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Bad vibration at around 70mph

Hi,

I've had a pretty bad vibration when driving my 02 Boxter S at 70mph or above. During the time I've had the car I had the tires that came on it balanced with no improvement. I've also replaced those Pirelli P Zero's with Michelin's all around and of course they were balanced and the car got a front end alignment. Again there was no improvement. Before I crawl under the car to take a look I was wondering if there are any more likely problems that could cause this issue? To me it doesn't really feel like a tire balance issue. I also wonder the odds of two separate places getting the balancing of the tires wrong so it vibrates at the same exact speed. Any suggestions would be appreciated!

Jeff
Old 05-21-2012, 04:15 AM
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Get a four wheel alignment at a quality shop and have them check the entire suspension for any worn components that might be the cause.
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Old 05-21-2012, 12:00 PM
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Does the vibration continue all the time after 70 mph? Why I ask is that a bad front motor mount will cause a vibration at around 3k rpm which may be at about 70 depending. It clears up as you get farther past 3k so you may want to see if the vibration is associated with a certain rpm.

Steve
Old 05-21-2012, 01:02 PM
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That's interesting. I have felt that it wasn't quite as bad when I got going a lot faster.....say 85 or 90. In other words it didn't seem to get progressively worse. It never really goes completely away though. This is a sign of the motor mount issue? I thought the stick shift jerking would also occur though I can't quite get my arms around how that will make a stick that uses cables jerk.

Jeff
Old 05-21-2012, 04:00 PM
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The gear shifter mounting points are in a fixed location. If the motor mounts are bad, the engine moves. If the engine moves, the transmission moves. If the transmission moves, the gear selector cables change positions. If the gear selector cables change positions, it moves the gear shifter in relation to it's fixed position (mounting points).
Old 05-21-2012, 04:21 PM
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I had a similar issue with my 2001. After replacing the control arms on the front it got a little better. then I took it to a shop that can do a "Road Force Balance" & had the alignment done. Problem solved, I had the car up to 100 yesterday & it was perfect. Look on this website Hunter GSP9700 Wheel vibration Control System solves wheel vibration and tire pull problems that balancers and aligners can't fix for a shop that has this equipment. I paid $112.00 for the balance job, well worth it!!
Old 05-21-2012, 04:42 PM
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Makes sense. I haven't noticed the shifter moving though. One last question about the motor mount possibility. Shouldn't I also feel the vibration when going through the gears when I take it up to 4k rpm say?

I know I need to take it and have the suspension checked for bad bushings. I just want to try and look for anything I can before resorting to outside help. I'm pretty tight on funds at the moment. The place I took it to for an alignment is one I've used for years on all of my vehicles. I can't imagine they didn't do a great job as usual but you never know I guess. If I were to go under to check for something obvious with the bushings do I do that with the car on ramps with the tires supported?

Jeff
Old 05-22-2012, 02:57 AM
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If the front engine mount has never been replaced install a new 987 mount before wasting time on anything else. Wheel alignment has nothing to do with ride disturbance issues. Have you set your tire pressure to Porsche specs?
Old 05-22-2012, 10:28 AM
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How many miles on the car?
Old 05-22-2012, 10:54 AM
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If that's the original motor mount there's no way it isn't bad. The early design had more voids in the rubber bushing and it doesn't last well at all. The 987 mount is more solid. The mount on my 2000 base was bad and caused a vibration around 3200k. It's a fairly simple weekend job for a DIY type and the part isn't too expensive.

Steve
Old 05-22-2012, 12:49 PM
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Probably tires

Do a search on the sister site 986forum.com. Search for "tire problem". I fought this issue for months. Was told it was bent rim. Had tires road force balanced 2 times before I found a person that knew what they were doing. Mine was at 70 mph and above. Could be other issues but sort this out before you spend money chasing a ghost.
Old 05-22-2012, 02:07 PM
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Tires set to 29 and 36 psi per Porsche. The car has 61k miles on it. I have no idea if it's the original motor mount but I have never felt the stick move even the slightest bit. I think I'll get up to where it's vibrating and push the clutch in, let the engine idle and see what happens. If it doesn't stop I imagine it's in the chassis and if it does more likely the motor mount.....seem reasonable?

I'll check for a Road Force Balancer in my area. That sounds cheapest to do for a start. I had to look it up....never heard of it before. I have heard that these cars are real particular on wheel balancing before.

Thanks,

Jeff
Old 05-22-2012, 02:28 PM
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I wouldn't assume that a bad motor mount will be entirely evident in the stick shift. My mount was totally shot but I "thought" shifting was fine. Seemed fine anyway. I replaced it for the vibration problem. The new mount made shifting much smoother but only after replacing it did I realize the difference. Not sure how to test the mount. My guess is at your mileage its original and shot.

Steve
Old 05-22-2012, 04:13 PM
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A lot of shops actually has the Hunter equipment. And, you don't have to spend a lot for a balance, either. I paid $60 for a lifetime balance at my local Firestone. And they did it on the Hunter machine. Without the lifetime balance, it would have only been $40. So, don't spend a lot. Shop around. And, if it's not much more, consider the lifetime balance option.
Old 05-22-2012, 04:17 PM
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There is nothing wrong with the tires so if you want to pursue the balance theory why not go back to the place that you already paid for that service? No balance machine can fix a bent wheel, but a good technician can compensate for it on any properly calibrated balancer.
Old 05-22-2012, 04:35 PM
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I took the car out on the highway and got it up to where the vibration was really bad and pushed the clutch in. The vibration didn't change in the least so I think that rules out the motor mount.

I could take it back to where I had the tires balanced but it sounds like the Road Force machine is more geared to finding issues like a bent wheel than the standard one.

Jeff
Old 05-25-2012, 03:28 AM
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Quote:
Originally Posted by Jeff396 View Post
I took the car out on the highway and got it up to where the vibration was really bad and pushed the clutch in. The vibration didn't change in the least so I think that rules out the motor mount.

I could take it back to where I had the tires balanced but it sounds like the Road Force machine is more geared to finding issues like a bent wheel than the standard one.

Jeff
I use my left eye (that's the dominant one) & a popscicle stick to verify a bent wheel on any wheel balancer.
Old 05-25-2012, 10:12 AM
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All you do to detect a bent wheel is spin the tire and hold something at a constant distance away from the rim moving it in until it almost touches. It the rim only touches the object intermittently, bent. If even and constant, not. Low-tech but effective. Detected 3 rims on the wife's car that way...amazing difference. But I could feel the vibration when applying the brakes and not just at speed.
Old 05-25-2012, 01:01 PM
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My bad rear wheel bearings created the appearance of a bent rim by the above analysis. The wobble went away with new bearings, and so did the most horrendous din.
Old 05-27-2012, 06:40 PM
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Hi all,

din?

I'll give the wheel bearings a look before heading off to have the tires balanced again. Yesterday I thought I felt a vibration at lower speeds also but it's hard for me to separate out road feel from a vibration until it gets really bad. I still need to sort out a CEL I'm getting after I had to remove the primary cats and install a set of the catless headers so it'll be up on stands anyways. It's funny, I like working on the car but I don't like putting it up on jack stands.......it's a pain.

Thanks,

Jeff
Old 05-28-2012, 05:19 AM
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Old 05-28-2012, 05:19 AM
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