Abstract: The present disclosure relates to an engine system having an engine and a cooling system. An engine vent line is fluidly connected to the engine for venting air from the engine system and bypassing the coolant flow circuit. The engine vent line has a hydraulic diameter and length selected to provide a predetermined engine head loss for the engine and a rate of flow of coolant in the at least one engine vent line at or below a maximum engine vent flow rate target during operation of the engine. The engine vent line comprises a helical section in which a portion is formed into a helix.