Abstract: An engine cooling system for an automotive vehicle including a liquid coolant deaeration and overflow bottle having plural cells constituting a degassing chamber assembly with the first cell having only an upper fluid inlet for initially receiving liquid coolant from the engine cooling system and a lower flow-through window which leads into an adjacent cell for receiving coolant from the first cell. The other cells are likewise connected together in series and the final cell has an outlet connected back into the engine cooling system. Coolant in the cells of the degassing chamber assembly creates a liquid trap arrangement to prevent any substantial back flow of accumulated air from the deaeration chamber assembly back into the engine's cooling system when engine operation is terminated. Most importantly, this prevents any formation of air bubbles in the heater core circuit which may restrict coolant flow and seriously degrade heater effectiveness, particularly at idle and low engine speed operation.