Harvey Yancey

Hi Everyone,

I just had the left rear tire changed on my '05 Cayenne S (screw in the sidewall) and immediately after leaving the tire shop my PSM Warning Light came on and I had a complete loss of power (acccelerator to the floor ... nada). Once I turned PSM Off the vehicle started functioning again, so clearly the PSM was detecting something was awry. Later I turned the PSM back on and everything seems alright and the Pig started driving normally.

Since the initial episode the same thing has happened twice ... usually preceded by the vehicle getting 'stuck' in 3rd gear.

Bad sensor somwhere? Thoughts?

I have a call out to my local dealership to have it checked out but want to educate myself if this is a known issue.

TIA

touareg

I don't know anything and strictly guessing, but yeah I would say a speed sensor that detects the rotation of the wheel got messed up and is affecting the stability control adversely. Flickering of the PSM light in my experience means the wheel is slipping, such as would be the case in gravel, ice, water, etc when accelerating.

Harvey Yancey

Yeah ... my initial thought was a bad sensor since it was flickering in a straight line below 50 MPH under moderate acceleration.

PTEC

Believe it not it sounds like a bad air mass sensor to me. Its probably just a coincidence that it happened when leaving the tire shop. If you have access to a durametric check your air mass sensor values. When revving the engine you should be both sensor voltages go up and down nice and smoothly in sync, or maybe off by .1V. Its likely you have one that's not reading correctly.

Harvey Yancey

Interesting.

The Pig is at the shop now. I will pass along the info.

Harvey Yancey

UPDATE:

Apparently a variance (>30%) between the tread depth of the new tire and the old tire is enough to throw off the PSM in the Cayenne. However in my case, the variance in the tread is not that much, 8/32 on the old tires vs 10/32 for the new. The dealership offered no other solution other than getting yet another rear tire to match the other new one.

So another $275.00 down the drain.

Harvey Yancey

Found this on tirerack.com. Wish I had known this before installing the new tire.

http://www.tirerack.com/winter/tech/...jsp?techid=18&

Matching Tires By Shaving Them to Maintain Equivalent Tire Tread Depths

What does a driver do if one tire has to be removed from service when it and the other three tires have already worn to two-thirds to one-half of their original tread depth? Simply installing one new tire runs the risk of drivability problems or expensive driveline damage. Replacing the other three partially worn tires along with the damaged tire significantly increases the cost.

The Tire Rack can provide a solution by matching the tread depth of the replacement tire to the tread depth of the partially worn tires that will remain on the vehicle by removing tread rubber from a new tire on a specialized machine that operates as a tire lathe. While this may seem counterintuitive, the value of the mileage sacrificed by the one replacement tire is considerably less than the cost of rebuilding worn driveline components.

The Tire Rack has offered a tire shaving service that has been primarily used for preparing competition tires for racetrack use. This same service can also be used to remove tread rubber from new pairs or individual street tires used on four-wheel drive and all-wheel drive vehicles to allow them to match the remaining tread depth of the other partially worn tires that will remain on the vehicle. In addition to providing equivalent tread depth to eliminate driveline stress, shaved tires will also better match the traction and handling qualities of the remaining worn tires.

While the cost of our street tire shaving service will range from $25 to $35 for each tire, it is significantly less than the cost of unnecessarily replacing the remaining two or three good tires with lots of mileage still available from them.

Here are recommendations from some of the manufactures that the Tire Rack currently serves for matching the tires used on their four-wheel drive and all-wheel drive vehicles. Additional recommendations from other Original Equipment Vehicle Manufacturers is pending.

Audi As published in their vehicle owner's manual, "rolling radius of all 4 tires must remain the same" or within 4/32-inch of each other in remaining tread depth.

Porsche Cayenne within 30% of the other tire on the same axle's remaining treadwear.

Subaru Within 1/4-inch of tire circumference or about 2/32-inch of each other in remaining tread depth.

PTEC

Is your problem gone with the new tire?

Harvey Yancey

No it isn't.

I have found a business in town that will 'true' tires and I am hoping that they will be able to shave 2/32's off the new tire. I will update as this evolves.

PTEC

How often does this occur? The next time it starts to happen, pull the plug off one of your air mass sensors. If the problem clears up you've got a faulty air mass.

Harvey Yancey

Does it usually after the pig has been sitting for a while and when I go around the first corner leaving my house. I do not think it is the MAF as this started happening right after the tire change. Plus with all of the other info I have found since I am pretty confident that the shaving will correct the condition.

PTEC

If it fixes your problem I'll be surprised. I've seen a few Cayennes running around with tread depth differences of more than what you've got and they have shown no problems. I never say never though, so good luck!

Harvey Yancey

You'll be the first to know!

Harvey Yancey

Well ... the problem turned out to be much simpler than I had thought. Discount Tire put on the wrong size tire. 275/45R20 instead of 275/40R20 ... found this out during the shaving process.

The replacement tire will be here tomorrow.

Acid467

I have the same problem the psm light flashes and i lose power did you get it fix