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The radiator may be plugged or the head gasket can let compression gasses into the radiator . To check, run the engine without the radiator cap and look in to see if there any bubbles in the coolant. If there is a lot of coolant moving in the radiator after the T/stat has opened, the water pump is ok. If not then suspect the water pump is not circulating enough water.
Drain radiator, remove thermostat reconnect housing, fill radiator halfway with water add radiator flush it's by Preston fill up with water start engine run for 20 minutes , drain radiator replace thermostat fill radiator with 50/50 mixture of antifreeze and water run engine and top off radiator as needed
you have a GM common problem and that is a bad intake manifold gasket. GM change the gaskets to plastic and they crank and leak causing over heating. here is a picture of one i took from a 2005 pon. bonneville with 74,000 miles on it
I would change the Thermostat. if that doesn't fix it? I would buy a Radiator flush and follow the directions exactly you will be surprised how much junk comes out. That should fix all your heat problems.
try priming the engine block & radiator with water though the hose's & any hose you can squeeze to pump out the air lock with the filler cap off when running, sounds too much air is is in the system & the pump won't work dry.
I had this problem a few times & I ended up practically forcing water into any hose I could re-attach quickly using a garden hose, as the heating system need's priming too !
How do you know the thermostat is ok? If in doubt replace it. Water pumps are a even bigger what if. Did he do a radiator flush? Cooling system flush of the engine? Cooling fans come on at what temp and go off when? Replace the engine temperature sensor? 6 years old is bound to have sensors and such go bad.
Might be a weak or failing water pump... Does it leak antifreeze or do you have to add antifreeze very often? When the engine gets to operating temperature squeeze the top radiator hose and see if you can feel pressure from the water pump forcing water through it.