I use the small oil filter wrench that I use for removing the Fiero oil filter. Perfect fit. Not the cup style, but the kind with a handle. The V6 hose clamp is situated such that it's like a scalpel and will slice a hand open as handily. Just put the clamp around the cap, turn it with one hand and press down with the other. It doesn't scar up the cap like a pipe wrench or large channel locks.
Thanks everybody. I will try your ideas and see what I come up with.
I love this forum. Everyone is usually so very helpful. When I ask a simple question on the Audi forums, everyone goes out of their way to try and make me (and others) out to be stupid. Then they get all defensive when you call them out.
Replace it with a new one, I put a thin wipe of grease on the new seal to help remove it next time.
That shouldn't be necessary with a new cap. I'm not sure about all brands, but it appears that the seal on the old (AC?) caps was firmly attached to the cap. In other words, when you turned the cap the rubber seal had no choice but to twist on the face of the thermostat housing. We all know how difficult it is to remove one of those damn caps if it's been on there for a long time. However, the seal on the new style caps spins independently from the cap. This makes it a helluva lot easier to remove one of these caps, as even if the rubber seal has bonded to the thermostat housing, the cap itself will still turn and release.
[This message has been edited by Patrick (edited 04-10-2014).]
I second the utility of channel locks for this purpose. I ended up buying a larger set of them on a road trip where I had some trouble with possible overheating issues. It turned out to be a bad sensor but I had the cap on and off at least five times.