Abstract: A combination heat exchanger and expansion tank are provided having an integral housing with a self-de-aerating design. Particularly suitable for cooling marine internal combustion engines, the housing holds an elongated heat exchanger core configured with at least one straight seawater passage, such that an embodiment of the housing facilitates access to opposite passage ends for cleaning through respective openings in the housing, eliminating a need to remove the core. The housing forms an upper expansion chamber, a lower collection chamber, and a coolant channel in communication with these chambers. The coolant channel receives and directs a main flow of engine coolant over the core; the coolant flow subsequently dropping into the lower collection chamber, from where the coolant exits for recirculation through the engine. The upper expansion chamber includes vents which receive vented coolant flow from remote engine components.