Get back to the pressure reference on the gas tank. Not being able to fill it properly. I am surmising you think there is pressure keeping it from filling, not an obstruction.
For grins, does it make any difference when you run the engine without a gas cap?
How many lines are coming from the gas tank - one for fuel, a vent line and do you have another going to some sort of canister in the engine compartment.
I am with cstephens, start by simplifying the environment.