AO

There you go... you don't even have to go outside your own garage! Let us know what you find out!

hupp

I have been experiencing the same issue with my '79. At 90mph plus a small vibration is evident. I can even feel the vibration in the shifter. At this point I suspect a flat spot in one of my rear wheels. Lots of interstate repair over here in the last couple of years and crappy asphalt patchs at 90 will do the trick.

deliriousga

Scott, wish mine was a small vibration. It's flat out shaking like crazy, drives you nuts because it's smooth as silk until 80, then there's a small vibration, then big-time shaking at 90.

I'll post what I find if I can get to it this weekend. My baby turns 19 so lots of things going on this weekend w/ the family, but I'm hoping to squeeze a few hours in.

UKKid35

I've never been able to resolve my vibration issues.

Balancing all four wheels at the same time makes a huge difference, but things get noticably worse within 1k to 2k miles. Leaving it more than 3k is just horrible.

I have spent big money (for me) trying to get all ride height and alignment issues resolved and this has helped a little, but not cured the problem.

Next I'm going to replace the engine mounts, as this probably makes a small contribution. Even so, this vibration is probably the most dissappointing aspect of owning the car. It can be felt in the seat, and back, and also as a vertical vibration of the steering column, there is rarely much turning effect in the steering wheel. It seems to have several "harmonics", 80mph is the first, it seems to come and go in 20 or 30mph increments.

As the car must be on original CV joints, is that where I need to look next?

AO

Paul-
If you can, without a doubt, rule out the wheels and tires, then I would say the next likely culprit would be bearings, suspension and possibly drive train (CV joints). Of course, it may not be in that order. And if you're as fussy as I am about vibrations and squeaks, it'll drive you MAD!

mark kibort

did you guys check the "true-ness" of the tires on the rims. lots of times, they balance out, and at speed go NUTS with vibration.

check that. also, again, you can rule out engine and related components by just putting the car in neutral and pushing in the clutch. pretty hard time believing its goingto be a CV joint, heck, those things are only spinning sub 1200rpm. but, any thing is possible. more than likely its the tires out of round, (not flat spot, but out of round!)
MK

bigs

Y'know, this is probably stupid, but since the vibration occurs at increasing speed, and you have to get up to a pretty decent speed for it to appear, I'd even check to make sure the rear wing is securely attached. Also, maybe put it up on a lift and make sure the front spoiler is firmly attached along with any various pans under the engine. Makes me wonder if it's just wind turbulence acting against something loose.

deliriousga

I didn't think about the rear spoiler. I have one of those "flip-up" spoilers on the back. I think I read they dropped them pretty quickly, so maybe that's the reason. Every time I close the rear hatch the wing bounces up about 1/4"-1/2" then snaps back down. If the tires/wheels check out I may take the spoiler off and take her for a spin and see what happens.

UKKid35

I did once find that I had a rear tyre that wasn't round (I'd bought it used) and it was very noisy, but the vibiration wasn't significantly worse than usual. This has happened for my entire 30k miles of ownership, and with every set of tyres I have put on the car. I have tried puting the car in neutral when the vibration is at its worst and it reduces by about 25% which is why I believe the engine mount change may make some difference.

I also make sure that the wheels are mounted with the valve, looking wheelnut and red stud all aligned. I am still getting uneven tyre wear despite my all my efforts, so I'm going to get another alignment check soon, I will post the results when I get them.

bigs

Any follow-up?

deliriousga

Not yet. I had a HUGE scare Friday morning as I checked the TB before going to work. It was moving over in one spot, so I can't take her out or up to speed until I pull the covers and see why the belt is moving. It's looking like the belt has a bad spot in it since it's only a small part that tracks wrong once each belt rotation.

It will probably be another weekend as I had to do the 60K maintenance on the Boxster today (still have to find the darn spark plugs) and my daughter's 19th b-day is Sunday and the whole family will be here all day I'm sure. I will post my findings as soon as I get the TB taken care of.

deliriousga

My first investigation was the belly pans and spoiler. I checked them and there were several screws missing. I think the PO replaced several of them as there are different types, some are hex head and some phillips head and different sizes. It looks like they are all the same except for two 10mm head bolts that go in the middle of the front pan. Please let me know if this is wrong.

I got replacement screws that match the majority (hex head w/ 8mm heads) of the screws and replaced three with larger bolts. The holes were larger than the others on those (two went into the car, not speed nuts) so the old screws would not fit.

After getting it completely in place and tightened down, I checked the front spoiler again and all of the speed nuts I had replaced were still in and firm. Took her up to 80 and shaking began. Still the same all the way through.

I'm working on a retreat this weekend where I'm "guarding" the folks on the retreat at night (the retreat center is in a remote, rural area and we're basically the alarm system with cell phones) so I'll have all day Saturday and Sunday to do a few things and take her out for a few tests. I think I'm going to remove the bouncing flippable spoiler first and go from there.

AO

John-

I think you're best off to look at tires. I'll put 2 beers (at SITM) on tires. Swap 2 at a time with the boxter. Again, if the shaking is in the steering wheel, then it's most likely the fronts, otherwise start at the rears. Let us know what you find out.

deliriousga

I'll be checking the tires when I'm back home the next weekend. Since I'll be at a retreat center, I won't have the Boxster and I can't lift or work on the 928 in the parking lot. I have to take it around behind the house guards stay in. It's an all girls retreat weekend so we are not supposed to be seen by them. I'm hoping it's nice so I can start putting in the royal oxford wood grain kit from Jager I got for Christmas. All the little things I can't get done while I'm at home. It'll probably be the tires, so I'll pick those 2 brews up this fall at SITM if I can get there.

AO

John-
by accepting my offer, you do realize that I meant, if it is the tires you'll owe me 2 beers. If not, I owe you. Either way... LOL!