Duramax fuel system question

[okeehandyman]

On a '05 2500HD D/A truck's fuel fill, there is the large hose for fuel into the tank, a real small (about 1/8 inch)tube that i think is for a tank vent. There is one other hose that is about 1/2 or 5/8 inch that appears to go to some sort of vent into the fill tube. What is that 1/2 inch tube for? Is it for air to get out of the tank when fueling? Any help will be appreciated.

[More Power]

Just a guess, but the '05 trucks may need to capture excaping gaseous hydrocarbons from the tank, like what the gas rigs have been doing for decades as part of the emissions control system. Up to this point in time, the fuel tank cap was vented (holding up to 2 psi pressure to 2" vacuum).

The middle size hose is the vent used while filling.

MP

[tpitt]

I think if you look real close you'll find that the small one is the vent for the rearend. The 5/8 is the actual vent. tpitt