Hi all, I have been chasing a resonate vibration in my 2005 C4S Cabriolet 70k miles for about 2 years now. It has really taken the joy out of driving this beautiful car. The vibration starts low and builds in intensity up and down in a one second frequency. It is felt in the steering wheel and the passenger seat shakes. I have tried different 18" OEM rims, 19" rims, 20" rims, new tires, engine mounts, new serpentine belt, road force balance, new upper and lower control arms, replaced front, rear differential and transmission fluid, had a wheel shop ream out the rims and put hub centric rings for a more exact hub and rim fit. Any insights to what this might be would be much appreciated.

 

Are all the hubs good? I note you have changed a lot of things but the hubs have remained unchanged.

Have any of the half shafts been off? Whenever one is removed it wants to be marked before hand so it can be installed in the same orientation as before. If it installed in a different orientation this can introduce a vibration under certain conditions.

Is braking action smooth and shudder/pulse free? Has anyone checked the axial run out of the brake rotors?

Does this vibration occur on all road surfaces? I have found some road surfaces seem to affect my cars more than other surfaces. Most often in making me believe a tire is low but at other times on other surfaces the car, the tires, sing or moan.

 

Hi Macster, I don't know about all the hubs, I would assume so.... but, there is danger in assuming. Braking is straight and true. As far as the half shafts. The car was in a fender bender before I acquired it. So, I have no idea if they were removed, replaced or? I'll have to see about replacing the shafts. I do all the work on her so I do know the stuff I have done has been correct. Thank you for pointing me in a good direction.

 

You don't have to replace the half shafts unless you find a problem with the CV bearings or a shaft is bent.

What I was referring to is if the shaft is removed it wants to be installed exactly like it was before.

The half shafts were disconnected from the transmission of my Boxster -- this years ago -- for an RMS job -- and they must have been reconnected correctly as the car was vibration free before the work and after. Might add years later when the CV boots eventually gave up the ghost and I had the shafts removed and the bearings cleaned, inspected when found to be ok repacked, the Porsche tech reported that all the fastener heads were nearly stripped (I think the fasteners are Torx headed bolts) and he had a heck of a time removing the fasteners. Seems the tech at the place (the dealer where I bought the car new) that did the work when I had the car in for the RMS really hosed the fasteners. But at least he got the half shafts back on right.

Or maybe it doesn't make any difference? Maybe Porsche half shafts are of such quality they have no difference and be installed and it not make any difference.

It just occurred to me you never mentioned alignment. A proper alignment is critical. 'course, an improper alignment should (but no guarantee) make itself know by accelerated or uneven tire wear.

In my 996 Turbo one night I pulled just a bit too far into a parking space and the front tires came up against the low sidewalk edge. Fearing the worse I drove the car around some and was relieved the car felt ok.

The the next day I continued some several hundred miles to my final destination. When I left there to head home after some hundreds of miles of further driving the car had developed a noticeable howl at highway speed that was severe enough I drove 150 miles out of my way to the OKC Porsche dealer to have the car checked out thinking a wheel bearing was bad.

Turned out the problem was the front tires were out of alignment. While the car steered straight after the tech pointed it out to me I could feel the feathering/cupping of the tire tread blocks. He told me I could drive it like this for the approx. 1500 miles I had to go to get home with no problem -- other than the noise.

Once home I had the car aligned and after a few hundred miles to scrub the tires in adapt them to the new alignment the noise/howl was gone.

Just a bit of a long story to highlight in some fashion how important proper alignment is.

Be sure to get a *before* and *after* printout. Often these show the desired settings/values and the acceptable ranges and the actual values/settings but you might want to arm yourself with the proper settings for your car beforehand so you can go over the after settings and compare to make sure all are within spec.