I own a 2005 Ram 2500 with the 5.7 hemi and I am having some heating issues. It all started when I thought I had a blown head gasket and added a product called "SteelSeal" The heater just stoped blowing hot air shortly after. A few months after adding the SteelSeal I disconnected the heater core hoses and flushed the thing with vinegar to remove any crud caused by the SteelSeal. After that some heat returned to the cab but only for short periods. I have since had the head gaskets replace and subsequently the cooling system was drained and replace with fresh fluid. Still I have been having the heating issue. With on last effort to fix the problem i tried flushing the heater core one last time today with prestone radiator flush and hot soapy water. It seems to be a lost cause as now after running a few thermal cycles to remove bubbles the problem still remains. The heater core hoses are hot once the engine warms up so I preaty sure its not the core. Thought it might be the blend door but if you turn the heat to max with the fan on low it will blow hot air for about 30-40 seconds before it starts pushing not cold but cool air. And If you turn the fan to full blast it will blow hot for about 10 seconds and then just push cool air. If I turn the fan off for a moment and then back on it starts blowing hot for a moment again. I am at a loss right now and I am hoping for some ideas before i go tearing the dash apart or buying something I dont need. I know this is a long post but I wanted to be as descriptive as I could. Please help!

 

I've been thinking about this a bit more and I still wonder if your blend doors are to blame. Even though you get good heat briefly, it's possible the blend doors are kind of loose and just flapping around inside the air box. With your truck off, but key in the on/run position, you should be able to hear all the mode and blend doors move around as you change the heat and vent settings.