My daughter's 06 Equinox started overheating. I have changed the thermostat and the water pump. After changing the water pump I bled the air until I was getting good heat out the heater. While bleeding the system, both fans come on and the temp gauge stays at about half way and it doesn't overheat while working on it. When I take it on the road to test it, it overheats and boils over after going a couple of miles. I haven't actually "flushed" the radiator, but when I changed the water pump, had a lot of the coolant run through it and out of the hose coming out of the bottom of the radiator. When I was bleeding the air (located above the water pump) got good spray out of the bleeder when the thermostat opens. Anyone else have this problem?

 

Have you changed the coolant lid? It doesn't sound like it's properly holding pressure which will allow your coolant to boil at a lower temperature than it should.