_____
I confirm this observation about heat up time being about 20 min with
the 5 cyl/3 liter turbo diesel engines in my 300SD and 300D turbo cars.

When i bought the 300D turbo last summer i noticed that the temp gauge
would not rise above 60C.  I pulled out the thermostat in the fall and
tested it against a known good one in a pan of water on the stove.  Sure
enough, the MB brand stat did _not_ move at all, and the disc showed a
fixed 1mm gap vs the housing; ie it was stuck open!  A Bilstein brand
stat is in the 300D turbo engine now, and it controls at a about 88C
after heat up.  Since a stat in a car engine is a controller with
proportional action only, the temperature of the coolant at the sensor
location will always vary with load and ambient air temperature; low
load -> 85C, high load -> 100C.