Blasted Leaky TPMS Valve Stems

Does anyone in this forum have any inking as to the root cause of leaky TPMS valve stems? They are driving me nuts.

I probably have experienced 8 stem leaks in three years, half them old OEM stems and the other half previously repaired with new Schrader p/n 20032 stem rebuild kits.

I have two Gen II Prii, both with odometers at 185k+ miles, both purchased new. In the first 100k miles neither car had leaky valve stems.

I keep in my garage a nifty Harbor Freight tool that breaks the bead on tires, so I can address these leaky stem failures myself. Tire shops refuse to deal with the problem. They love to sell $130 tires, but not fix leaky stems.

The stems generally leak through the threads at the top of the lock nut, occasionally at the interface between the lock nut and the rim. These leaks aren't "whistlers". I lose about 5 psi per week.

The TPMS sensor itself plays no role in this dilemma. On Prius sensors the TPMS electronic module is a separate subassembly bolted to the bottom of the metallic valve stem. It does not participating in the pneumatic seal. I could omit the electronic module entirely and the stems would still leak. So the vendor for the TPMS sensors is not relevant.

However, it is true that I have been buying Dorman replacement sensors when the batteries in the factory sensors die.

While I have my hand inside the tire, I wipe down the rim in the area of the valve stem. I never detect corrosion here. I always replace the OEM rubber grommet with a new one from the Schrader rebuild kits. I torque the lock nut down to the 35 inch-pounds printed on the Schrader rebuild kit.

Recently I have started lube'ing the threads and grommets with some quasi-liquid stuff called Nylog Blue that HVAC guys use on their hose joints to get tight evacuations when they pump down refrigeration gear. Nylog is a clear, thick syrupy goop that never dries. It seems to help a little, but I've even observed some of the Nylog'ed stems leaking within a couple of months.

Any thoughts you might share with me on this are appreciated.

FWIW, I have observed that the rubber grommet begins to extrude/flow and deviate from its normal profile when you exceed the torque spec even a little bit. I've wondered if the slippery Nylog is lowering the torque threshold at which the grommet begins to extrude.

My gut instinct is that there isn't enough "meat" in the rubber grommet given its importance, but they seemed to work fine for the first 100k miles of the cars' lives.

Jim Olson
Indianapolis, IN