What does it mean when the overflow tank is bubbling yet car not overheat?

If the overflow tank is bubbling but the car is not overheating, it could be a sign that the overflow tank is not completely sealed, or that it has a leak. This can cause air to enter the cooling system, which can cause cavitation and potentially damage the water pump. It's important to have the overflow tank inspected by a qualified mechanic to ensure that it is sealed and functioning properly.