Pontiac Repair: fueling causing problems!, jug of water, vent valve

Question
I just bought a used 03 grand prix gt with the 3800 engine in it. Fuel gauge was almost at the "E" mark. After 3 to 5 seconds of fueling the nuzzle jumps like if the gas tank its full. I kept fueling slowly and it started spilling it back out. I was only able to fill it above half tank. When I started the car it started having problems trying to stay running. It would die or the rpms would jump up and down. What surprises me is that as soon as the gas level is weeeell below half full, the problem stops and runs like new! So rite now I'm stuck with only being able to fuel 3 to 4 gallons at a time. I would really apreciate any suggestions. Thank you.

Answer
Hi Cesar:

there is an issue with the evap vent system either the vent valve is stuck closed or the valve on the top of the gas tank is bad. there is a valve on the top of the tank that vents the air out of the tank and into the charcol canister. If the air can't excape from the gas tank it will not fill. It's like trying to fill a gallong jug of water when the jug is right up against the faucet it will not fill quickly. same thing with the gas pump the back pressure cause by the air comming back up the filler pipe cause fule station pump to kick off. so something in the evap system on you car is stuck or blocked and what's happening is the fule vapors are backing up inside the hoses and pipes and when you start the car up it runs poorly due to this.