Expansion tank cap (Audi 80 B4)

The expansion tank cap is important for the coolant circulation system, maintaining the necessary pressure for the system by means of the valves installed on it. If the pressure in the system rises above 1.2-1.5 bar, the outlet pressure reducing valve opens, through which the excess of the formed steam comes out. When cooling, the coolant is compressed, creating a vacuum in the system. In order to remove the low pressure in the system, a second valve is installed on the cap - the inlet valve. It opens when the pressure in the system drops to 0.06-0.1 bar, letting in atmospheric air.

To check the outlet valve, use a device with a pump that allows you to create the necessary pressure. A normally functioning outlet valve on the cover should open at a pressure of 1.2-1.5 bar.

The functionality of the inlet valve is checked as follows:

1. Remove the cap from the expansion tank.

2. Squeeze the thick coolant circulation hose.

3. Replace the cover and tighten it.

4. Release the hose. If the hose does not remain in the compressed position, but returns to its original shape, then the inlet valve should be working properly.

Flat coolant circulation hoses observed in the morning indicate a faulty inlet valve on the expansion tank cap.