had the 348 out for a long drive today, after running for a few hours i started to smell fuel. , checked for leaks and found none. heard a slight hissing from the gas cap, loosened it and it released what seemed to be a lot of built up pressure. drove a little more and the same thing , smell and built up pressure. has anyone run into this?i'm a mechanic so i can fix it myself , , just wondering what the most likely culprit is.