Coolant circuit (BMW 3 Series E30)

The coolant circuit has thermostatic control. When the engine is cold, the coolant circulates only in the cylinder head, as well as in the engine block and - with the heater on - in the heat exchanger. When warmed up, the thermostat opens the large coolant circulation circuit. In this case, the coolant is supplied to the radiator by a constantly running water pump. The coolant passes through the radiator from top to bottom and is cooled by the oncoming air flow passing between the radiator fins. A fan installed behind the radiator ensures sufficient air flow, additionally sucking in a stream of oncoming air through the radiator.

The fan contains a viscous clutch. As soon as the air coming from the radiator reaches a temperature of approximately 75°C, a bimetallic contact engages the fan's viscous clutch. The fan then starts to run at increased speed and increases the air flow until the air temperature at the radiator outlet drops to approximately 65°C. The viscous clutch then disengages and reduces the fan's speed. Models 316i from September 1988 and 318i from September 1987: the switch-on and switch-off temperatures are 95°C and 70°C, respectively.

Thanks to the non-permanent full connection of the fan, the useful engine power increases and fuel consumption decreases.