The thermostat should be a 190. In my experience if you run it any cooler than that you get problems with accelerated wear. Cummins says the minimum operating temp is 158 the maximum is 212. Make sure your radiator cap is good 15 lbs. To test you thermostat with the engine cold, buy one of those inferred thermometers, cover the front up with plastic run the motor at 1800 RPM when the top rad hose gets flow threw it test the heat below the thermostat that's the thermostat opening temp. Then remove the plastic and see if the temp goes way down if it does you should get some kind of cover to keep the heat up. If it is real cold besides a winter front get a tarp from the bottom of the radiator to behind the transmission.
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JP 3
Licenced HD mechanic for 35 years. RETIRED!
worked on engines all my life. Spent last 19 HAPPY
years at Freightliner