Off/on Vibration above 70
 

I have a new to me 07 Boxster base, 58k miles. The PO put new Michelins on the car recently.

When I am driving above 70 mph, I get a quick shake, felt in the seat not the steering wheel, then back to smooth. Cycle repeats at about a 1 to 2 sec interval.

Looking around the net, sound like maybe a motor mount, since it seems to be rear end problem. Raising the spoiler earlier than 75 does not seem to make a difference. Speeding up, the cycle does not noticeably change in frequency. Top up or down does not seam to make any different either. Since it is not continuous I assume the tires are probably not the issue.

During the oil change I did not see any obvious issues with any suspension problems, but that does not mean the old rubber bushings needs replacing.

Any thought on what I need to look at.

Thanks.

Try this….
 

….Loosen all the lug bolts and then re-torque them.

Perhaps rebalance the wheels/tires.

I had a similar issue, for me it was rear toe, when was the last alignment?

+1

Since it's not in the steering wheel, chances are a rear tire is out of balance. Just bought a new set of tires and trip to Dallas was my 1st test drive. Now have a vibration at 70+ in the steering wheel that wasn't there with my worn out tires. Grrr. Will be going back for a rebalance when I get home.

FYI....although it's a rarity, some tires are defective by being manufactured "out of balance". If that's the case it's almost impossible to balance them and they should be returned for replacement.

Years ago, a name brand, steel belted radial on one of my cars was impossible to balance correctly.......the reason: some of the steel belting "shifted" either while in use or during manufacturing.......it was replaced (at no cost) and I never had the problem again.

I’ve had wheel weights fall off within a couple days of installation and had to take them back

Had some extra time while visiting, so I had a Discount Tire in Plano rebalance my tires. Vibration gone and trip back (other than the snow in Wichita) was great. When I was pulling the car out of DT, I asked the tech mgr if there was a tire out of balance. He said all 4 were!

Been chasing a vibration in the shark. Thought is might be worn out parts and have been parts exchanging chasing the vibration. I didn't start driving the shark a lot until I had the timing belt replaced, which was after I'd bought new tires (same Bridgestones as I bought on the CSS) from the same Discount Tire. Will be going to a different DT for a rebalance since it very well could be they didn't get them balanced correctly to begin with. Occam's razor.

I would make sure they are doing "road force" balancing and at the some time I would have them check to make sure the rims are all straight. When I got my 911 I noticed a very slight shake at high speeds, and it turns out all four rims were out of round. Got them fixed and now the car is smooth as silk. Had the same issue with the 2001 Boxster S

I chased the same problem for a long tome. Turrned out to be bent wheel.

Were you able to figure out the cause?

How about worn suspension/weak shock absorbers?