Heater core issue...or is it working fine?

It sounds like water is not circulating through the heater core, or circulating very slowly. Do you have heater shut-off valves, and are they open?

I do have the valves and they are both open.

It would make sense that a blockage could be the cause. Is there a way to "blow out" the core without damaging it? Air pressure perhaps. Anyone know how much air is too much (semi warm air is better than a leaking heater core)? Perhaps a solvent flushed through?

Sometimes, if the manual valves are left closed too long, or maybe if only one valve is used, trapped water, silt & antifreeze in the heater core seems to form a type of green jelly that blocks things up. It's a good idea to purge the cores by opening the valve(s) every year or so..... and I'm as bad as everyone. My 2 valves have been closed for 10 yrs- I should take my advice!

As far as using air pressure, remember the cores were kind of thin & fragile in 1971, they are now 43 years old and are completely unrepairable if they perforate. Ideal would be a good soak in a big pan of hot water but pulling the dash is a chore. Eaaassy does it!

I would try the ol' reverse flush on the core. I've had to do that even after installing a brand new core. It took awhile to get all the crap out of the system. That would be a lot easier than getting the core out of the car.

Is there a way to know which tube is intake and which one is the output? What happens if I hooked the tubes up backwards?

Jimlah,
I should have been more specific. I didn't back flush the core in my cat. It was another vehicle. That being said, the principle should be the same. I do have my cat torn apart in prep for engine and trans remove. So I had to check it out. From the block a hose goes to the tube on the passenger of the tunnel. From there it moves to the drivers side and connects to the valve. It travels through the valve and core then back out on the drivers side and travels through the tunnel in the tube on that side where it connects to the water pump(suction). So I 'd say run the water back through the driver side tube. If you look under the dash you should see the heater core connections on the driver side. They do look small and I'd keep water pressure down. As Bosswrench says, they're old thin & fragile. It may be best to remove it. Hope I got it all right! Sorry if you didn't need all this detail.

I appreciate all the details. I'll check the tubes and try and reverse them to improve flow. I also noticed that the heat selector slider only goes about 3/4 to the right, indicating that the valve may have some gunk on it keeping it from fully opening. I'll let you know what my troubleshooting reveals.

My slider moves, but the valve under the dash doesn't. I plan on doing a complete teardown in the next few years. But for now I'm just trying to get it drivable so we can take a few road trips before that happens.

Jimlah,
It looks like I have steered you in the wrong direction as far as the tubing in the tunnel is concerned. The post from fordgt has pictures of the tubes actual path through the tunnel. I reversed the path of the tubes. I assumed that the tube on the drivers side started in the back and ended there up front. That doesn't seem to be the fact, sorry to miss-lead you. I'm still learning the ins and outs.

Well I swapped the tubes (turns out they are backwards now). It seems to work a little better. Perhaps I will run it a few more driving days like this then swap it back. Hopefully the reverse flow will free it up a bit more. Its winter in Colorado, so not much driving these days.