03 buick rendezvous heater blows cold air at stop light

My 2003 buick rendezvous is freezing. If I take my foot off the gas, the temp. gauge goes up and the heater blows cold air. When I start moving again everything returns to normal after about a minute. Makes for very cold rides in town and my auto start pointless. Coolant levels fine, what’s wrong?

The problem might be trapped air in the cooling system. If there’s air trapped in the cooling system and you stop, the water pump slows down and the air collects in the heater core because it’s the highest point in the cooling system and you get no heat. Once the vehicle is moving again the water pump spins faster to force the air out of the heater core and the heat returns.

To find out if this is the problem, get the engine up to operating temperature. With the engine idling, slightly loosen the upper radiator hose clamp. Take a small flat bladed screwdriver and slip this between the upper radiator hose and the radiator hose neck. Allow the engine to idle until all that comes out of the upper radiator hose is coolant. Remove the screwdriver and retighten the hose clamp.


Are you checking the coolant level at the overflow tank or the radiator? If you are just checking the bottle you could still have a low coolant level. If it is air trapped in the system there is usually something you can open or remove at the highest point to let the air out until coolant comes out.