[ReelMagic]

As a prior owner of a 31 Open (1997) w/twin 454 FI Crusaders I can provide some info regarding the cooling system. The cylinder on the top of the engine you described is the "heat exchanger".. That transfers the heat from the engine antifreeze to salt water (or fresh water based on where the boat is kept) via cooling rods internal to the "heat exchanger"... The overflow tank allows antifreeze from the "heat exchanger" to expand into the tank as the antifreeze gets hot during engine operation keeping the system at or around 14 PSI (could be 7 PSI based on the pressure cap used).  The overflow tank has 2 marks, one shows where the antifreeze should be when the engine is cool and the other mark is where the antifreeze should be when the engine is hot.... When you first shut off the engine (and the engine is hot) the overflow tank level should be near or at the high level. Once the engine cools off the level should return to the cool mark. As the engine cools down the pressure in the cooling system goes down thus sucking the antifreeze back into the "heat exchanger".... Once the engine is fully cooled the "heat exchanger" should be full or nearly full if checked..... The way you should be filling the system is through the overflow tank..... When cool you can add to antifreeze through the cap on the heat exchanger till full although I recommend keeping an eye on the level in the overflow tank. When cool ensure the antifreeze is no lower than the cool level... Hope this helps.......