Engine temperature drops while driving – what is the reason and how to fix it?

BMW driver’s question

At low temperatures, the engine is too slow to reach the optimum temperature. It reaches it after about 15-20 km, but after turning the air vents on to heat, the temperature drops again and stays at 3/4 optimum. I replaced the thermostat and the problem remains. I heard that this model uses another thermostat for a small system. Will replacing this part fix the issue?

The problem is with a 1999 BMW 3 E46, with a 2.0 diesel engine with 136 horsepower, but it may also occur in other models.

Response from BM Cars service

On an E46 with a 2.0 d engine, the engine temperature gauge should indicate half of the scale (vertically), after a relatively short mileage of about 7-10 km, depending on the outside temperature and load. In no case should the indication fall or rise beyond the middle level.

Reaching the temperature indication in the middle of the scale does not yet mean that the correct – optimal engine temperature has been reached. The cooling control system of the so-called small and large coolant circuit is implemented by means of the main thermostat. After a “cold” start of the engine, the thermostat is closed and the coolant is directed in the small circuit. As a result, it is able to warm up very quickly to the thermostat’s opening temperature of about 85℃.

Once it is opened, the fluid is directed to the large circuit, which also includes the radiator. If the engine continues to run and the temperature of the fluid in the radiator rises to about 95℃, the radiator’s main fan is activated, using either a viscous clutch or an electric fan. Operating the cooling system in this way ensures that the engine temperature is properly maintained within a range that protects it from both high and excessively low operating temperatures.

If the fluid temperature drops while driving, either the thermostat or the radiator fan control (Visko) is defective. The additional thermostat used on the 2.0 d engines in the E46 covers the cooling of the exhaust gas recirculation system and has little effect on engine temperature.

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