Welcome aboard. Assuming no one's gone and swapped engines, your 82 300 SD does have the OM 617 engine. The proper thermostat for that engine is one that opens at 82 degrees, I think. I'm sure that you shouldn't be running @ 78 if the engine is fully warmed. If your temperature gauge is accurate, its time to check the thermostat. When the engine runs cool, you lose some mpg and increase engine wear. The themostat is a very easy swap out if it needs to be done.

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Tjohn

82 300 SD
77 450 SL (gone)