Lately it seems that when I run either of my trucks down on fuel from a full tank and I open the cap to refuel It starts venting. Im not sure if its in or out. In 50K miles on the 93 I never noticed this. Ive swapped caps on trucks and it still does it with either? Am I crazy and never noticed it before or? It would seem if the tank isnt venting right it would starve the LP/IP for fuel. any thoughts on this one??

 

I know the fuel tank has a vent hose on it towards the front, but I never followed it to see where it goes. I think it just goes up into the crossmember behind the cab. It will have a check valve on the tank to close off in the event of a roll over. The cap should not be vented. The gassers will have a vent hose that goes into a charcoal canister for evaporative emmissions control. These tanks should not build pressure or vacuum.