Description

Thermostat – engine coolant thermostat for the 2.5L 6-cylinder BMW M51 diesel engine, part of the engine cooling circuit.

The thermostat is fitted in the coolant outlet housing at the cylinder head, between the engine and the radiator. Below operating temperature it stays closed, restricting flow to the radiator so coolant circulates through the bypass and the engine warms quickly. As the coolant reaches the rated temperature the wax element expands and opens the valve, admitting flow to the radiator and holding the engine at its regulated running temperature.

A thermostat stuck open gives slow warm-up, a persistently low running temperature and weak cabin heat output; one stuck closed causes rapid overheating. Renew the housing seal on refitting, then refill and bleed the cooling system.