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Here is some cracker barrel discussion and my idle thoughts:Heat plays a huge role. Dependent partially on fuel characteristics, combustion can suffer significantly when an engine is cold. Diesels rely on the heat generated from cylinder pressure for effective combustion rather than an artificial source such as the spark employed in a light fuel engine. Initially, when a diesel is cold, supplemental heat from the glow plugs improves combustion until cylinder temperatures alone become sufficient to support it. Lower combustion efficiency at cold startup means that any "normal" amount of oil present in the cylinder won't burn as well either - hence, short-lived blue smoke until cylinder temperatures rise sufficiently. At the very least, you should verify glow plug health and their proper operation. You might even consider using a block heater or heated pan blanket if the smoke is really bothersome. Replace the t-stat; 120° F at operating temperature is too cold.However, in your first post you stated, "On startups at 20F; there is sometimes blue smoke for the first second. After the first second, it clears up and doesn't smoke at all." Frankly, that doesn't sound like an issue regarding a diesel powerplant in icy weather. Nonetheless, there maybe sufficient accumulated hours and normal wear on the engine to pass slightly more oil into the cylinders, manifesting as blue smoke - more so when cold and combustion is less complete. Collectively, worn valve guides and seals, minor ring wear, cylinder glazing, carbon accumulation in the ring grooves, increased fuel dilution of the oil, etc, are more likely "combined" causes contributing to a minor increase in cylinder oil, especially with moderate to high mileage.Essentially, aside from the thermostat, it doesn't sound like you have a significant issue to be overly concerned about at this point in time.
I had assumed that being a diesel owner he knew smoke was normal and he was concerned about an excess blue smoke. I must have been wrong.
im going to add this. idk about the 6.2 that much but my cummins would hardly reach 100° till i blocked off the radiator. first i blocked off 100% but the temps would be about 210° trans temp would be about 70° in 30° weather. i unblocked it a tad now its maybe ½ blocked maybe a little more. but the OT of her now is about 140° AND she will smoke for about one minute really bad during these cold morning