A method of replacing fluid in an automotive system requires interconnecting a radiator fill port of the radiator with a first external tank through a closed control valve, disconnecting an upper hose from an upper radiator port and reconnecting the upper hose to a second external tank containing a replacement fluid. The system is sealed so that fluids are constrained for movement only within and between the engine, radiator and external tanks through the various hoses. When a pressure differential is applied within the system, fluid is forced from the automotive system into the first external tank, while automatically refilling the automotive system with the replacement fluid from the second external tank.