doctormc2

Hi there Porka-lovers.

I've had my 991C 2S for 11 months now. Covered 10,000 kms. The car itself is relatively standard: 20" Carrera Classic wheels, wheel spacers, no PASM/PDCC, standard steering, Sports Chrono Pack. All good! With the exception of the 0.5kms on the dash when I picked the car up I can 100% guarantee that I'm the only person to have driven (or abused) the car. It's never been bumped, crashed, or bashed (apart from a miscreant in a car park into my driver's door... Grr.)

I love the car, use it every day. I live in the South of France so temperature is pretty constant (20 to 30 degrees C). My driving is a combination of back-roads and motorways/highways. On the back-roads the car is amazing - handles beautifully.

However, on the highway (motorway) it's horrible - whilst it feels flat and firm, and most of the time it is fine. Every now and again it will wobble/veer off track without warning. Almost changing lanes. I have never experienced this in any other car (I counted 'em - this is my 20th car, but my first Porsche). Our other current car is a 2012 BMW X5 which has a much higher side profile and is rock solid - even in very strong cross winds.

I've taken the car back to the dealer a few times and can never replicate the issue. They did check tyre pressures and one of the rears was a little bit low (5k miles ago), which they inflated - and now I check tyre pressures every time I fill up - but I still get this occasional wobble.

I now regularly put the car in Sport Chassis mode which 'seems' to make it happen less - and definitely makes it more stable on the motorway - but this is surely madness?

My cruising speed is around the legal limit: 110-130 km/h (70-80 mp/h) - the car simply doesn't feel good or pleasant on the motorway. My X5 however is very different, it's big and high - and I often look down at the dash and see that I'm doing 160-180 km/h and it feels like I'm travelling much slower than the 911!! If I have any distance to cover I opt for the Beamer!

Is this me? Is my car broken? Would changing the tyres make a difference (I've seen some mention this on posts) - would changing the tyre pressures make a difference? Like I said the car is on the standard rubber. Should I be more pushy with the dealer?

Any thoughts gratefully received!

jester

Hi Doctormc2,

Sounds like tramlining to me.
(http://en.wikipedia.org/wiki/Tramlining)

I had this happen many years ago on a BMW 328 - brand new goodyear tires. Very disconcerting and only happened on certain parts of the highway.

Have you noticed ever so shallow longitudinal grooves on your highway where you had this problem?

Anyway, I fixed my problem when I went to Michelin Pilot Sports and dumped the goodyears.

Cheers
Jester

Justin N

I agree with the tramlining possibility. I don't have goodyears on my C2 although on previous cars had lots if problems with them and some early run flats on a Z4. I haven't had this experience with the Pirellis on the C2 except for an area where they were prepping the road for paving.

doctormc2

Thanks guys - I've had tramlining before - I'm pretty sure it's not that. What I have noticed is that it can be the result of going over a bump or an expansion joint on a bridge. But sometimes it feels like wind as well. Other times it just feels like the car isn't connected to the wheels! These roads/conditions are the same I've driven regularly for 3 years in 5 other cars and never experienced it any of them.

lspain

I recently had a slow leak in one of my rear tires. Until we could get to a gas station and refill the slack tire, the difference in tire pressure caused the traction control system to occasionally grab on that wheel and caused the car to suddenly veer a bit. The tire was repaired and everything was fine afterward. Your circumstance with a slack tire does sound similar and I wonder if the traction control system is (still) grabbing on one of the wheels.

With that in mind, I understand you are checking the tire pressures. What does your TPMS system say the pressures are?

docness

I'm having exactly the same problem with my 2012 C2S (7,500 miles). In fact, it's at the dealer for the third time right now because of it, and they're still mystified by the problem. In my case, the veering out of line is often accompanied by an abrupt snap of the steering wheel which brings the car back into line.

My tires and pressures are fine, and tramlining is most definitely not the cause as it happens even on perfectly smooth roads. It also only happens with PSM enabled. The dealer has already replaced all four wheel speed sensors on my car and is planning to replace the PSM control unit next, which I'm hoping will finally fix it.

Try disabling PSM on the highway, and see if you still have the problem.

doctormc2

Oh dear - that doesn't sound good.

Do you mean turn off traction control? I guess so if you think it's grabbing a wheel because it's not detecting the spin speed properly?

I had the 5mm wheel spacers on mine (no longer an option I can see on new 991s) - it always felt like they'd only put them on 3 of the 4 wheels. But I had them check that

Let me know on the PSM = Traction Control? (I assume this doesn't turn off ABS)

docness

Yes. PSM=Porsche Stability Management, which is the traction/stability control. Turning it off will not affect ABS, and I believe in normal (non-sport) mode, PSM will still function if your ABS activates to keep the car stable under hard braking.

If you don't get the strange behavior with PSM off, then you will know for certain that the problem is caused by PSM braking a wheel. Although, that won't really tell you why that's happening.

doctormc2

Thanks docness. I'll do it this morning and see how it goes. Will have to keep an eye on my right foot around corners though

///MMB

Go to the dealer and test drive another c2s to see if its just your car that drives that way.

doctormc2

Thanks MMB. Problem is that it's not very repeatable. I would certainly say that the car does not feel 'planted' on the motorway, the steering doesn't feel quite right. So yes, maybe you are right a test drive of another will show the difference. I will give them a shout this morning.

I did follow docness's suggestion this morning and turned off PSM. It didn't make any difference. I did get a couple of little wobbles (although not the big ones - but these are rare anyway). It was unusual driving with the coasting feature disabled!

I'll call the dealer (again) and arrange a test drive.

I assume that all you other 991 drivers are happy with your motorway/highway 'plantedness'? The car feels good on these types of roads?

Jay-S

I've experienced the same symptoms but I have chalked it up to tramlining. The steering wheel does move on me. Roads weren't in bad condition when it happened. I just realized I was catching "small" seams when I started really paying attention to it. Doesn't happen to me very often and it really doesn't bother me.

Also, I've never had this happen in other cars other than when the road conditions were pretty bad.

Just my experience...not saying it is what is happening to the OP.

stealthboy

I've had mine up to 135 mph on the highway (long stretches near 120mph) and it felt even *more* planted as the speeds increased. A sudden veering seems to me like it's some control system trying to correct, and not an inherent issue with the chassis, but that's just my guess. This is a very interesting issue and I'll be following this to see what comes of it.

On a related note, I had two minor nitpicks about my car so I took it into my dealer, and of course they could not replicate the issues. That seems to be a common thing. The car never exhibits the behavior in question when a mechanic is looking at it... there must be some quantum-level observation state collapse going on.

Dyim

I thought it could be the PDCC but just re-read your post and you do not have it.

SanibelSpeed

http://www.6speedonline.com/forums/9...0-70km-hr.html