Found this info on another MB diesel list. This was news to me and it is certainly a 'good to know' bit of info it it's accurate.
"To block the thermostat open (use an OLD thermostat as it may NEVER work correctly again) you can heat it until it opens and block it open or dismantle it. Running without a thermostat will allow the coolant to flow through the by-pass and NOT flow through the radiator and make a joke of the entire excercise. The thermostat must CLOSE the by-pass passage and open the passage through the radiator. The heater must be ON full throughout the process (put it into defrost - that's full heat). "
1985 300SD - 'Grace' (198K mi.)
2010 Honda Fit Sport
2013 Honda Accord Sport
1996 Lexus LS400 (Retired)
1995 Ford Contour SE (Retired)
1976 Porsche 914 (Sold)
1972 Datsun 240Z (RIP)