I believe someone may have messed with it, possibly put glue or sealant on the threads. No matter how I turn it, it clicks. I don't want to ruin my fill neck. Is my next step to crush the cap with channel locks?

 

id probably screw a screw through it about a half inch from the center.

 

This should lock the cap together as one piece and stop the clicking. If need be then I would carefully use channel lock pliers. I would try to keep from crushing the cap as that could put pieces of the cap in the tank. Though I would think it would be unlikely those pieces might eventually damage the fuel pump pick up screen or 'sock'.

 

Thanks guys. Drilled two shallow holes near eachother and put screwdrivers in each hole. Worked great.

 

For future reference, this shouldn't be an issue since the filler neck has the spring loaded door to keep foreign bodies out of the tank.