The Stovebolt Site Forums The Shop Area General Truck Talk Vent tubes in gas filler necks

Some gas filler necks like on an AD has the vent tube routed to the filler neck a few inches behind the cap. I don't see how that can vent anymore than the filler neck itself. Can someone school me on that?

Dennis

the vent tube allows air to exit while you're filling the tank to avoid causing 'splash back', the vent hole in the cap is to allow for expansion/contraction as ambient temperature changes, which causes air movement up/down the filler

Bill

Thanks Bill. I was looking at a filler neck (generic, not GM) fuel neck and vent that fits to the tank and has a hose barb for the vent hose. That got me to wondering how these things work.

Dennis

The vent on the "generic neck" (linked above) is not for filling specifically, but rather to allow air to enter/leave the fuel tank. I.e. expel air due to expansion in hot weather (or during filling) or allow air to enter the tank as fuel is used or the tank/fuel cools down. The reason this is required is that the cap for the neck is billet aluminum without a vent (hole). I imagine this type of setup must be filled with fuel slowly.

Normal gas caps are self venting (this vintage anyway). The vent tube in the AD factory tank is for filling purposes only and will only work properly if the gas nozzle is inserted in the fill neck past where the vent pipe connects to the filler pipe.

My $.02