Use the right thermostat and be done with it. Diesels run cooler than gassers in the first place, and they need to be at the right temp to provide good efficiency, that is why they lack power when first started, the pre-combustion chambers have to get to the right temperature.
DDH

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'95 E320 Wagon my favorite road car. '99 E300D wolf in sheeps body, '87 300D Sportline suspension, '79 300TD w/ 617.952 engine at 367,750 and counting!