John Veninger

I finally purchased a Cayenne last weekend. Flew into Pittsburgh and drove it back 6 hours and 375 miles. Everything was good but......something has gone wrong.
Here is what happens and maybe someone can shed some light on what the issue could be.

First the car:
2009 Turbo with 82K miles
305/35/22 on back and 285/35/22 on front w/ Pirelli Scorpion Zero Asimmetrico (9/32 on them). No TPMS in them.

Here is what I was told was done by an independent:
Feb. 2014 Complete Transmission oil filter and fluid change with all Genuine Porsche Parts.
Feb. 2014 Lower Front control arm was replaced
July 2014 Front and Rear Brake pads and sensors were changed and Brake fluid flushed using Motul RBF 660 Brake Fluid
July 2014 New plugs and Ignition Coils
July 2014 Coolant pipe O-rings replaced
July 2014 Thermostat was replaced

PROBLEM:
Was driving on highway and anything above 55 is starts to have a drivetrain "shutter". doesn't seem to be wheel/tire related.

Here is the interesting part. It I get off the gas and just touch the brake (just enough for it to sense it's touched and not engaging the brakes at all) it will stop. If I keep my foot on the gas and touch the brakes the same way it continues to shutter. This is why I don't consider it brake or wheel related.
Feels almost like a engine issue/transmission related issue.

Anyone ever come across something like this??????

Checked the cardan shaft and rubber looks good. No rip or tears.

I have a Durametrics on order and should be here Thursday.

One other strange activity. Yesterday morning the car was cold. I drive on a road that has several curves as soon as I leave the house. Doing 40 in a 30 mph area and the PSM kicks in hard. The motion and speed should not have triggered it. Later on when car is warmed up there is no PSM trigger on the same section. I even drove it harder and no PSM.
Repeated the same thing this morning. Clicked off the PSM and all was good. Don't know if it is at all related to the above. Turning PSM off at highway speed didn't change the shutter.

Thanks for any help or comments.

civerson4

Cayennes are REALLY sensitive to tire balancing. Find a good tire shop and have all four tires road forced balanced (I think most people recommend a Hunter machine). I went through similar issues years ago with an earlier Cayenne and it turned out to be a bad tire balance. I now insist on road force balancing on the Cayenne. $100-$125 is pretty cheap insurance to start digging into other things like the Cardan shaft, etc.

957austin

Before doing anything else, I would check the tires. You're running a staggered setup with different diameters between the front and back set of tires. From my experience, the Cayenne has a pretty sensitive AWD system that can have problems just when running brand new tires on one side and a worn out pair on the other. The difference in tire diameters from your staggered setup could be enough to cause problems to occur with the transfer case.

wrinkledpants

Is that a typo or are you running two different sized tires? The cayenne's AWD system is not calibrated to run different rolling diameters front and rear. Staggered wheels with same sized tire? Yes. Staggered tires with same rolling diameter? Yes. But, you have a 1/2 inch difference in diameter there!

John Veninger

I was also given a set of stock 21"wheels with 295's. I'll put them on this weekend and see if there is a difference.

wrinkledpants

Hopefully it's not too late

John Veninger

Too late???

wrinkledpants

No no - you should put your other wheels on now, and you need to replace one of the other sets of tires. You can't have one axle spinning faster or slower than the other. The reason your PSM keeps kicking on is that it thinks you are spinning one of the axles because they are rolling at different rates (like you're on a slippery surface). When you're at speed, the transfer case locks because both axles should be spinning at the same rate. If you have two different sized tires on the car, and the front axle is spinning faster than the rear - you can see how this will be a problem when the transfer case goes into lockup.

If you're already getting a bad shudder, my guess is either one of your diffs, or transfer case, has taken on significant wear, or is already failing.

I can't think of any AWD system where it would be OK to change the rolling diameter between the front and rear axle from something different than what the factory specs.

wrinkledpants

In fact - I would take this back to the dealer that sold it to you. Since this is kind of a big no-no, I'm guessing you didn't get it at a Porsche dealer. Used car lot?

John Veninger

I'll change them before driving it again. Tires are really new so hopefully not a lot of diff wear.
Private sale. PPI by Auto Palace in Pittsburg. Nothing mentioned about tire size.

User 4221

Don't get too worried. Change the tires/wheels and see if the behavior changes.

Having two different sizes would likely make the system more sensitive vehicle dynamics.

wrinkledpants

It would do that through throttle cut, PSM flashing, and the ABD system clamping down on the brakes. The vibration at speed is either a failing component in the driveline, or windup in the transfer case.

It's the equivalent of driving around on pavement with the t-case locked for off-road use. And, it's why the manual strictly forbids you from doing this on pavement as the driveline has no way of getting rid of the windup.

Doug_B_928

I bought a used Ford Escape that the PO had put on new rear tires that were what appeared to be a slightly larger diameter (I didn't know any better at the time). Well, the difference was significant such that I had to have the transmission rebuilt

John Veninger

Update:

Durametric came today. Scanned and no faults found.
Changed over to the stock wheel/tires and went for a drive.
All is good. Nice and smooth

wrinkledpants

Nice! I'd also suggest you change out the fluids in both diffs and the t-case. It's easy and fairly cheap.