I would replace it with a good thermostat. The long slow warmup is hard on the engine. A Diesel needs to get up to temp as soon as possible especially when the weather gets cold. You need to keep the cap on it too! And run approx 45 or 50% coolant mix (antifreeze to water ratio).
If it runs much over 85 C under normal driving conditions then you have other problems, like the radiator, water pump or ???

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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!