Last night on a short 5 mile return trip my E320 temp shot up to 110 deg C with both aux. fans running on high speed. The outside air was 63 deg. A/C was not on. There is no coolant leak and fan clutch seems to have proper resistance when I tried to give it a spin. Serpentine belt looks OK.

I did a search on overheating and will replace the thermostat first. However, I remember reading somewhere in the past that MB thermostats are designed to fail only in the "open" position to prevent overheating. Is this not true? Any other suggestions to attack the problem?