I have a 92 F-250 with a 460 with dual tanks. Every time I try to put gas in the filler neck backs up a the gas bubbles out. I have removed both filler tubes to check for any blockage, every thing is clear, this happens with the truck running also. Even after letting the truck sit for a while, when I remove tha gas caps its like the tanks are under pressure because the gas still bubbles out from the filler tube. Any help would be greatly appreciated.