Expansion Tank Threaded Cap

The threaded cap on the expansion tank plays an important role in maintaining pressure in the cooling system:

When the cap is closed, its sealing surface is pressed firmly against the edge of the expansion tank opening, and as long as the pressure relief valve is closed, there is no pressure loss.

If the pressure in the cooling system rises above 1.2 – 1.5 bar during heating, the pressure relief valve opens and some water vapor is released. The pressure is equalized.

When the system cools, the coolant contracts and a vacuum forms in the cooling system. To equalize this vacuum, the cap has a second valve. It opens at a vacuum of 0.06 – 1.0 bar, allowing outside air to enter the system.

Checking the Pressure Relief Valve
Anyone who does not want to deal with testing should simply replace the relatively inexpensive threaded cap if there is any doubt.

A workshop checks it with a pressure gauge:

At a pressure of 1.2 – 1.5 bar, the valve must open.

Checking the Vacuum Valve
It is only possible to roughly determine whether this valve is working.

PROCEDURE

With the threaded cap removed, squeeze the thick cooling system hose firmly.

Fit the cap and screw it on.

Release the hose.

If the squeezed hose returns to its round shape, the valve is most likely working correctly.

If the cooling system hoses look flat and collapsed in the morning before the first start, the vacuum valve is almost certainly faulty.