The drive of the water pump in a 4-cylinder engine is provided by a V-belt, in 5- and 6-cylinder engines by a toothed belt.
The path of the coolant depends on its temperature at a given time:
If the engine has not yet reached operating temperature, then the coolant flows through a small circuit passing through the engine and heat exchanger. In the four-cylinder engine with the Mono-Motronic injection system, an intake manifold is connected in addition to the small coolant circuit.
When the temperature reaches approximately 87°C, the thermostat opens a large circuit, which includes the radiator. If the temperature of the coolant rises further, so that even in the radiator the temperature is above 92°C, the radiator fan comes into operation.
Overpressure refrigeration system
High pressure prevails in the cooling system at operating temperature (9.2-1.5 bar) which raises the boiling point of the coolant well above 100°C. This creates a certain reserve. The engine can reach operating temperatures in excess of 100°C without the danger of boiling in the cooling system.
The correct pressure in the system is created thanks to the screw cap of the expansion tank, which is equipped with a safety valve (1.2-1.5 bar) and vacuum valve (0.06-0.1 bar). The vacuum valve lets in air if the coolant gets colder and thus takes up less space.
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