s4r

So here is the setup: I have a 2008 C2S cab still have the OEM Pirelli tires. Wednesday night on the way home from work I get the warning about low air in the drivers side rear tire. Wait until about 10 PM when the tires cool off and set the right pressure in all four. The drivers side rear was down to 35 psi, so I set it to 41. Monitor for a couple days and it stays right at 41. This involves just commuting 6 miles to work on city streets.

On Saturday, it is finally rather hot here about 90 degrees. I jump in the car to run some errands and am on the interstate cruising at 70 or so and the Flat Tyre! warning comes on, I pull off and into a gas station and put a bunch of air into it up to about 43 psi. Drive on city streets carefully home and after it cools down set the tire pressure to 41 again. Check it that night and again in the morning and it is fine.

On Sunday, go for a drive to a friends house about 15 miles away (watching the TPMS computer carefully the whole way), and it stays right on 41. On the way back take a detour with some spirited curves and the warning comes on. Smartly, I have brought along my portable air tank and fill it up. Stop for a burger and come out and the pressure is holding. So I jump on the interstate and within 1 mile of hitting 70, the flat tyre warning comes on and the TPMS air pressure is down to 34 psi.

I used my accu-tire guage to check the pressure every time and the air really is down - precluding a faulty TPMS sensor. So the question is:

What would cause the tire to hold pressure until you hit interstate speeds and then rapidly deflate?

Le Chef

Check side walls - inner and outer - for a small tear.

Carrera Mike

Take your wheel/tire off the car, lay it on its inboard side and pour soap water solution along its lip/rim, on the valve and around the jam nut of the valve. Some leaks are so minute that it will take few minutes to see small bubbles. Do the inboard side if needed.

If Im a betting man, the o-ring for the valve is pinched.

gota911

As you stated, you obviously have a leak. It could be a nail in the tire, or a bad valve stem, or ???? One way or another you are going to end up at the tire shop, so you might as well take it there and have them diagnose the problem, and if possible, patch the tire.

Edgy01

This is where I never question the TPMS. (That's why we have it--for the naysayers). You have a certain leak and you need to remove the wheel, get a bunch of dishwashing soap and some water and start your hunt. It's there. Don't forget to check the valve,--but it should be okay.

s4r

And the answer is:...........a loose and leaking valve stem. The valve stem on the outside of the wheel has a nut that holds it on and that was loose. When you pushed on the stem it leaked. Simply tightening it up solved the problem.

No charge from Garrard tire in Charlotte. Probably the only free thing that will ever happen to the Porsche.