Seems to me I read somewhere that engines are more efficient if the temperature difference between the engine and the ambient air is greater rather than less. Obviously you can't overdo that, especially with our alloy-headed diesels, but the principle is sound. I remember seeing a change between pre-smog and post-smog cars when I was driving GM iron. The older cars ran around 180F (82C) while the newer cars ran much closer to 200F (93C). With an 80C thermostat, an OM603 engine should sit at about 90C depending on ambient temps and load.

Keeping the cooling system clean is probably the best way to make sure the temperature stays below the danger area. In addition to Hit Man X, as previously posted, both Sixto and I have the red top aux fan switch (S25/5) that turns on the fan at 100C instead of the factory 105C and additionally turns off the a/c when the coolant hits 110C (instead of 128C in the original blue top factory switch used in the 1987 300D Turbo). There are at least four different temperature combinations to choose from.

Jeremy

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"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970