Seems the heater core like to plug up internally, it usually starts with the driver's side blowing colder air. The most reported is the HD trucks with the 6.4.
Be sure to rule out the thermostat, coolant should get up into the 200s and both the upper and lower radiator hoses will be hot.
As for a fix you could try to back flush the heater core, this has worked for some but others have had to replace the heater core
Here is an interesting scene: I bought my home in late 2015, it having been built in 1972. Over the years, I had been practicing a ritual related to me by a plumber, who gave me a money-saving tip over THE LONG HAUL! When washing my hands in the kitchen sink, he said "use Dawn classic blue dishwashing liquid". He further stated that this miracle degreaser will keep my pipes clear in spite of insulting them with grease down the kitchen drain, and in my case, further down stream, lint from the washing machine!
Well, I got away from that habit, because my little bride took away the Dawn dispenser, and replaced it with a dispenser which emits a foamy, foo-foo scented soap. In keeping with the "Happy wife/ Happy life" dictum, I went along with this new norm.
Fast forward just over a year (about 9-10 months ago), and I arise in the morning and was greeted with greasy, slimy water all over the kitchen, due to the dishwasher attempting tp drain through the garbage disposal port FOR a dishwasher drain hose, and over filling said sink.... yada yada yada.
So, I took a toilet plunger and attempted to clear the blockage, and all I could accomplish was a VERY slow drainage. So, I went outside and removed the cleanout plug from about a foot off the ground, in the brick wall, and nasty water gushed forth. (I'm getting to how this is germane to a truck heater core blockage).
Vinegar/baking soda volcanoes, which typically perform miracles did almost nothing. Ditto for plunging the drains over ans over and over. I just could NOT get the drain to move faster than, oh, maybe a gallon per 20 minutes.
Time to revisit the "Good old days"! I went out to my garage and found my sodium hydroxide stash (LYE). I donned my PPE and commenced to apply this corrosive and deadly cleaner into the drains on both sides of the sink, adhering STRICTLY to the instructions to NOT apply this stuff more than 4 attempts, because it will eat PVC pipes! But, progress was just about, maybe 25% better.
What to do? Well, I contemplated.... then, the light came on and the possibility sharpened that the problem was further downstream.
So, I went downstream and applied this same lye treatment to the washing machine drain hose opening 3 times... and improved the overall problem to about 50% or so. HMM!
I then took my drain snake, pushed it down into the cleanout port by the kitchen sink (outside) and it hit something that felt solid about 20 feet down stream... and I augered it, and pulled back a lint ball/grease ball concoction and the drain went back to 100% function.
NOW TO MY POINT!!!..
Once upon a time, in the days of yore, we mechanics AND DIY'ers could go to AutoZone or O'Reilly's and get a product which used Oxalic acid to address the most stubborn of radiator/heater core blockages. I used it routinely to address the "peanut butter" goo caused by mixing old fashioned Green coolant with Dexcool (HOAT).
It worked like a magical elixir! Problem is/was, the EPA had a dim view of this corrosive toxin being washed into drainage systems... and banned it in THAT application. I get it from the hardware store, due to it's usage in furniture finishing. (It's been a while, so I do not know if Oxalic Acid is still available there). I will tell you that, when I ordered LYE, they made me feel as though I was being interrogated by the Deep State!
Oxalic acid WORKS! HANDS DOWN! BUT BUT BUT BE CAUTIONED!!! IT IS DANGEROUS! And it requires copious amounts of flushing with baking soda (arm & Hammer) followed by MANY gallons for fresh water flushing. If the flushing is short changed, then Bye Bye head gaskets and soft metal aluminum components such as radiators, heater cores, and engine components.
P.S. I'm back to using Dawn in the kitchen. That pump dispenser sits right next to the Fu Fu stuff my wife so loves!
