So it's kinda a long story, but here it goes. A while back I started having heater issues on my 2012 Ram Hemi. I back flushed the heater core multiple times and kept getting debris out. It appeared to be either the coolant had went bad or gotten mixed with the wrong stuff somewhere along the line and was wreaking havoc on the heater core. After trying pretty much everything I could think of to avoid replaced the core (and removing the entire dash) I was left with no choice. I replaced the heater core, but due to the contamination I found I replaced pretty much everything: Radiator, water pump, heater core, thermostat, and radiator cap. I also pulled the block drains and flushed them out as best I could. Put everything back together and did one last flush, bypassing the heater core with a clear vinyl tube so I could watch flow and keep any remaining contaminants out of the new core. Flows were fine and temps good, so I drained that off and replaced with 50/50 premix, bled (bleeder screw removed) and burped the system numerous times with the heater core connected to the system. While air plenum was out I also cleaned the evaporator, installed a cabin air filter and installed a new recirc door actuator (because it's almost impossible to get to once the dash is back together. I also did the reset on the heater controller and verified proper movement of all the doors and actuators.

The problem is that I've had intermittent heat ever since. Sometimes the heat will be great at about 120ish coming out of the vents, then in just a few minutes it will fall to about 60 (ambient temp 40ish). It can be heating well one day, and not the next. When the heater goes cold I can pull over and turn the engine off for only 2 seconds, restart and get heat back instantly (no idea why) But it will only last for a little while. It's not a blend door issue (that I can tell) as I have the glove box removed and can monitor its position and movement as the heat comes and goes. No apparent motion of the actuator or door as the heat fluctuates. I've burped the system numerous times, both on level ground and on an huge incline. I've used a block tester to test for combustion gasses from the head while system was both hot and cold. Test revealed no combustion gasses in the system. It seams like a big ol air pocket somewhere in the system that is moving around, or that it's pulling air from somewhere. But since it passed a block test I'm not sure how that is possible. About the only thing I have not done is a vacuum check of system. I'm not loosing coolant anywhere and have no combustion gasses in the coolant, so I'm skeptical this will tell me anything I don't know. It seems unlikely that anything is clogged with debris since I can get really good heat at times. The truck itself does not overheat at all. If anything it runs a little on the cool side. Rarely does the gauge temp get over 200, it usually hangs out around 185-190. I've been chasing my tail on this for weeks now. As soon as I think I get it figured out, it fails again. I'm open to any suggestions. Thank you very much for your time!