I would suspect its time for thermostats replacement.
I'm not sure on yours but most of the older cummins engines used two thermostats.
If one sticks in the closed position, the operating temp will stay higher than it would if you had two correctly flowing thermostats opening when needed.
Think of it as slowing the flow, and the fact that you are not over heating means the rest of the system is in great condition.
DTW
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Dan & Loretta, US Army Retired Aero Scout Pilot
2012 Fleetwood Providence 42P Class A/DP
Spartan Chassis, 8.9L Cummins 450HP
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