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It's a well known problem that once the TJ's engine oil fill cap is hot, unscrewing it from the valve cover is difficult at the very least. I recently broke my own rule about changing the oil when the engine was hot....but I basically had no choice. I snapped a couple of pics of the filler cap when it decided not to come off in the usual manner.
The first thing that happens is that while you attempt to unscrew it, it starts to click and ratchet and will not release itself from the valve cover. When this occurs, pry the top of the cap off. I used a screw driver to get a little leverage. It will separate into two pieces. Pry it off and set it aside.
With the "top half" of the cover removed, you can now grab a pair of pliers and carefully unscrew the "lower half" of the filler cap from the valve cover.
Success! With this half removed, you can snap the two pieces back together. Note, you will notice that the teeth in the upper half are probably a bit rounded off and it may not work quite as well as it once did. If this becomes a problem, pick up a new one at the auto parts store or the dealership.