Faulty gauge.
Rad too small in those cummins side rads..( known issue)Install bigger rad.
Head gasket leak in coolant port..
Dont run in overdrive or eco mod for trans...
All I could think of...
Mike L ... N.J.
2006 Silverado ext cab long bed. 3:42 rear. LM7 5.3 motor. 300 hp 350 ft lbs torgue @ 4000 rpms
2018 coachmen Catalina sbx 261bh