Hey I just bought a 2001 2500hd with the 6.0l, and my issue is when fueling it only fuel up about half way then the pump stops, alittle fuel cones out the overflow, has to go really slow to fuel the rest of the way. Seems like the tank vent isnt working? Has anyone had this problem and how do you fix such a thing. Thank you for your time.

 

Yes I can see this is a really old post. I have the same problem on my 2006 2500hd 4x4. It takes forever to fill the tank. I emptied the dust out of some plastic thing on the rear of the tank and put it back. I checked the fill hoses and neck on the tank and with the vent lines removed the tank seems to fill just fine. Does anyone know what the big plastic box way up against the bottom of the bed is? it has three vent hoses attached, and when removed to the bench, it seems as though one of the three stubs coming off the assembly does not allow much air to go through -- is it a filter????

 

First post: box is the vapor canistor. Second post: tank vent, sounds like ur on right track.

 

I found a strange looking plastic box the size of a cereal carton, black with three hoses attached, mounted to the underside of the bed, accessed easily after dropping the drive shaft. There is a photo of one, in a slightly different position, in the Haynes manual 24066, page 6-17.
I think it's called the evap canister (evaporative recovery canister) and it appears to be an integral component in the fuel tank filling system as it is designed to recover the fuel vapors in the tank and vent them to the engine for combustion. In the case of my truck, the canister has two separate compartments filled with activated carbon, prilled to about 1/16" diameter, with one compartment sealed with a spring loaded filter and both compartments having one filtered port.
Two of the three filters were completely filled with fine dust, and it was nearly impossible to get air to go through the entire sealed assembly.
It must be the reason why the fuel did not want to leave the gas nozzle, because the lip on the gas nozzle makes a seal with the filler neck, and the nozzle is designed to shut off when it detects fuel somehow, and I think that there is pressure in the tank and it tricks the nozzle into thinking the tank is full. ??? I don't know, only opinion.
Anyway, if you're reading this six years from now and don't have a lot of money to throw around, I was able to use a hand saw to cut through the factory seal at the top of the canister, open the canister, empty and clean and recharge the charcoal, clean the filters and canister, reload the whole mess, wrap galvanized wire around it in two locations to recompress and seal the assembly, and finally apply two beads of hi quality silicone in 24 hour intervals.
I put the whole thing back in and the gas tank problem is solved!
The whole operation took about two hours total.
Cheers.