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FWIW - my truck seems to take FOREVER to warm up when it is cold outside. However, once it does completely warm up, it runs between 198°F and 202°F pretty much regardless the time of year. Like I said, when it is cold it takes much longer to get there but it does indeed get there. Lots of BTUs are required to raise the temperature of that much liquid from low temps to 200°F.
Regardless of the outside temps, The thermostats job is to regulate the engine temp. I was taught to call it a regulator not a thermostat. But regardless, The engine may take longer to warm up, but the temp should come close to normal operating temp, My 19 6.7 ( I know not the same engine, But point is the same ) sits around 190 to 198 all year long. Unless I am towing, Takes longer in the winter to get there, but it does get there. I would replace the thermostat, and depending on miles, coolant too. I am doing both this spring with 82000 on the clock,
Regardless of the outside temps, The thermostats job is to regulate the engine temp. I was taught to call it a regulator not a thermostat. But regardless, The engine may take longer to warm up, but the temp should come close to normal operating temp, My 19 6.7 ( I know not the same engine, But point is the same ) sits around 190 to 198 all year long. Unless I am towing, Takes longer in the winter to get there, but it does get there. I would replace the thermostat, and depending on miles, coolant too. I am doing both this spring with 82000 on the clock,
It should still be functioning properly, but at this point the process of elimination should assist with the issue. Its easy to do and cheap enough, plus you'll end up with a new t-stat. It could be the problem.