An apparatus is provided for filling an engine cooling system with coolant fluid and for controlling fluid flow through the cooling system using three dimensional weirs in response to changes of an engine cooling system temperature. The apparatus includes a fill housing having an inlet section, an outlet section, and a fill section configured to receive a cap thereon. The inlet section is formed to include a valve seat and a flange for coupling the fill housing to an engine. The apparatus also includes a thermostat and means for coupling the thermostat to the flange to hold the thermostat within the inlet section beneath an outer surface of the flange. The fill housing is formed to include a venturi portion located between the inlet section and the outlet section to provide a pressure drop in the fill housing as fluid flows through the fill housing. The venturi portion is located adjacent the fill section to subject the fill section to the pressure drop as fluid flows through the fill housing.