BMW X5 E53 Engine Won't Warm Up or Temperature Fluctuates
Why does the X5 E53 not reach normal operating temperature?
The most common cause is a faulty or incorrectly working main thermostat. A missing or broken thermostat in the automatic gearbox oil cooling can also keep temperatures low. Replacing the main thermostat has raised temperatures clearly, but replacing all the thermostats does not always suffice.
Where is the gearbox oil cooler thermostat?
It is under the expansion tank. Its plastic cover can break, and then the thermostat is renewed. The thermostat often breaks precisely when the expansion tank is removed, so have the part ready.
What is the normal coolant temperature?
Readings vary by engine and gauge, and no single exact value can be given. With a healthy main thermostat the temperature settles steady during use, and at idle it may drop, which is normal. Compare the reading with the same car's earlier behaviour.
What else should be checked if the temperature does not rise?
The temperature sensor has separate circuits for the instrument cluster and the engine management, so the sensor is worth checking. Gentle driving leaves the engine cold, and faster driving helps it warm up. The engine must also reach operating temperature for particulate filter regeneration to succeed.