if your thermostat isnt working right those might be symptoms, have you ever replaced it? most all of us have replaced ours once at least.

Maybe you have too much coolant in the 50/50 mix and its allowing the engine to run cooler?

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2018 Ram 2500 6.7L Cummins 68RFE
19k miles -Bright White/Black - Big Horn Sport - Crew Cab Short Bed
2013 X5 35D (CEO's) - Born on 5/17/2013 -
82k miles - Alpine White/Cinnamon Brown/Premium Pkg, Sport Activity/Premium Pkg and Sound/20" Style 214/Running Boards