We recently purchased a 08 equinox and it had problems over heating n heater core went out with last owner. They replaced heater core , however still had an occasional problem over heating. We replaced the coolant cap and thermostat and coolant temp sensor we flushed all the lines because they had put stop leak in and it started corroding the water outlets. We have now flushed the system around 12 times getting alot of rust gunk out. We bled the air out and had hot heat then you drive it awhile and the heat goes away. You rebleed it and get heat again but then it goes away again. Just wondering if anyone thinks this could be the blend door because when u switch to defrost the 3 vents in middle on dash for defrost not all 3 wrk. Drivers side doesnt wrk right ,middle wrks passenger barely wrks. Then when u switch back to vents whether on ac or heat driver wrks better than passenger. We have had it tested and everything comes back as not head gasket cause it doesnt over heat everytime. N so far almost a week with no over heating just the heat goin out. At our witts end just need a little help not use to working on the ones this new.