I need to determine if I have a thermostat issue in my 2004 X3 2.5i. On a typical cold winter day here in Quebec Canada, with a warm engine, and driving at city speeds, my temperature gauge will indicate a normal "middle " warm reading, and I will get warm air inside the car. If I then drive on the highway, the gauge will wind up in the blue lower area, and I will hardly get any heat in the car. Is this the classic symptom of a bad thermostat?

The BMW TIS seems to indicate that this is a simple enough part to replace. Is draining the coolant really necessary since the thermostat+housing seem to sit high on the engine block?

Thanks for sharing your experiences and comments.