New to the forum, but I have a 2013 Ram 1500 Larami, Hemi, with an 8.4AN. When I turn the heat on first thing in the morning and set temp to high the system will blow hot 120' air. After you adjust the temp to a lower temp and subsiquiently take the temp back to high it will not give you hot air as it did when you first started the truck. Have a themomiter in the vent and when on high it show the temp going back and forth between 70 and 110' but its not very hot. To date dealer replaced heater core and the valve body right of the water pump. They are now waiting for Chrysler to tell them what to do.

Has anyone had this same issue or know a fix

 

Isn't there some sort of gate in the heat/cooling system that meters out how much air passes through the heater core?

Though I suspected a plugged heater core, I also find your comment that the heat works when first started then you turn it down, then when turned back up it does not heat up. This screams the door or gate is not actuating properly. Bad/sticky/fussy solenoid probably.

This is all a guess as I am not very familiar with the ram environmental control system.

 

They already replaced the heater core which originally was blocked. Mechaniclly I believe everthing is working, when you change location of discharge those doors are working, in terms of blend doors for controlling temp I like you am not sure how that part works. Coming to work this morning got 120 degs out of it for about 15min then it dropped to 70 all by itself and would not go any furhter up or down. Once I slowed down from 70mph to 30 the system started cycling between 80 and 120 and remaind that way until I put it on defrost where the temp started cycling between 60 and about 78.

Still waiting for the techs in Abron Hills to tell the dealer what to do next being there are no codes to read.

 

I wonder if there is a bubble stuck in the coolant line?

 

I was going to suggest a burping may be in order