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I noticed a tire was a little low and went to check the pressure. The valve cap had corroded to the stem. On all 4 wheels. Part of the valve stem sheared off when trying to remove the cap. I tried a penetrating lubricant, but I don't think it would have ever made a difference after inspecting the cap after it snapped off. Surprisingly, the tire remained inflated with half of the valve exposed.

The valve stem seems to be made of aluminium (light and non-magnetic). I'm not sure what the valve cap is made of, but it is slightly magnetic, so it's obviously dissimilar to the stem.

Part of the wheel around the valve stem had corroded badly enough that the tire shop couldn't get the valve stem to seal using a normal stem. Audi want $27 for the stem pictured below.

Annoyingly, the OE 18" wheels for the 4.2 seem to be orphaned. I went to the <a href="http://www.trademotion.com/partlocator/index.cfm?siteid=215021">Riverside parts site</a>, but I couldn't find the wheel or $27 stem.

The valve stem and cap look like this after removal:
<a href="http://picasaweb.google.com/enzeder/CarStuff/photo#5208079020451087586"><img src="http://lh3.ggpht.com/enzeder/SEbQRogmxOI/AAAAAAAACm8/MM9zg6kxj94/s144/IMG_0917.JPG" /></a>

Trango

...Mix aluminum, steel, and a little road salt or mag chloride (or transient environmental ions or electrolytes), and voila, you have a cathode, an anode, and metal salt creation.

Some stainles Steel is slightly magnetic, which is what your valve caps sound like being.

My best guess as to what happened.

cbons

corroded valve stem (due to TPMS) from the 17" twin-spokes this past winter. Resorted to lock-jaw pliers holding the inner part, square nose of these pushed right up against the inner rim. Impact wrench on front outer nut. We ripped apart the inner sleeve initially in trying to break loose the outer nut.

Used a normal valve stem (even had a faux chrome cosmetic outer sleeve) to replace it. I wouldn't think the 18's would be any different. This of course does remove the TPMS. The shop did hassle me about this, but I told them 'I'd' remove the sensors if they'd simply unmount the tire for me, and that 'the vehicle is equipped to disable this system' (ie. optional). Did kick them a couple 6-packs for their troubles, and they ended up doing it all for free.

So, it would appear some shops do fret about this due to US laws (something about not removing standard safety equipment).

quackroad

same here

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Are standard rubber stems ok? I'm not going to rouse the Audi-Gods am I?

want

Yes, standard rubber stems are ok. The original one is for tire pressure sensor.

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Great!!! Thanks for the reply - I feel a little better...

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$22 for that stupid valve stem...

derekpowell

at least they were on my wheels. I mistakenly got the 17" stems and had to exchange them.

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Got the 18" parts (parts guy asked what wheels I had).