Let your engine get warm, and while at operating temps, try loosening it then. My car has done this a few times to me. Warming the engine up def helps...ALOT!
I'd recomend cleaning it with a non-harsh chemical. That means no brake cleaner. Use Seafoam if you have it and make sure you lubricate the cap seal with some fresh oil.
Ohh, and clean the filler neck out too. It's prob gunked all up with a film of semi-slimy/hardened parrafin wax.
Last edited by dirtydog; 05-26-2012 at 08:57 AM.