I and many others have shared your experience.  Put a rag on the cap
then grip with a large set of pliers.  Once you get the cap off I find
it helps to bend the two metal tabs on the bottom side of the cap down
just a little.  This will relieve some of the pressure (grip) so you
won't need a tool next time.

Of course you still want a little resistance so it won't vibrate
loose.  Adjust the tabs, replace, adjust,.... till it's just right.

Bob

use a tool... try a pair of  channel locks  ??

thanks to all for help.  unfortunately by the time I got home my son broke
off the grip on the top of the cap and i'll have to replace the whole thing.