If your vehicle is showing coolant boiling or bubbling over in the reservoir tank, here are the most likely causes and the parts you'll need to fix each one.
The cap maintains system pressure to raise the boiling point of the coolant. If the spring weakens or the seal fails, the system loses pressure, causing the coolant to boil at a lower temperature and bubble into the reservoir.
If the cooling system was recently opened for maintenance and not properly bled, trapped air pockets will expand as the engine heats up, pushing coolant out and causing bubbling in the tank.
A thermostat that fails to open prevents hot coolant from flowing to the radiator to be cooled. The trapped coolant overheats and boils, forcing its way into the expansion tank.
If the radiator fan fails to engage, the engine cannot shed heat properly at idle or low speeds. This leads to overheating and boiling coolant.
A water pump with a degraded impeller or slipping belt cannot circulate coolant effectively. The stagnant coolant inside the engine block boils and pushes into the reservoir.
A breached head gasket allows high-pressure combustion gases from the cylinders to pump directly into the cooling system. This often looks like violent boiling in the reservoir even before the engine fully overheats.