A marine vessel overflow tank system for use in the hull of a marine vessel that has a fuel system including a fuel vent line connecting a fuel tank to a vent that communicates with the atmosphere. The system comprises a first vent line fitting adapted for direct parallel communication with the fuel vent line. The internal cross sectional area of the first vent line fitting is approximately the same size as the internal cross sectional area of the fuel vent line. The system also comprises a second vent line fitting adapted for direct parallel communication with the fuel vent line. The internal cross sectional area of the second vent line fitting is approximately the same size as the internal cross sectional area of the fuel vent line. The system further comprises a fuel expansion tank. The fuel expansion tank comprises a plurality of walls defining an enclosed chamber and a first restrictive orifice located in one the walls. The first orifice is in direct communication with the first vent line fitting and has an internal cross sectional area less than the internal cross sectional area of the fuel vent line. The fuel expansion tank also comprises a second restrictive orifice located in one the walls defining the enclosed chamber. The second restrictive orifice is in direct communication with the second vent line fitting and has an internal cross sectional area less than the internal cross sectional area of the fuel vent line. The fuel expansion tank further comprises means for attaching the chamber to the hull with the first restrictive orifice being positioned above the second restrictive orifice and proximate to a top edge of the chamber, and the second restrictive orifice being positioned above the fuel tank and proximate to a bottom edge of the chamber.