whenever i go to fill up the tank, somethin happens. it acts like there is a huge air bubble or somethin in there, and it makes a gurgling sound and spits some of the gas up and onto the side of the truck. it does it real soon, like i can only put about 1/2 gallon in there, and then it does it a few more times. after 3 or 4 times of this, then it acts normal and lets me fill it up the rest of the way. anybody know what could be causing this????