Any ideas what this could be?? - 986 Forum - The Community for Porsche Boxster & Cayman Owners

Was driving back from a softball game yesterday on the freeway. Cruising at 80 and all of a sudden a vibration/shuddering starting happening with the car. It wasn't the road surface and I stopped and checked the tires and they were ok. Thought maybe I had a flat. Anyhow got back on the road and took the first exit, which was within a few hundred feet and didn't feel it again. Of course I wasn't doing 80 again either. It didn't feel like the engine misfiring. Got home and checked the tires which are new and made sure the balance weights were still there. The only things I've done recently were installing new spark plugs, tubes and seals about 2 months ago. Would worn strut tower bearing plates cause something like that? Car was fine on the trip to the ball fields. Any ideas?

engine or Trans mounts..may have to go 80 again to duplicate the vibration any new play in the shifter ?

Were you decelerating or accelerating? 80mph on a S with a 6spd trans gets you about 3000rpm, which is usually when the engine vibration from the motor or transmission mounts start vibrating when it is decelerating. I bet if you go about 85 and let off the gas to decelerate, the vibration will come back right around 80mph and stop somewhere below 77mph. It does that with mine. Flmont is right it could be engine and trans mounts. Or the coil packs might be going bad, it didn’t say you replaced them, do you know how many miles are on the existing ones? I’m still trying to figure that vibration or shudder at 3000 rpm (on any gear) in my car. I’ve replaced the motor and trans mounts, spark plugs, tubes and coil packs. Some people here say the motor has natural harmonics at that engine speed. LMK if you figure it out as well.

I'm the second owner. Had no maintenance records when I purchased 7 years ago. I'm thinking the mounts are all original. When I replaced the plugs and tubes I did examine the coil packs and all were good. First time I ever experienced this vibration at any speed. When it happened I was going a steady 82mph. No excessive play at the shifter. I'll look at getting the tranny and engine mounts replaced 1st.

If you dont have any pending codes then it wont be the coils/plugs. vibration at speed is going to almost always going to be something physical. the most common is going to be a tire, especially at speed. Can you tell if it is front or back? Did the steering wheel also shake/vibrate? Feel it in your butt?

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The tranny mounts are oil filled so if they are not leaking oil they are most likely fine. The center mount (engine mount) on the other hand has a rubber center that is certainly shot after 20 years. If it is shot and you are in tune with your car you can feel the engine rotate in a front to back or back to front manner when slowly letting the clutch out. The shift lever will sometimes move in a forward to back manner as well.But not always. Or if you slowly go over a speed bump you can feel the play in the mount, sort of a slight forward to back rock of the engine. Sometimes a little clunk or thump. As for your vibration at speed check the bolts that mount the drive axles to the trans for tightness. Six per side I think. I had a similar thing happen but at around 75 MPH but I could feel it in the steering wheel. All new Michelin P4 tires. Took road-force balancing the tires to sort it out. Regular balance method didn't cut it.

Took the car out on the freeway yesterday and could reproduce the vibration. Seeing as my Box is 20 years old I'm going to replace the tranny and motor mounts.