GM Genuine Parts Engine Coolant Reservoirs are designed, engineered, and tested to rigorous standards, and are backed by General Motors. These engine coolant reservoirs hold any excess or overflowing coolant within the cooling system. If a situation arises where the engine needs additional coolant it is utilized from this reservoir. GM Genuine Parts are the true OE parts installed during the production of or validated by General Motors for GM vehicles. Some GM Genuine Parts may have formerly appeared as ACDelco GM Original Equipment (OE).