Coolant Expansion Tank

It has MIN and MAX marks for checking the level. Often the pressure cap is on the tank. Pressurised tanks raise the boiling point of the coolant. The cap contains valves that release excess pressure and let coolant return.

Coolant flows into the tank as it heats and returns when it cools.

Never open the cap when hot; a falling level means a leak. Bubbles or oil in the tank, or a tank that pressurises quickly, can indicate a head gasket problem.