my 450 hp yanmar looses coolant into the overflow tank but does not draw coolant back in to the system when the engine is cooling. it consumes about 1 qt of dexcool per3 hrs run time. i removed the heat exchanger and replaced the gaskets and had it pressure tested . i have no coolant in bilge. the engine runs at 160 f. the low coolant light goes on after 2-3 hrs running . i replaced the thermostat and gasket. same problem continues. ideas please