Coolant
The cooling system is filled with a mixture of water, antifreeze and anti-corrosion agent all year round. This prevents rust and corrosion, salt deposits and, in addition, increases the boiling point of the coolant. In the cooling system, due to the expansion of the liquid when heated, excess pressure is created, which also leads to an increase in the boiling point of the coolant. The pressure is limited by a valve located in the expansion tank cap, which opens at a pressure of 1.4–2.0 atm. For the perfect functioning of the cooling system, it is necessary to insure against boiling of the coolant, which can form a vapor lock, as a result of which the circulation of the liquid decreases and engine cooling deteriorates. The boiling point must be increased. To do this, the cooling system must always be filled with a mixture of water and the listed additives to it.
(Information copied from the website: MercedesMAN.ru)