i have a 2005 duramax 6.6 temp quage will go from normal temp to 260 back and forth real fast i have replaced the temp sensor and the pig tail still does it, it will also do this right away even if the truck sits for days and you start it up cold will happen right away can be -10 seems like it will go days or weeks and will not do it then when it starts it does it for awile the truck is loosing no coolant

 

is the upper rad hose hard after sitting over night? if you slowly loosen the rad cap on the Degas bottle is there a large pressure build up?

You have air in your coolant system, thats why you're having the huge temperature swings

 

no the upper hose is not under pressure if i squeeze it you can watch the coolant level go up and down a little bit in the tank