I'm trying to hunt down an issue I'm having with no heat at idle or yet alone just cruising on my 2000 GMC Yukon Denali. About the only time I get heat is when I'm either accelerating or when the engine is under additional load. It was doing it before I changed the heater core and still doing it after changing the heater core. The heater control valve is in good shape and working. I have bled the cooling system multiple times and yielded no result change. Also the inlet and outlet are pretty warm as well.