My Z4 has a map controlled thermostat. When you stomp on the gas the coolant temperature drops extremely quickly and when the pedal is released jumps back almost as quickly. We are talking 4/5 secs to shave almost 20 degrees celsius.

At cruising the thermostat is barely open to maintain operating temperature, particularly in a diesel that generates little heat and with the massive radiator that the X5 has. If it opens all the way, for example because the spring can't hold the increased coolant flow the temperature will tank. That's why I took that swag about the thermostat opening up...

__________________