Daily reader here but never really posted. I've learned a lot from this forum, and so I come looking for some advice from the knowledgeable.

I picked up a CPO 2012 991S about a month ago, ~10000 miles on the odo. Platinum on black, PDK, SPASM, sport chrono, PSE. Specced nearly identically to my launch Racing Yellow 2012 991S that was stolen from my garage after my home was burglarized at Christmas time during a blackout. Good price for a CPO Canadian car with those options and mileage, so I grabbed it. Counting the stolen car, this would be my 2nd 911, so I don't have much experience with trouble shooting these.

I drive it daily on a reasonably fast moving highway, a pretty good stretch of road for where I live. Long and short of it, day two or so of ownership I notice there's a vibration occurring at ~65 mph, and worsening up to about 95 mph, the fastest I've had a chance to test it so far. (I did test drive it quite a bit before purchase, but the highway local to the dealer is a crappy rutted toll road, and I just didn't feel it there I guess.) It starts after the car is stably moving on a smooth road for a few seconds, 7 or 8. It disappears under acceleration, and when the car hits bumps it takes a few seconds to set up again. It's palpable through the steering wheel but definitely not coming from the steering, not worse under braking, and not enough to actually shake the wheel. It feels clearly like it's originating from the rear half of the car. I can feel it through my back/backside, and when it's really vibrating, you can feel it through any solid surface in the car with a finger or flat palm; sometimes I think I can almost hear it as a low pitched beating, almost akin to wind buffeting. Starting around 80 mph, the passenger seat starts vibrating so much it's visually distracting, at about 5-6 Hz. The seat moves so much I've begun to wonder at times if it's the seat shaking that is being transmitted through the chassis, lol.

Hell, even my wife, who's hopeless at feeling vibrations or hearing squeaks and usually doesn't care enough to try, can feel it through her feet in the passenger side.

The dealer has tried, and replaced both rear tires which apparently weren't perfectly round. That didn't fix it, and they've roadforce balanced all the wheels, which has lessened it, but it persists. Of course, the techs and service manager swear that they could feel it before but that it's now 'perfect' following the balancing. I'm becoming 'that guy' at my service dept. They've had it in the shop for more days than I've had it so far.

So I come looking for suggestions. I've searched the forums, and most of the time it does seem like the wheels are the culprit in the threads I've read, but mine don't seem to be. Should I take it for re-balancing somewhere else anyway? Does anyone else's passenger seat shimmy on the highway? Any other suggestions for things I could look for or do, to try and isolate the source? Or am I just over reacting to vibrations in a race bred car?

Does anyone know of a good place to get another opinion in Toronto/GTA? I don't mind paying out of pocket at this point for more diagnostics or a fix; it's a pretty dismal driving experience right now, and since I'm on the highway every day, it's inescapable.

Thanks for reading and for any thoughts.

 

My car does something similar. I've had the wheels balanced twice, and while it makes it better at 60-70mph, the car vibrates noticeably at 25-45mph. I think it's a slightly bent wheel.

 

There was a post on here about the rims being "defective" causing vibration issues.

 

I notice nothing like what you are talking about.

To address your final somewhat rhetorical question.. about over reaction. you said you have a 991 before.. so I assume your comparison to that is that this is significantly different? So sounds like there is a real issue of some sort that you aren't imagining...

Doesn't sound like it is coming from the brakes somehow since you mentioned it isn't affected by application of brakes.

Assume it wasn't tires since you've replaced them. Possibly something funny with the wheels.. I suppose if one of the wheels was deformed slightly or had an inconsistent weight distribution the balance wouldn't completely do it's job. There is only so much balancing can fix. I'm wondering if the dealer might swap on another set or rear wheels/tire and test it just to eliminate that variable?

If it isn't wheels/tires, steering or brakes... I suppose you start to move to looking the power and drive train. I would first try to isolate the vibration and see if it is it related to a) the engine RPM or b) the speed of the car.. If it is related to the speed of the car and not the engine RPM, it might point to something out of whack in the drive train from the transmission and the wheels. If it is related to the RPM, then it is probably related to the engine and drive train through the transmission.

You indicated it sounds like a 5 or 6 hz beat. Is there any aerodynamic surfaces under the car that might be damaged. There are some baffle/diversions that send air to the front brakes.. I suspect there are some towards the rear of the car.. I would imagine that could cause some odd sounds at certain speeds.

If you changed your PASM settings, do you notice any differences? That changes how the car sits as well as the suspension stiffness.. As well as maybe changing the aerodynamics it might indicate some kind of oscillation that is occurring in the suspension.

If you had a typical front engine, rear drive car.. with a traditional drive shaft I'd be having you look at the drive shaft U-Joint as when they get cracked, what you are describing with a low HZ oscillation is exactly what you feel. Obviously not that, but it makes me think that maybe something is going on in the trans-axle. This it would be important to isolate if it is at engine speed or wheel speed.

I'm hoping it is just a bad wheel.. and swapping them as a test would eliminate that very quickly.

 

For starters welcome. Secondly I had a similar issues several years back and it was remedied by new tires. My suggestion is never to replace two, always 4. Therefore, the vibration can still be with the different harmonics between the new rears and old fronts.

Next the wheels are likely fine as you've had them balanced and they would have found an issue if they were requiring too much weight (which is what would happen with out-of-balance wheels). I actually am surprised that your dealer would replace only two tires without urging you to do all four.

 

Stuff like this is a process of elimination. Since you are in Toronto, and there are other 991 forum members there, see if you can meet up and do a tire/wheel swap with another 991 owner. A 5 minute run after than will tell you if its wheels/tires or not. If you were in the Wash DC area I'd loan you my stock "S" wheels and tires for the day to determine as I'm running an HRE wheel set now. I'm sure there is a Toronto Porsche enthusiast that can help you out, just see if you can connect. If you still get the vibration after trying four different tires and wheels, then you know its in the drivetrain, control arms, bad shock, etc.

 

Did you have them check run out on the rims to make sure it is within factory specs? Road force balance tries to match high to low spots. But if the wheels have a wobble it can be tuned down but not completely out. I would ask the dealer for a wheel swap or take it to a shop with Hunter road force equipment and have them measure runout on the rims. It is not in the dealers interest to replace the rims unless they are outside the tolerance limits that porsche considers acceptable. You did say the problem improved but did not go away entirely wouldn't that suggest you are on the right track? Good luck, it sounds like you have had a run of bad luck.

 

Did you check if your car has spacers?

 

This happened to me on my bmw 3series when I installed wheel spacers. I removed it immediately.
Check if thats the same case with you

 

Been a busy few days and just got around here to read responses. Many thanks for the numerous thoughtful replies. I needed some third party input; I'm so frustrated by the problem that I am not thinking critically, that's obvious.

To answer some of the questions posed: it's perhaps a bit worse with SPASM in sport, no spacers, and yeah, changing only two tires was a bit surprising. No obvious aerodynamic causes I can see underneath.

The bulk of the wisdom here is that it's still a wheel problem, and thanks to those of you that pointed out that it improved with balancing, strongly implicating a bad wheel. It reminded me that when I picked it up last time, the tech actually told me one wheel was pretty unbalanced and that they got it close but not totally perfect, but they thought "good enough".

So, that's the place to pursue further, obviously. I'm going back for a trial wheel swap next week. This is a great suggestion, one I hadn't thought of.

Again, I much appreciate the perspective and advice.

 

I have a friend who had a Lexus with a rear vibration problem that the dealer tried multiple times to solve incl changing tires, road force balancing , etc and who swore it was not related to the wheels which looked " perfect " to them
long story short, the rear wheels were eventually replaced after much drama and the problem went away
sometimes an out of spec OEM bushing in one of the susp arms can do this too and is hard to diagnose , since rebalancing the tire can improve it but then lead one down the wrong road ...
If you can isolate it to front ..felt in steering wheel ...or back ...felt in seat of the pants , at least have service guy just swap out a similar wheel/ tire from a service or used car to see if a wheel is the problem .
good luck !

 

Hi megacondenser, did you ever resolve this issue? Was it a bent wheel?