Hi, I have the “phenomenon” with my Smart 451 mhd, i.e. the petrol engine. He got a new engine (now run 3,000 km) and I used Geizhal’s old thermostat again. From time to time I log the water temperature with an app via Bluetooth on the mobile phone. I read the data directly at the temperature sensor during the ride, because the Smart has no water temperature indicator. The water temperature fluctuates between 90 and 70 degrees on these cold days at short notice. So the thermostat works, but the temperature fluctuation is already high, if this drops by 20 degrees and then rises again. It is a constant temperature fluctuation in the motor-side small cooling circuit. . It is just the question whether the overall system with such a small engine behaves technically due to that. Large water quantity, small engine, fast thermostat opening and probably too much opening path of the thermostat per degree of temperature change. Could it possibly be a misinterpretation of the desktop developer? I have no similar observations?