Originally Posted by JimN
If the sound occurs and the smell of gas in really strong immediately after, it's positive pressure. If it was suction, it wouldn't be so strong. With a pressurized tank, you could easily feel the air rushing out.
I guess the question is "is a marine fuel tank supposed to EVER have pressure in it, either positive pressure or negative"? I suspect his fuel cap, which is supposedly where his tank is supposed to vent, is malfunctioning and accumulating negative pressure in it as the fuel in the tank draws down.