I got some nifty-looking aluminium valve caps sometime last year, and they’ve been left on my car all winter. Unfortunately, I can no longer get them off. 
I’ve tried pliers (the stem just turns), oil, and heating them with a lighter.
Reading around, a lot of other people have had this problem, but nobody’s come up with a solution.
A person could carefully use a fine cut-off wheel and slice the thread area until they would come off. I would do a little bit at a time and pour water on it before continuing. Then switch to a plastic cap.
You may be stuck getting the tire stems replaced if the metals have welded together.
I think Albundy has the idea. You can buy a cut off wheel for a Dremmel. You don’t need to cut into the threads of the stem Just get it started and then finish it with a screwdriver.
Try using another pair of pliers to keep the stem from turning by holding it lightly just below the threads while turning the cap.
Take two pairs of vice grips, a hammer and a can of penetrating oil. Have the valve in a position where the oil will run into the threads, so between 10 and 2 o’clock, and apply small amounts of lube to the join. Give it 5 minutes to work its way in, then clamp the vice grips good and tight (though not so tight that you squash the valve) on the valve and cap, with minimum distance between them, placed so as to hold both in one hand and undo the cap by squeezing them past each other, whilst lightly tapping the grips and cap with the hammer. If this fails to work, try tightening the cap the break the corrosion before slackening, and put plastic valve caps on in future.
…Or have them replace the valve stems when you go in to have the tires balanced.
That’s an expensive solution around here as they have to remove the tire and remount it to get a stem in. That would run at least $6 per wheel with the balancing on top of that. Now if the tires are fairly new and the same shop is responsible for installing those aluminum caps, we’d have something to talk about. And perhaps a tire shop would have a technique that works pretty well in removing them to begin with. Maybe they could spray the valve with nitrogen or AC refrigerant to break the bond. I’d consider it a personal challenge and would never take it to a shop. That’s just me.
Try spraying with Liquid Wrench. Let soak in for a long time (several hours) before trying to remove.
Drill into the top of the cap and spray some shit in from that end.
The tire stem is actually a solid brass piece covered in rubber. use pliers on the upper end to get a good grim. The problem with aluminum caps is that road salt and water act on them against the brass valve. Tape some paper towel to the top of the valve and soak it with penetrating oil. That allows the oil to work itself in and keeps it off the valve stem.
My first thought when hearing the word “Dremel” was: cut a slot in the top of the cap with the Dremel, then hold the stem still with pliers and undo the cap with a flathead screwdriver in the slot you cut.
Thanks for the suggestions. I tried more oil, but it wasn’t happening, so I split them down the side with a dremel, torqued them open with a flathead screwdriver, and could then unscrew them. (The only other thing I can think of that I didn’t do was drip an upside-down canned air on them).
Taking them off, you can see that maybe 1/3 of the thread was corroded, which explains why they were seized to the stem.
If anyone is considering using such caps, I’d highly suggest teflon, or (as some other board suggested), maybe vaseline or some oil on the threads. I’m going to switch back to plastic myself. 
It happen to me too. I had those stupid aluminum caps i tried double pliers, heating it and nothing just ended up spinning the valve stem. Finally had to use the Dremel…
The other option is tireshop they will charge you like a tire installation.
http://fdo-workspace.blogspot.com/2011/11/aluminum-wheel-cap-nightmare-removing.html
