EVAP problem

I dont remember the code number but my 97 aurora throws an EVAP code. It was smoked out and noticed no leaks... but i could have told the technician that. The problem is when i fill the tank full, gas shoots out of the inlet, then the pump finally shuts off and you can listen as the gas gargles down into the tank... whats the problem here? the students at the Tech College dont know whats going on neither do i but i havent checked it out that much...