A coolant bottle defines a first chamber and a second chamber fluidly coupled to the first chamber at a valve seat. The first chamber is fluidly coupled to a source of heated engine cooling fluid, while the second chamber is fluidly coupled to an engine water pump. A thermally responsive actuator is disposed within the first chamber, and has a thermally actuated sliding member having a valve seat engaging surface. The thermally actuated sliding member is movable from a first open position to a second closed position when the coolant is above a first temperature. The thermally responsive actuator is disposed within the first chamber.