When I fill the tank at a reasonable speed, the gas overflow/vent tube overflows. I took the wheel well cover off and pumped to find it spewing from there. In order to keep all of the gasoline that I pump, I have to pump slowly and intermittently. I took the filler neck tube off. It has no obstructions. There is some type of box that connects to the back of the has tank that also connects to the vent tube. It appears to have some sort of solenoid between it and the vent tube. Does anyone know what this box is and if out may be the problem? I presented this to the chevy dealership without paying them to look at it. They say they've never heard of that before.

 

that should be the canister...sounds like your vent tube is working well