The heater in my 08 will not get hot until you drive for several miles no matter how long you let it warm up while idling. This is my son's car and he drives 9 miles to school one way. His brother's school is about 5 miles away. He tells me that he can turn the heat on full blast and it blows only cold air until he's almost at his brother's school to drop him off then it starts blowing hot air. I have a 2004 chevy Avalanche and it has the same problem. I found that in my Avalanche I have a slow coolant leak and the problem happens mostly when the Dexcool is low in the overflow. But I don't know if it's the same with his impala. Does anyone know what this could be? OAN: Once he was at McDonald's drive through and it was taking awhile and he said steam started coming from the hood. He even videotaped it on his Iphone and showed it to me. He called me when It was happening and I don't him to check the temp gauge. He said it was reading normal. And after he left Mcdonalds and got on the highway it stopped steaming. When he got home and I opened the hood and can see where the area above the overflow was wet. What could be causing these issues? Do I need a radiator flush? New thermostate? Is there a leak or what?