Be sure you have enough coolant in the system by filling the coolant reservoir to the max line. Close the cap tightly. Start the engine and turn the heater on. Allow it to reach operating temperature. Looking at the engine from the front of the vehicle, you should see a solid metal coolant line running across the front valve cover. There is a vertical stem attached, with a bleeder screw. Open the bleeder until no air bubbles come out and only coolant comes up. Close the bleeder. Wait a couple minutes. Repeat. Do this as many times as you need to until you are confident that all air is expelled from the cooling system. If you consistently see air coming out of the bleeder, you may have a bad seal on the coolant reservoir cap. It should be replaced ASAP, because it could cause poor engine cooling or coolant back-up/overflow out from under the cap. New caps are available at most auto parts stores or your GM dealer.
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