Fuel tank vent question

So can someone tell me if the fuel cap vents why does the fuel tank neck vent also.

The reason i ask when i was doing a smoke test over the weekend the smoke was pouring out behind the gas cap. I thought it was odd for it to have 2 places to vent....i thought that a gas cap would be plenty for venting. OR is there something wrong with my fuel neck ? I removed it to see if there was any pitting from rust and it looks fine.

 

Year of vehicle?

03-04 and maybe 01-02 have a air vent here. It's about 18mm in diameter coming off the charcoal can that's under the bed. Pinch this hose closed Carefully during a smoke test.

 

The only time the tank should vent is with the Gas cap removed.

All other times it vents through the Evap system.

You have a leak!

 

Its an 03 , ok well ill try again with that line plugged . I guess i just wasnt understanding why it was vented twice. Gas cap vent is for tank and that line im plugging is for charcoal venting i guess. Thanks