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When struggling to figure out why something is not working as it once did, and for all my Diesels the normal operating temperature, regardless of how hard it was working or not, was always withing a needle width of the 80 degree C or 175 degree F thick mark on the gage, it is best to have a good idea of how it was designed to work.
The thermostat does see hot water from the engine, and that is what initiates its movement. However, as the engine warms up and the thermostat moves around, it responds to water flowing at engine temperature and whatever the temperature of the water is coming out of the radiator. Its intended design is outlined in the manual I have for my 240D (as well as the 300D) from 1975, which applies to the W123 versions as well since the update to the manual identifies no changes for those models.
I scanned the flow diagram, some illustrations of the thermostat operations and some text. I will try to attach these here, in a sequence of posts unless the quality (black and white text and illustrations do not do well at the 60k size limits) is too poor.
I hope this helps get a solution for what is ailing these cars. Jim
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Own:
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles
Owned:
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)
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