I did replace the T-Stat, T-Stat gasket, Water Pump Gasket and Engine Block to T-stat housing gasket.

The new Bosch water pump looks fine and the shaft looked nice and tight. The belts are tightened per the Mercedes spec using a Gates Krikit I & Krikit II to verify force as best I could.

I am aware that the dash sensor can drift over time and the sensor wears out. I have an Omega TC-K-NPT-G-72 which is a 1/4 NPT thermocouple I can screw into the block. I have not yet screwed it in to check the sensor accuracy (I will post back results when I do). I did learn that the engine coolant sensor thread is a 1/4 - 18 (AKA 1/4 NPT) thread. That info is nowhere to be found on forums. I may replace the stock temp sensor with the thermocouple permanently and wire up and calibrate a new circuit to the dash gauge.

I have been driving it more and see a consistent reading of 90C on the dash gauge. The fact that is nice and steady in all driving conditions is reassuring. I will use my thermocouple to see what the real temp is in case of sensor or gauge miscalibration.

Govert: What is your typical dash reading? What is the "ideal" temperature for a properly operating 617.912 engine?

Thanks again for all the replies! You guys are great!

-matto-