2007 Uplanders were having problems with heaters not working when idling or at low speeds. This was caused by seals seeping or leaking anti-freeze. The heater will become "air locked" The seals were defective. I fixed mine in half an hour without removing the seals. Her is how I did it.
1. Make sure engine is cool, not running for at least 2 hours.
2. Remove coolant cap from reservoir, it will look like there is sufficient coolant in it but go to next step.
3. Remove radiator cap, top of radiator. You will notice you can not see any coolant in top of radiator. Fill with proper mix, before replacing cap, carefully clean any gunk in cap with 409 cleaner or similar cleaner, rinse. Replace cap.
4. Squeeze radiator hose along top of radiator with both hands, squeeze and release several times. Remove radiator cap and fill again. Repeat until radiator stays full. Now replace radiator cap and rill reservoir with anti freeze mix to proper level indicator on tank. Replace cap. When all air is out of radiator heater will now work.
If this happens too often you will need to put in some radiator sealer or have gaskets replaced. Chevy knows about this problem and will charge you $$$$ just to remove the air from cooling system.
Wednesday, January 12th, 2011 AT 2:18 PM