I don't know anything about the Duramax but I replaced the thermostat on my 2005 Dodge diesel about a year ago when I replaced the water pump. Ever since then the temperature fluctuates. It usually ran a steady 200 degrees once warmed up no matter what the outside temperature was but now it sometimes fluctuates between 200 and 210 whether I'm driving in town or on the highway. Maybe a Dodge guy has an idea of what is causing this. I guess it could be an air bubble but the holding tank is always full and always has been.
