Completely normal operating range for the M104 engine. The thermostat partially opens at 87c and is fully open at 102c. I have basically the same motor in my C280 and I experience the exact same thing. It was quite a shock to me when I first got the car as I was used to the standard "needle in the middle of the gauge until the car has already overheated" read out that you get with lesser cars. The gauge on the MB shows actual fluctuations in temperature.

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Jonathan

2011 Mazda2
2000 E320 4Matic Wagon
1994 C280 (retired)