The coolant inside a car’s cooling system drastically fluctuates in temperature, which means its volume also changes. … Once the radiator is completely full, this expanding hot coolant will overflow into the reservoir. When the coolant cools back down it contracts, which creates a vacuum.

Why does my coolant level go up and down?

This is caused by the expansion of the coolant as the engine gets warmer, the expanding coolant builds pressure in the system until the cap is overcome. The remaining expansion pushes coolant into the tank.

Does my coolant level keep dropping?

If you notice coolant levels are dropping by the day, this is because of an internal or external leakage in the cooling system which needs immediate attending to. External leakage may be any leakage in the hoses, radiator tank or throttle body(depending on the car you drive).