I think J cat might be on to something here, a mechanic at the local Chevy dealer told me their having a lot of issues like this.
They werent having any luck getting air pockets out by putting the truck nose up though, they said you need to bleed the system with a Radiator Pressure Tester/Bleeder to get air pockets out.
hmm ok time for a coolant flush I guess
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Well I tried the parked on a hill trick again and this time a little steeper and longer and sure enough that seams to have fixed it. Curious now though, my dealership said my coolant lasts 100k miles and I have 80k on it, does anyone change sooner than 100k miles?
It cant hurt and is fairly easy to do, just a little messy.
so you did the coolant purge proceedure ? did the level drop ?
this is removal of the pressure cap when cold. run engine until hot , then rev engine to 2,ooorpm. fill to the hot mark.
using a vacuum gage and a propane torch to direct propane gas to the area of your suspected leak area will show as a surge in rpm and vacuum gage readings.
you place a hose over the torch end and open the valve slightly so gas comes out alittle..
Electric blend doors since at least 99 so no vacuum issues to worry about. Dexcool does have problems building up crap in systems, even the new stuff. I would change it around 100k and take the heater hoses off and run a hose though both hoses to force the crap out of the heater core. Also water pump impellers that rot off cause the same prob but since your truck is newer that wouldnt be it.
I had a similar problem a few years ago. It turned out one hinge on the air diverter door (may not be the correct name) broke off and it was rotating around in an uncontrolled manner. I could not control the temperature, regardless of the position of the temperature control. It was most noticeable during the winter, when the heater would be blowing hot, then suddenly blow cold air for a few minutes.
Just my two cents worth,
the water pump impeller corroded to stubs does occur with using non distilled water and or not enough dexcool ..some have had this TYPE water pump failure for sure !