A Plea for help with a Zombie of a Problem

Hello all. For the past two years, I have been chasing a ghost in my car. The problem is subtle, but infuriating. Basically the car feels unstable at speed, and nobody can figure out why. The faster you go, the worse it gets, and I recently moved to Germany which makes it much worse. Three different Porsche Specialists have looked at the car now, none have been able to tell me what they problem even is. For a little history, when I first got the car, one of my favorite things about the driving experience was how unflappable it felt at speed. It tracked like a laser, always going exactly where you pointed it and following that line as if on rails, even over bumpy highways. One day, a few months after purchase, I was driving home from a visit to family, which was about a 2.5 hour drive. The car had been perfect on the drive up, and nothing of note happened on the drive back, except the car suddenly felt like it was moving around under me all the time. I thought some wind had kicked up, but when I stopped for gas, there was no wind whatsoever. basically it feels as if the car is tram-lining, but it does this all of the time, even on pristine fresh poured pavement. In fact it is most noticeable then, as other inputs are filtered out. The feeling is as if the alignment were randomly shifting around. It's mostly felt through the hips. At 100 mph, it feels as if the entire car is shifting nearly a foot in varying directions, at random. This problem has now remained with me through countless attempts to correct it. During the first attempts, the only thing they found was slightly worn front LCA bushings, and a worn motor mount. Those were replaced with no effect on the problem. Next they found that the rear shocks were worn out, so the dampers were replaced with Ohlins R&Ts. Unfortunately the first set of R&Ts had a bad damper out of the box, so eventually they were replaced. Problem is still there. I started going through all of the suspension wear items. Rear anti-squat arms, rear LCA bushing replacement, new toe control arms, even tried solid ones with heim joints. Problem is still there. Next , the car got shipped to Germany, and as some might already be aware, was severely damaged in shipping. The second set of Ohlins were destroyed, and replaced with KWv3 as Ohlins isn't available in Germany and as such are not TUV certified. The steering rack was also damaged and replaced, along with the tie rods, and associated hardware. Problem is STILL there. So, at this point, basically the only things in the entire suspension system that have not been replaced are the front anti-dive links, and the anti- roll bar equipment. Mechanic swears they are good. One odd item of possible note. Recently, something seems to be happening with the driver's side CV axle. The outer boot started leaking, but I couldn't find a hole in it, so I replaced the clamp, as it was leaking out through the clamp itself. Mechanic checked CV axle and says function is perfect. Oddly, the wheel carrier on this side seems to be having some sort of issue as well. The hub-centric portion has changed size slightly, with the wheels BARELY fitting over it anymore. They also don't want to come off after finally being worked into place. It feels to me as if this piece has been warped by heat somehow. Bearing on that side was replaced out of an abundance of caution. No change. So, I am begging, someone. Anyone. Help me out? What they freaking hell is this? It's driving me insane.

It sounds as though you’ve looked at virtually every possible mechanical cause...but have you looked at aerodynamics? It’s a long shot, but is your rear spolier actuating? The downforce generated at 75 mph when it’s designed to deploy would be small, but at 100 mph no rear spoiler could cause some skittishness.

All under body trays are present and accounted for. I know the spoiler works from the button inside the car, but I can't see it from inside to know whether it is deploying at speed or not. I guess I could ask a friend to follow me and check. Personally, I find it difficult to believe that sort of issue could cause the type of effect I am feeling, partially because the problem can be felt at speeds below the speed at which the spoiler system deploys. But I don't have experience with similar issues to know one way or the other.

Have you tried swapping out a different set of wheels/tires?

Shooting in the dark here.... Can you get it onto a 4 wheel dyno? Run it up to speed and see if you can spot anything. Get two friends, one to drive and the other to look, and have them follow you at speed to see if they can spot anything. I had a similar issue with an RX-7; replacing a faulty front splitter (which looked perfect, even when removed) solved the problem. Good luck! Joe Henley

I had a similar problem with my 986 Boxster . It turned out to be the tires. Maybe borrow a set of wheels and tires and try them out.

+1 for tires. A bad belt can do strange things. My other thought is caster. Alignment should have taken care of that but caster has a huge effect on straight line stability.

Change your Tires even if they look good. Had the same problem with my Boxster ,

Have you changed tires at all through this evolution? If not maybe a bad belt?

Sounds like you've checked or replaced all the undercarriage links, shocks (I assume front and rear), and tires. So as far as I can see the only thing you did not mention was a 4 wheel alignment. Also as far as the rear wing goes, assuming it does go up and down at the push of a button then push the button and drive it. There is no need to have anyone follow you. Good luck. PS you didn't put anything heavy in the Frunk did you?

I think your car is lowered and wondering could the sway bar be pre-loaded somehow? I'm not an expert but have been researching adjustable drop links and when they should be implemented. Mine car's started tramlining more and also squirming under brakes has increased so I'm thinking about a refresh strategy right now too

From the driver's seat I could always see the spoiler on my 987 when it deployed. However, I would still check the tires.

So, it appears I may have stumbled upon to cause of, and solution to the problem today, purely by chance, and Joe here seems to have nailed it on the head. It appears it was aerodynamics related. Today I swapped out my temporary front license plate for a permanent German style plate. The temp plate was an American sized plate mounted in a tow hook mount on the right side of the car. Coincidentally, I have been using that plate mount since roughly a few weeks before I noticed the problem. The new plate is center mounted, currently just adhered to the center of the front bumper with 3M double sided tape, as I would prefer not to drill my bumper. Looking for a drill hole free solution that is more permanent. Coming from Pennsylvania and intending to move back there at some point, I never wanted to drill the front bumper, as front plates are not required in PA. So here is what I am thinking happened that caused the source of the problem to be obscured. I had moved to MD a few years back for work, and found the tow hook mount a great solution on my previous ride, a BMW 135i. When I got the Cayman , the previous owner, who was also from MD, said he had never run a front plate, and never had an issue with the cops in 6 years, so I thought I would give it a try. 6 weeks or so later, I got pulled over for not having a front plate. I ordered a tow hook mount immediately, and installed it on arrival. For the next several weeks, I just drove the car back and forth to work, and as the problem is really only apparent at highway speeds, I didn't notice it. Then I drove home to visit family and friends, and on the way out, I didn't notice a thing. The only thing I can think of is that traffic must have sucked that day or something, I honestly don't remember, but that happened a lot as I was typically traveling to PA on a Friday evening. Then on the drive home, I got on an isolated section of the PA turnpike which had recently been repaved after 30 minutes or so of back-roads, and immediately noticed the problem. As mentioned above. my initial impression was that wind must have kicked up a bit, as it felt to me like the car was being pushed around by the wind. After ruling out high winds, my brain ignored that similarity from that point forward. Several times when I was trying to explain the problem to someone, I used the analogy that the car seemed to move like a knuckleball does, unpredictably changing directions at random. I never figured I was so close with that analogy though I am utterly dumbfounded that this could be the source of the problem, but after roughly 2 years of my car bobbing and weaving at anything over about 70 mph, I hit 110 on the way home today, and suddenly realized the car was going in a straight line. I had to drive some extra miles on the way home for confirmation, and while I am still not 100% certain, I am starting to really believe this was my issue all along. Makes me wonder what exactly could have been the aerodynamic cause?

Wow, that is amazing. I can only think that the front plate, mounted off-center and perhaps angled a bit, acted as a small wing that pushed the front end around. Congrats, hope that was indeed the problem and it is permanently fixed!

Should have linked you above, Sir. You were in the right ball park, it was aero related.

Glad you got it figured out; and glad I could help. But I think gcurnew gets the credit, he was first! Joe Henley

I'm happy that we "collectively" got it figured out. This situation shows how finely tuned our cars are, and that even a seemingly minor change or variation can have unintended outcomes.

Aerodynamics, yup...remember what happened to the Superbird and Daytona cars in NASCAR'S 1971 season when they stuck on that long nose cone and bolted that GIANT wing on? It was so ffective, the cars got banned from racing due to their extreme efficiency and resulting dangerous high speeds. :drivingskid::drivingskid::drivingskid:

Was looking for another old post and realized I never properly updated this one in case someone else ever has the same problem. As it turned out, the license plate mount was not the true source of the problem either, thought it may have been at least in part the cause. The actual problem was broken mounting tabs on the center blanking plate in the front bumper. The center blanking plate covers the location where the third radiator goes on some higher performance models, and clips into the front bumper. The clips in the bumper had broken off on the passenger side in my case. This resulted in a situation where the blanking plate would be pushed back by the airflow while you were under way, open up and allow air flow on one side only, and possibly also flap around unpredictably, creating the knuckleball effect I was experiencing. When you stop the car, the elastic nature of the plastic would return the part to it's normal position and close up this opening, making it invisible. Eventually it finally broke another few clips, which allowed it to stay open even when the car was stopped and be visible. I repaired the problem by installing a third radiator kit. I have no way of proving it, but I suspect the license plate mount might have been the cause of the clips breaking, as they broke on the side where it was mounted. I suspect the plate mount may have created a focused pressure point on the corner of the blanking plate, which was enough to break the clips.