I have a 2011 Chevy Silverado 2500 with a 6.0 L. I never noticed this before because I was living in the south and never really had to increase my cab temp too much. Well I just moved to northern MI. Shortly after I moved here I had to replace my oil pump. Just before winter. once I was done I noticed when I would drive or when truck was at idle if the temp setting in the truck was above 75 the engine temp would rise and come back down. Now the hotter I kept the inside temp setting "say 80-90 degrees" the fluctuation would be more often and stay hotter longer. I have flushed the coolant and added the desired amount. It still happens. I cant seem to find a leak but with weather conditions its been hard to check. Any kind of helpful information would be great.
most modern vehicles now have two thermostats .and they are integrated in the coolant flow.and preset.. usually one will be directly in the collant flow from the top of the engine and the other will be fitted to the oil cooler exchanger . Its the one fitted to the oil cooler tha i would suspect as sometimes this will fail and start opening early or late depending on the way the capillery in the stat is oriented . add its the way id first direct attention .. also ensure you are using a correct type coolant ..