I don't have a specific problem at the present, but want to avoid one, once my car is complete. A roll-over vent is nothing but an open hose and it will stink, unless a carbon canister is used or the end of hose valved-off when the car is not in use.

I once had a fill-through gas cap from Tanks, combined with a short filler neck and it did not work at all. The fill-through hinged opening was wet with splash, after every ride. The folks at tanks denied that these gas caps had a problem, but offered to sell me another with no freight. I tried that and the second one worked just like the first. That's when I changed to an external filler cap mounted high, on the deck ahead of the trunk lid. Fill-through caps, if built right work great. I have two relatively new cars that came with them from the factory. Those are totally sealed and must be vacuum tight to work with a EVAP system.