I'd flush the core itself, disconnect the hoses from the engine if you haven't. It's the only way to ensure that the core is getting flushed completely.

Then, the thermostat. That's it. Nothing else should affect the heat.

 

that right there is the key of confusement.

Let's go back to the FSM:

Where exactly is the needle, if it's in the same place? Is it to the left, saying it's not getting above 130, or is it just above, showing "real" temperature?

I've lost track of who has done what since this thread went haywire.

What percentage of coolant are you running? All water, all anti-freeze, 50/50, or some custom concoction/combination of those?

Here's what I'm coming up with. There's only 2 working scenarios here.

1) The engine simply is not getting up to operating temperature when cold, the thermostat is not opening, and the gauge is completely unreliable

2) The thermostat is opening and the engine is reaching operating temperature, but something is blocking flow to the heater core when cold.

 

I agree with magnethead you have a few things going on.


You are 100% sure on the temp of the thermostat you installed pull it and check it should be stamped on it.


Next replace the temp sensor if you have not done so


You may have installed the wrong thermostat

 

i am 100% sure the thermostat was installed correctly you cant install it backwards on the 318 the thermostat housing wont let it the temp is 190 and i pulled it and tested the thermostat it worked properly as it should 190 it opened and closed below 190 the blocked flow idk how it only happens when its cold my mixture is 50/50 i usually run 70/30 not because its so cold but if i ever need to add coolant i can always use water if i dont have coolant but i just mixed it 50/50 this time heres another thing i removed the condensor core right around the time i had issues with the heat it was rotted and no good i am wondering if that might have something to do with it

 

Had the same trouble. heater core partially clogged. Here is how I resolved the issue on a 3.9

https://dodgeforum.com/forum/2nd-gen...ow-to-fix.html

 

Garrett,
I assume that you getting very little heat means that you still get lost of air out of the vents, but it isn't hot.
2 things that could also cause that condition are the controls not working properly and the blend door not working correctly to switch completely to heat.

I realized you tried the cardboard to block the cold air. I had a 1977 full size van that I had to hang a rug over the front grill to make it heat up in very cold times.
Dave