The mark on the expansion tank of the cooling system shows the maximum level of liquid in the system at a temperature of 20°C.
Draining the coolant must be done in the following order. Prepare a container for draining the coolant and a clean rag. Unscrew the cap (30, see Fig. 3.17) on the expansion tank of the cooling system.
1 - radiator; 2 - temperature sensor; 3, 17, 20, 22 - hose; 5 - bolt (M6x25); 6 - thermostat in the housing; 7 - gasket; 3 - cylinder head; 9, 13 - ring (20x3); 10 - heating hose; 11 - nut (M6); 12, 15, 16 - bolt; 14 - pipeline; 18 - heating pump; 19 - holder; 21 - Air bleed screw; 23 - timing belt cover; 24 - ring (68x5); 25 - engine pump; 26 - pulley; 27 - bolt (M6x16); 28 - Nut (MB); 29 - expansion tank; 30 - tank cap
Loosen the screw (21, see Fig. 3.17) to remove air from the radiator expansion tank (only for cars equipped with manual transmission) or hose (22). Remove the stiffening plate. Unscrew the bolts (see fig. 3.56) and remove the protective panel of the engine auxiliary units.
Remove the drain plug from the bottom of the radiator (fig. 1.137) cooling systems...
...and a cork (arrow, Fig. 3.138) drain hole on the cylinder block on the right side.
Drain the coolant and dispose of it. Replace the cylinder block plug sealing gasket (M14x1.5) and tighten it to 25 N·m (2.5 kgf·m).
Filling the coolant through the expansion tank neck must be done in the following order: Replace the sealing gasket of the drain hole at the bottom of the radiator and tighten its plug to a torque of 2.5 N·m (0.25 kgf·m).
Turn on the ignition. Set the heater control to the maximum temperature. Set the fan to the lowest rotation speed. This opens the valves of the heater unit, and its additional pump starts working (18, see Fig. 3.17).
Remove the screw (21) to bleed air from the engine cooling system. Cover the neck of the expansion tank with a clean rag. Slowly pour coolant through the neck of the radiator expansion tank to the maximum mark (2, Fig. 3.139).
Wait until the radiator vent (or screw holes 21) the coolant will not start to come out without air bubbles. Close the plug on the expansion tank and tighten the screw on the branch pipe (22) to a torque of 8.0 N·m (0.8 kgf·m). Remove the rag and send it for disposal.
Bleed the engine cooling system and check it for leaks. Start the engine, warm it up to operating temperature until the thermostat opens. Check the tightness of the cooling system and the fluid level in the expansion tank of the cooling system.
[Information obtained from this online resource: BMWman.ru]