I am freezing my butt off here in PA. My heater quit working. Here is what I know.
1. There is some heat but not much coming from the rear heater.
2. I replaced the thermostat.
3.The heater core is not blocked. I removed the heater hoses and attached a garden hose to thestubs and water flows through the core in both directions with good pressure.
4. I removed the inlet heater hose and outlet heater hose from the mechanical control valveand joined them with a piece of clear plastic hose to by pass the valve,still no heat. So It probably is not the valve stuck shut.
5. I pulled the hoses apart with the engine running and the inlet hose pumped water out. 5. With the raditor cap off, I quick squeezed the plastic hose on the heater discharge side and and water squirted up out of the radiator.
6. The fan works.
7. The air discharge selection works.(floor, dash, floor/dash, windshield/floor,windshield)
8.Having completed these tests I assume that the water part of the heater is all open and functioning OK. That leaves me with the idea that the problem is related to the air control system. The air seems to be blowing straight in from the outside. The colder it is outside the colder the air is that theheater is blowing.

Here is what I don't know.
1. I can't tell if the outside air and the recycle inside air mechanical selection control is working. Where is the passanger compartment air intake opening for outside air located? Where is the flap located?
2. Where is the recycle inside air intake located? How can I tell if the flaps are all opening and closing properly? I can hear flaps slapping open and close but I can't see if they are working.

Thanks for reading this and I would appreciate some advice.