jon928se

Not so unlikely- I had the exact same symptoms on my SE a few years ago. Discovered the cause of the problem when I had 2 passengers. Wife sat in the back and noticed the hatch was vibrating at Kerching Kerching 76mph. 30 seconds to adjust the rubber bumpstops up a little to take out the slack cured the problem completely.

The rthymical nature of the problem is due to vortex shedding off the trailing edge of the roof .

Leon Speed

Based on the side to side shaking which always starts at the same speed, I would think something is moving with air flow. I can't see the logic in side-to-side vibration from wheel or axle related issues when it starts at a certain (high) speed. I know it doesn't give you much to go on unless you can get access to a wind tunnel In any case, I'd be taking a real close look at the tranny and the exhaust mounts.

Mrmerlin

So try removing the rear wing and see if it make the noise go away.
Bill the halfshafts might be something to consider,
i wonder if hitting the piece of metal may have damaged anything,
also consider replacing the trans mounts

White Lightnin'

True... but I did them with the age of the vehicle & components in mind. That's why I suggested the new halfshaft assemblies, new rear tires (he needs them anyway) and a 4-wheel alignment.

But in my case, the 20+ year old suspension would have made the problems reappear, wear the tires unevenly, put unwanted pressure on bearings/CVs.

In my 'shotgun approach', by replacing the suspension with new -I eliminated the issue of incorrect drive angles (on axles, CVs, bearings, tires). By replacing the halfshafts, I prevented the further disintegration of the CVs by now running them in the correct position. By replacing the brakes/rotors (big brake upgrade) and bearings, I ensured a true smooth running track. When I went with larger brakes, I chose 18" wheels for greater tire selection... therefore starting with new unworn (even) tires. With the 4-wheel alignment and corner balancing, I further ensured everything was in the correct position and the wear patterns on everything would be even and predictable.

Most of these components on Bill's car are new (somewhat), with the exception of 'rebuilt' axle assemblies. 'New' comes with warranties; 'Rebuilt' depends on the skill of the rebuilder... often without recourse of replacement (except from same rebuilder).

Bill Ball

I did once have a noise with the rear hatch which was wind-related, but no vibration. The hatch has new tongue and has been carefully adjusted.

The axles are on the suspect list. I checked them once and could not find anything wrong, but will do that again. This started before I hit the road objects. The tranny mounts are new and as I said, this vibration started soon after I replaced the old ones, so the new ones are on the suspect list, but they look fine and were installed per WSM. You know, there was nothing wrong with the old mounts - the tranny sat a good distance above the crossmember. It was just one of those things. They had 200K miles on them, easy enough job. Another vote for if it ain't broke don't fix it.

Bill Ball

I have been considering something vibrating in the wind. It's not clearly wind-speed related, but since it's not always there at high road speed, I have been thinking it's not exactly mechanical. That's the problem with this vibration. It's not consistent with anything enough to direct the search. In the past, it's always been rear wheel balance with new tires - 3 times over the years - and it was always resolved by a rebalance. This time that didn't resolve it and 2 other sets of wheels/tires that worked fine before showed the same symptoms.

It "feels" to my gut instinct like a worn suspension bushing that doesn't show itself except when the road inputs are at a certain level and then the loose suspension arm develops a resonant vibration. I've already replaced the front upper A-arms even though the old ones were not worn or broken. I have bought all the bushings for the rear. I've examined and pryed on all the rear bushings trying to find decent candidate to start with and they all seem quite firm and I have no oddball handling issues. I could just go ahead and tear apart the rear suspension and replace all the bushings, but I'd like to have SOME indication that's a good bet. I've seen cars that had terribly worn or distorted bushings and the indication was clear. Mine is not one of them. I'd say 3/4ths of the repairs I have done to this car were not indicated by any observable defect (steering rack, tie rods and balljoints, A-arms, tranny mounts, TC bearings, etc.) The only repairs that were clearly indicated were throttle switch, LH brain, cam cover seals, pan gasket and rear main seal.

928mac

Well, interesting read so far Bill. Looks like you have checked and rechecked.

So you say the whole car shakes? like a speed wobble?
originally I was going to get you to place something next to the tire on the floor and turn the tire by hand to check for runout, but if it shakes the entire car?

here is what I think happened.
When you lifted the car to replace the mounts, somehow the rear caster changed which may cause a speed wobble. put 50 psi air in the rear tires and take it for a drive to see if anything changed.

question, did a curb get hit at any time that may have shifted the suspention?

after thought, you may have to set it on scales and make sure that you have equal corner weight

Tom928

Hi Bill,

When I replaced the trans in my 84, I did the mount spacing/shimming like the WSM specified.
I had to order a shim that was kind of thick, but that is what I measured.
After getting the car running, I had this vibration that started at about 60mph but it was not as bad as you experience.

I talked with an PCA member who told me to remove the trans shim, loosen the TT bolts (at the tranny and the bell housing) and lower the trans onto the mounts without shims, then tighten all the bolts.
Recheck the spacing and reshim is needed.

I checked the alignment/spacing and there was no need for a shim.
After doing this my vibration was gone.

I can see where this would make a difference with a thick shim; however, if you have a thin one or no shim at all it is likely something else.


Good luck,

inactiveuser1

If you had the old mount it would be a good test to throw it back in with the same spacing as you had.
The post Tom wrote above about shimming and setting it up with the TT seems to point in the direction of what the cause may be.Could try different shimming to see if it changes anything.
Read alot of post and if it did it right after you changed the mount I would be trying different settings with that to see if any changes.

Bill Ball

Rear caster? I've had 3 different sets of wheels and tires on the car, so I don't think it's runout, but this deserves checking, along with a more careful look at the axles, rotors and hubs.

Bill Ball

Original mounts are gone. There were no shims and there aren't any now. Although I can't see what I could have done wrong, it's on my short list.

Tom in Austin

Very interesting Tom, with all the torque tubes that are replaced, I wonder how many people ever think of the alignment of the entire drivetrain, either via shimming at the trans mounts or loosening the bolts to let everything take a 'neutral set'.

Tom in Austin

btw, caster is not adjustable at the rear, could only be an issue if something were bent or damaged ...

FredR

Bill,

Is there any chance your torque tube bearings are on the way out? Apologies if I have missed anything in the thread. I believe Porsche had to pull some tricks in fitting a vibration damper in the TT- whether that applies to both auto and manual variants not sure. I understand the auto tubes have two bearing assemblies and the manuals 3. Presumably the rubber bushes that carry the TT bearings may be getting tired by now- Maybe Constantine's TT refurbishment kit is on the cards? If you feel the vibration is somewhere in the TT area I would think there has to be a pretty good chance what you are sensing is close to the root cause of the problem. There is plenty to cause issues in there. I cannot explain why I only felt vibration in top gear when the flex plate clamp slipped but I did.

Regards

Bill Ball

Fred: I had the TT out and it was in such good shape I simply reinstalled it. The shaft spun true, had no play, the bearing carriers were where they were supposed to be, the bearings spun quietly. Plus I feel no vibration reving the motor in park or neutral and the vibration is road-speed, not engine RPM related. Still maybe I fooled myself into thinking everything was OK when I found I could not get the bearing carriers to budge with a PVC pipe and a 5 lb sledge. It would seem expected that at 200K+miles these bearing should be on their way out, but when I found the torque converter bearings were like new, I lost interest in making a tool to press the TT bearing carriers out.