Thermostat assembly for an engine cooling system

A thermostat assembly for an engine cooling system comprising a housing having a coolant flow passage therethrough having an inlet and an outlet with the outlet vertically higher than the inlet, a thermostat valve positioned in the inlet and operable in response to the coolant temperature to block or permit the flow of fluid through the flow passage, an air bypass passage having one end operatively connected to the inlet on the upstream side of the thermostat and having the opposite end connected to the outlet downstream of the valve, and a bleed valve in the bypass passage movable to an open position to allow air flow past the thermostat valve when the latter is closed as the engine cooling system is filled with coolant and movable to a closed position by fluid pressure, characterized by the housing having a fluid fill passage that connects to the coolant passage and has an inlet at a level higher than the coolant passage outlet and through which coolant may be introduced into the flow passage, the bleed valve being located in the housing at a level higher than the highest-most point of the thermostat valve.