Warm up the engine while driving until the coolant temperature gauge shows normal operating temperature.
Check and top up the coolant level if necessary, see section "Maintenance work".
Remove the expansion tank cap.
Caution: When the engine is hot, before opening the expansion tank cap, to avoid scalding from hot liquid or steam, place a thick cloth around the cap. Open the cap only when the coolant temperature is below +90°C.
Place the testing device with the adapter to the filler neck of the expansion tank. Using the hand pump of the device, create excess pressure of approximately 1 bar. If the pressure drops by more than 0.1 bar within two minutes, you should find the leaks and eliminate their cause. Leaks are detected by the leaking coolant.
If the coolant leaks are not visible, and the pressure in the system drops or even increases, then we can conclude that there is a loss of coolant in the engine itself, for example, as a result of a defective cylinder head gasket or a crack in the housing.
To check the steam-air valve in the expansion tank cap, the cap must be connected to the device. Create excess pressure of about 2 bar with the pump. In this case, the steam (safety) the valve in the lid should open, otherwise the lid must be replaced.
Create a vacuum under the lid. At a vacuum of 0.1 bar, the air valve in the lid should open.
(This article is based on information from the website www.bmwman.ru)