out.

It is important that every step be done correctly. As I touched upon in my earlier post often times a tire is just not balanced correctly, or as a recent thread in another section ended up, the new tire was defective and had to be replaced.

In another case an owner suspected wheel/tire balance was the problem but upon returning to the shop (a dealer IIRC) he was assured the balance was just fine. Finally after exhausting other possible explanations he visited another dealer and a wheel/tire balance as found to be wrong. Apparently the first place made a mistake and thereafter just fed the owner some BS.

Be sure tire inflation pressures are correct. As I touched upon in my earlier post I have found at least my Boxster to be sensitive to tire inflation pressures.

While on the subject of inflation pressure I recall at least one of these vibration problems turning out to be water in a tire. So you might want to when you have the wheels/tires off give the wheel/tire a good shake to check for water.

Be sure the wheels are true. I do not know what wheels you have on the car but some wheels can be rather easily bent. I'm not sure balancing will correct this nor will road force balancing flag a bent wheel. Hopefully the tech will carefully observe the wheel/tire spinning and spot any runout either on the balance machine or back on the car giving the wheel/tire a spin and looking for runout but one never knows.

Certainly the tech can use this opportunity to also observe the brake rotors checking for any signs of run out.

Be sure the wheels center properly on the hub. While I would find it hard to believe factory wheels would have this problem I have come across owners with aftermarket wheels that were found to have a hub register diameter wrong -- could be too large but also too small in diameter -- and when installed the wheel did not properly center on the hub. This resulted in a vibration but of course the wheel/tire balances just fine.

Fortunately I've never had this problem, but my preliminary research into Turbos turned up a few instances of the cardan shaft (its bearing) failing and one sign was a vibration when the car was moving in the highway speed range. To check this you need the car in the air and then you can grasp the cardan shaft and check for any play.

While you have the opportunity you might as well check the front (and rear) half shafts too for any play.

You can give the wheels a push/pull test to check for excessive play which can be a bearing. While my only Porsche bad wheel bearing experience produced just noise I would not put it past a bad bearing to create some kind of vibration. And of course Dock reports this was the case with his car, so there you go. Might mention while there was noise there was not any excessive play. It was with some difficulty that the shop owner and I identified, more guessed than anything, the noisy wheel bearing but the identification did not have 100% confidence level. Fortunately we "guessed" right and replacing the bearing had the noise gone.

At some point you might consider getting a well regarded Porsche tech to test drive your car and offer his opinion. Techs encounter a variety of abnormal behavior and often times can save one a lot of thrashing about trying to identify the root cause of a car's misbehavior.

Really the important thing to keep in mind is you can't overlook or ignore any explanation. And you must be sure when you eliminate a possible explanation you have done so correctly and thus have complete confidence that the problem lies elsewhere.

Last but not least, just so it is clear you should try to eliminate those causes that are easiest to do, that require little or no expense. Obviously if these prove to not be the root cause you have to continue but at least you do so knowing you have been diligent, deliberate and disciplined.

Dock

Correct. I had no play in the push/pull test.

Here's one of the bad bearings: