Duovalve or thermostat? Low heat 97 E420
 

My 97 E420 operating temperature is running low around 60 C degrees. The heat is not that hot even on HI. I top off my coolant level about a 1/3 gal which improve the heat a tad. So are these symptoms indicative of a duovalve problem or a thermostat problem?

T-stat

Just had the exact same problem on a '97 S500. Temp was very slow coming up and rarely got above 60 degrees centigrade. A new thermostat fixed it. Now it warms up faster and the gauge goes up to a hair width above 80 degrees centigrade and stays there. The climate control is much better as well, especially on days like today when it's 8 degrees Fahrenheit!

Sounds Like T-stat to me too. My duo/mono valve was spotty...symptoms were low heat from the vents at idle/low speeds and heats up at highway speeds.