To drain and fill the cooling system, a technician will first remove the radiator cap or or overflow tank cap to allow gravity to pull coolant from the system. Next, either a valve on the bottom of the radiator is opened or the lower radiator hose is disconnected to allow the old coolant to drain out. The used fluid is properly collected and disposed of according to environmental regulations. Next, the valve is closed or the hose is connected before the technician refills the system with a mixture of fresh coolant and water. This is often done with a special device that applies vacuum to the system to remove all of the air before the coolant is installed. (Air that becomes trapped can lead to engine overheating and serious damage).
Once the coolant has been refilled, and if a vacuum device was not used, the engine will be allowed to run for up to a half hour to purge the system of air. This process can be difficult on many modern engines. Finally, the system is topped off with fluid and closed up.
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