Problem filling gas tank - charcoal canister or purge valve?

[jpilk99]

I have what seems to be a fairly common issue and that is, when trying to fill up at the gas station, the pump constantly 'clicks' off.

After reading a bunch of stuff on this site and others, I can't tell if I should be chasing:

1. Charcoal canister, under rear of vehicle next to gas tank and fuel pump, and the hose between it and the filling spout; or
2. Purge valve at front of drivers' side of engine

And, is there something plugging the vent hose opening in this picture? That white plastic plug/cap at about 2:00 in the picture? I'm finding more and more that the previous owner did some ...pretty ridiculous things to this vehicle.

If anyone can help guide me in the right direction, I'd be most appreciative. thanks,

Jay