Join Date: Feb 2011
Location: San Francisco Bay area
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You can try to take advantage of the difference in thermal expansion between plastic and aluminium.
The aluminium should be 20ppm/C and the plastic could be anything from 25 to 200 ppm/C. (Nylon is 50). That means that cooling the cap and deck fitting might free up the filler cap as the cap will shrink more than the aluminium part.
Try pouring some very cold, icy water over it and see if that frees it up. If you can cool the plastic part more than the aluminium, so much the better.