Coolant Recovery Systems: Operation
WARNING: This page is about a different car, the 1988 Pontiac 6000, 1988 Oldsmobile Cutlass Cruiser, 1988 Oldsmobile Cutlass Ciera, 1988 Chevrolet Celebrity, and 1988 Buick Century. However, it is still accessible from the selected car via links, so may be relevant.
As coolant temperature rises and pressure in system exceeds pressure relief valve of radiator cap, excess coolant flows into overflow bottle. As engine cools and coolant contracts, vacuum is formed in the system. Vacuum draws coolant, stored in overflow bottle, back into radiator. In a properly maintained cooling system, the only coolant loss will be through evaporation.