did you try running it with a old cap with a 1/4 to 5/16 hole drilled thru it ? i do not know if you can do this on your car if the filler neck is hi like in the rear 1/4 them you can try it see if it still will do it .if not then it a vent deal. drill the hole in the cap off the car......... or the pick up in the tank on some cars have a filter on the pickup