O.k. Thanks, XXXXX XXXXX gets in if work was just done, or if it had hi miles, I would say heater core
starting to clog, but I don't think so in this case.
There is an internal G.M. bullitin for this problem. It involves Purging the cooling system. Here are the steps;
•With the engine cooled off, remove the coolant pressure cap.
•Remove upper heater core hose from the metal pipe on the engine.
•Add coolant to the metal pipe, until full.
•Raise the rubber portion of the heater core hose as high as it will go with it still attached at the heater core. Using a funnel, add coolant until the hose is full.
•Reconnect the heater core hose.
•Start engine, have assistant raise the RPMs to about 1500 to 1700 RPMs, top coolant off at engine, then before lowering engine RPMs, install the coolant pressure cap.
•Fill coolant reservoir as necessary, then check the heat at idle.
If this doesn't help,A Coolant Vaccum fill system might help. But I think this will take care of it.