The coolant reservoir tank collects the heated coolant from the engine and recycles it back into the coolant system once it cools down. The coolant reservior bottle or tank works in sync with the radiator cap to protect the engine and prevent coolant loss due to overflow. Over time, your factory coolant reservoir tank may crack or break due to old age or overheating and coolant may leak out causing all sorts of problems to the engine. It is a simple but very important part of your cooling system.