A vane-type water pump mounted on the front of the cylinder block circulates coolant throughout the engine. The coolant flow washes over each cylinder and is directed to the rear of the engine. Cooling galleries cast into the block direct the coolant flow around the intake manifold ports and exhaust manifold, spark plug areas, and exhaust valve guides.
The solid wax thermostat is built into the lower hose on M10 engines, housed in a shroud at the front of the engine on M20 and M30 engines, or behind the elbow under the upper timing belt cover (on the front side of the cylinder head) on M40 engines. During engine warm-up, a closed thermostat prevents coolant from circulating through the radiator. As the engine approaches normal operating temperature, the thermostat opens and allows hot coolant to circulate through the radiator, where it is cooled and returned to the engine.
The pressure in the cooling system increases the boiling point of the coolant and increases the efficiency of the radiator. The sealing of the system is ensured by a special sealed cap. If the pressure in the system reaches a certain value, it acts on a spring valve inside the cap, which opens and allows excess coolant to flow out through the overflow pipe.
The pressure cap on four-cylinder models is located on the top side of the radiator; on six-cylinder engines, it is located at the top of the transparent plastic expansion tank. The sealing values of the cap are indicated on the cap itself. They are usually 1.0 bar or 1.2 bar.
Do not remove the radiator or expansion tank pressure cap until the engine has cooled completely and the cooling system has been completely depressurized. Removing the cap when the engine is hot can cause severe burns.