Factory AC/Heat not getting hot

[fireguywtc]

Its the season now for cooler weather and I am actually needing some heat here in Texas. My 70's factory AC/heat system has never gotten very hot since I have owned it despite several attempts to fix it. It does get warm, but just barely warm enough to take the nip out of the air when the truck is fully warmed up.

Here is what I have done so far.

New heater core with new hose lines
New 195* thermostat. Truck temp gauge shows to be around 185-195 when it is fully warmed up.
has new temp control valve and is opened.
Checked the operation of the blend doors and such when I replace the heater core.

I cannot figure out why it won't get hot, seems in theory it should. I have done everything I can think of so now it is time to elicit some expert help and ideas. TIA

[flyboy2610]

Could be that the water pump is plugged where the heater hose goes, not letting much hot water through.

[68RGR]

Are you certain the heater valve is correct and "plumbed" properly? I think there may be two valves and you need to be sure you have the correct valve.

Have you checked the hoses going into and out of the heater core (or, at least, the firewall) with the heat on and the truck up to temp? Naturally, both should be hot.

That's all I can think of other than an air flow issue.

[Engineer]

Could be that the water pump is plugged where the heater hose goes, not letting much hot water through.

I agree if you have replaced the core and the switch you either need new hoses or a new water pump. They are cheap and easy, about 40 bucks for a water pump and a couple more for the hose.

[fireguywtc]

Thanks for the replies, I haven't been forgetting you guys. The heater hoses are relatively new as well, replaced when I rebuilt the motor not too long ago. The pump has been what I am leaning toward as well. It seems like the heater core in the AC setup is a poor design for flow. It is upside down with the water coming in and going out from the bottom, so it has to fight gravity to work.

The pump that was on it was a used one of my old 352. It only had 20k miles on the pump but a few years of sitting around. Guess I just got to tinker with it and hope I can get it blowing hot.

[tac]

Split the chore in half. With it up to temp and the heater on, feel the heater hoses. Are they hot on both sides of the control valve and on the exit of the heater core. If they are hot all around then it is a problem with air flowing across heater core.

[1972hiboy]

another idea to bring up. What shape is your cab in? And this is what I mean. do you have holes in your cab? bulkhead fittings rotten away, poor weather stripping around the door allowing air to come in? Alot of small holes makes it hard to the heater to keep up. I've had some trucks that came in for a/c issues but there a/c worked fine. lots of small holes and cracks here and there would let the cold air out thus not cooling the cab as desired. The same could be true in opposite. Just food for thought since it seems like you have covered the basics already.

[fireguywtc]

No my cab is sealed up like new with all new weather stripping, anti-rattle kits, insulation under the new floor mat. All the pass through holes in the firewall have been filled and no rust holes. I did it for my AC and it gets cold in the cab, I'm just having a heat problem. I even deleted the fresh air intake from the HVAC box so it just recirculates.

[1972hiboy]

wow, its gotta be something simple. I mean my heater will get me sweating running at full heat on a cold day and my cab is very loose. Have you flow tested the heater core? its only going to be two root problems, a circulation issue or a convection problem. i.e the fan is pushing enough air? nothing blocking the fan, ect.

[sargentrs]

Might want to check your cable to diaphragm lever connection. If hot water is getting to/through the core and the fan is blowing/circulating air, maybe the diaphragm is closed preventing the heater air from being directed properly?

[cole71]

I had something similar happen. I run an electric fan and I had my sender for the fan in my intake and my heater hose in my thermostat neck (302). My heater never would get hot until I switched my fan sender to the thermostat neck and the heater hose fitting back to the intake. Maybe this can help.

[fireguywtc]

I replaced the water pump, its quite a feat with AC system, just more stuff to remove. Anyways, it seems to have fixed the problem and seems to blow much hotter now. Heater hoses got hot also. Later this week will be the true test as it is going to get cold so I will see how hot it really gets. Thanks everyone for the help!

[fireguywtc]

I was wrong, water pump didn't seem to help when I drove it. On to the next thing.

[hazelnut]

Sounds like one of the doors isn't opening up from the A/C side to the heat side to let the heat out into the cab. I know you've put a lot of time into the factory A/C reworking it so hopefully it's something simple and easy to fix.