Coolant Bypass

The bypass hose allows coolant to recirculate within the engine, without passing through the radiator, as it does when the engine is warmed up and the thermostat opens. The bypass hose connects the thermostat housing and the water pump. The water enters the bypass tube through the bypass valve, when such a valve is fitted. The bypass valve is sometimes operated thermostatically; it closes off the bypass hose when a certain temperature is reached. This increases the circulation of the coolant within the engine. Many cars don't need a bypass valve, because there is plenty of coolant going through the radiator hoses due to the thermostat.