Coolant expansion tank and its cap (Skoda Octavia 1U)

The expansion tank is located in the engine compartment, is made of white transparent plastic and has a spherical shape. In addition to the outlet hose at the plug level, it has a drain hose nipple.

The tank contains a device for monitoring the coolant level. It operates on the principle of electric current flowing between two electrodes while they are immersed in the coolant. As soon as the liquid level falls below the specified level, the current is interrupted and the driver is informed of the lack of coolant by a simultaneously lit indicator and a sound signal. A signal is sent in a similar way when the permissible operating temperature (approximately 119°C) of the coolant is exceeded. (Coolant thermometer on the dashboard of the classic type.)

The expansion tank filler cap is made of blue plastic, with a thread and an excess pressure valve that opens at 0.14 MPa and a valve that opens under vacuum, letting air into the tank when the liquid level drops during cooling. This valve operates within the reduced pressure range of 0.002–0.01 MPa. The tank and cap are the same on all cars, regardless of the engine type.

[Source of the article web resource skodabook.ru]