Hey guys/gals, I've got a bit of a confusing one. At least, confusing to me. First some specs on the truck so you know what we're dealing with;

2003 Dodge Ram 1500
Short Cab Full Box
4.7L V8

Since I purchased the truck in 2014 it's had little to no heat from the HVAC system, which can be problematic when it's -40 outside and you're trying to defrost your windows. This year (after going back to school to join the mechanical trade) I decided to do something about it.

I started out by changing out the thermostat (for an unrelated issue) as the truck was genuinely taking forever for the coolant to reach operating temperatures. This had a noticeable difference on the speed the truck warmed up, but the HVAC system was still only blowing cold air.

Next I went and gave the heater core a good flushing. Made sure it had really good flow in both directions (starting with a back flush). Afterwards I ran the truck for a while with no noticeable change to the heating situation. I didn't think to check the temperatures of the lines beforehand (see if one was hot and one cold) but after the flush, and running the truck for a while, both lines at the firewall were pretty hot to the touch.

Finally I tackled the dreaded HVAC box, gaining access through the dash. I removed the blend door actuator and manually manipulated the doors (2 doors linked together by a small metal bar). I was able to clearly tell that both doors were working so I decided to replace the actuator, since it was inexpensive and I didn't want to tear into this dash again. For those wondering, it's the electronic type with no vac assist. After a quick calibration I tested it and everything moved perfectly. While I had access I checked all the other actuators as well, they all seemed to be moving fine.

That said, I still have no heat... Quite frankly I'm stumped.

One more thing to add. When I shift to the cold setting the air does get noticeably colder, like very cold. When I move back to the "hot" setting, the air is still cold but not AS cold.

 

Poor airflow thru the heater core, or, you have an air pocket in there. I don't know if the third-gen trucks have a valve on the heater lines, so that it only flows if you actually want heat... but, that would be something else to check.

 

It appears that the coolant is direct flow through the heater core and the actual heat is controlled via the blend doors. There's no valve on the heater lines. Also, with the temp setting to maximum heat and the fan on full there is a pretty high amount of air flow out the vents.

I also have 2 hot lines at the heater core. If there was a clog or air pocket wouldn't I have one line a lot cooler than the other?

Someone on another forum suggested to blow out the heater core with some shop air and pour some CLR in there, let it sit for about 30-60 minutes, then flush it out. Not sure if that would do anything either since I have 2 hot lines, but it might be my next step if nobody else has any ideas.

 

if there is no flow, the coolant would be the same temp on both lines. with no flow, there is no coolant movement...the hot coolant from the engine transfers heat back to the outlet side that is not flowing

if you had good flow there would be a slight temp drop on the outlet hose. the heater core will drop the outlet coolant temp. take a heat gun and test this on a vehicle with good heat.

the CLR is a good next step. if it doesn't work replace the heater core.

 

Very good possibility you have a broken blend door inside the hvac housing. Actuator and arms would move properly but doors inside are not. Common issue. Search this problem

 

That sounds like the issue that I had too. The fresh air door broke off and blocked most of the air flow through the heater core. I was able to remove the broken door but would have had to take apart most of the components under the dash to put a new one in so I am running without a door and it seems to work well. My truck is a:

2003 1500
Laramie
5.7