I thought I had a bad thermostat at the beginning of this heating season. This Safari is a 2001 with 64000 miles. I put in a new thermostat flushed the system. Replaced the heater control valve, no luck. Ordered a new heater core had it installed. No luck. Took it back to shop they replaced all bad vacuum hoses took nipple out that feeds heater core said all look good motor clean. I have about half the heat I should have. When I stop to idle heat goes to almost nothing. Shop foreman says he does not know anything else to do. I am not getting enough flow but nobody knows why. GMC has no technical support. I was thing of drilling out the nipple that feeds the heater core. It is a 1/2 inch nipple with a 5/16 inch hole I thought of making it 3/8 inch. Guy at shop says the larger I make it the more risk of blowing heater core.