2005 987 2.7 base model with automatic. Convertible. Car runs fine and is in great shape. Mostly driven in suburbs and to the Florida beaches and islands so I usually don't get it up to high speed very often. On a recent long distance trip on the Interstate I noticed (barely) there is a very light constant vibration above 70 mph that comes on for about 2 seconds and then stops for one or two seconds. Then repeats continuously. It is very quiet and almost unnoticeable but can be felt from under the seats and steering wheel. I am highly sensitive to anything odd or unusual so it bugs me and I'm wondering if something might be amiss in the drive train. I just got new tires and thought that would solve the mystery but the vibration is still there. Tire pressure is okay and double checked. 32psi front (a little more than normal) and 39psi rear. Any thoughts out there????? Thank you !
I don't have any idea what you're feeling/hearing but I would say your rear tire pressure is too high. I'm running 19" on a 2008 Cayman and Porsche calls for 32# front and 36# rear. However since I'm also in Florida, I run 31 and 35 and with our weather, they very quickly become higher, and at 39-40 # in the rear, the ride becomes pretty jarring. I wouldn't think of starting off at 39#. If you're running the more normal 18" tires, I'm not sure what the pressure should be but I am sure that 39# is too high since the pressure is supposed to be set on a cold tire.
Bud may be on to something here with that rear pressure. The recommended values are based on wheel size, and if I recall correctly, the 20" rims like I have run higher pressure than the smaller 18-19" sizes do.
The Porsche recommended cold settings on 20s is 33. Even that is too much for my liking, so I run the the lower comfort setting of 30 so that they settle at about 35-36 in the Summer once fully heated up.
39 just seems really high to me, even more crazy if that is the starting cold temp, and keep in mind those settings listed in the doorsill are the recommended values for tires if fully loaded. Its a max loaded value, not a starting point
Its easy enough one way or the other to experiment with a different value just to see if anything changes. I also start to notice some vibration in my tires when they start to get some wear on them and are getting close to needing replacement. The balance can start to go out a bit at certain speeds/harmonics if they are have uneven wear on them, and all goes back to normal once I have them changed out
Speed sensitive vibration is common with an unbalanced wheel as well.
Yep, typically a harmonic resonance of some sort if it only occurs at one speed but gets better at other speeds
Or in the worst case sometimes a wheel that got bent so slightly you didn't notice at first, rare but not unknown.
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Thank you Gentleman. Problem solved, lesson learned. I lowered the front tires to 29 psi and I lowered the rear tires to 35 psi and the problem went away. My recommended pressures are 30 front and 37 rear. When I had the problem...... The 30 front seemed too low to me so I added 2 pounds and put 32 in the front. Then I had a brain fart and added 2 pounds in the rear to make it 39 when I wanted to just leave it alone. I'm not a fan of different sizes of tires and pressures from front to rear but I guess I'll get use to it.
Great to hear it was an easy fix!
