Why does water come out of the expansion tank after the engine is shut off?

The expansion tank is a critical part in the heat dissipation circuit of a water-cooled engine, and serves the purpose of balancing pressure of the radiator.

It works on the following principle: When the engine radiator absorbs heat from the cylinder and becomes hotter, the liquid will expand and result in higher pressure. The coolant in the radiator will flow via a bidirectional valve and the radiator cover to the expansion tank. When the temperature drops and the pressure of the cooling system becomes negative, the system will suck the coolant from expansion tank via the radiator cover back to the radiator. In this way, it maintains a balance of the radiator pressure.

Then why is there water coming out of the expansion tank after the engine is shut off?

Because once the car is stopped, both the water pump and the fan will cease to run. As a result, the engine water temperature will continue to rise. We’ve learned from the working principle of the expansion tank that the coolant in the radiator will flow to the expansion tank. If the expansion tank is filled with too much coolant, flow of coolant from the radiator into the tank will naturally cause spill. This is why water is seen coming out of the expansion tank after the car is stopped.

The expansion tank is marked with level graduations that show the amount of water intuitively. Please take note of the water level to avoid spill.