Engine Overheating

New day, new problem... I sware this 1980 American truck is turning into a 2022 Chinese import, one part at a time! Got my new calipers installed and took it for a victory drive around town. Within about 5 miles, the engine temp was pegged on high. I replaced the thermostat a few weeks ago so that can't be it. What's my next move? Radiator has fluid.

First, I wouldn’t rule out the new thermostat. Could be bad out of the box.

Do you have an infrared thermometer? Nice to see if it’s overheating or the gauge.

Could be the radiator, or the water pump.

how long did this truck sit before you got it and got started on it? radiators can get clogged very easily from rust a crud that corrosion "grows" in the block. it just sits there until put back into service and circulation, vibration and heat cycles happen. if you do decide to replace a rad be certain to flush the system very well including the heater core. another item to check is the thermostatic fan clutch. when running at temp you should be able to hear it loosen and tighten, cycling as the temp demands it. if it never "loads up" and the engine is getting hot then it's not cooling the radiator.

1980, 300. sorry. I think you have a solid fan. skip the fan clutch.

Truck was sitting for 3 years before I got it and got it running. I actually have an extra new thermostat. I'll try plugging that in. Trying not to replace the radiator as it is a pretty expensive part. Radiator had a leak in in it when I got it. I'm embarrassed to say that in my thriftyness, I used Bars Leak to fix it. Do you think that could be causing my problems?

if the rad was dirty and the system was not flushed really, really well before hand then yes. it could have made matters much worse by gluing the trash together. I'm sure that you have the small brass rad also and it "may" be to your benefit to have it rebuilt if there is a competent shop available. im not saying that it would be cheaper but it may be better. there is a lot of junk out there. if going new i would look at an aftermarket all aluminum type and more capacity is not a bad thing if you can get it . especially in texas!

Looks like you and I have very similar trucks.  I had a problem a while back with my temperature reading high, but using a laser thermometer on the thermostat housing showed me that the engine was actually at the correct temperature.  Replacing the temperature sender that is screwed into the block ultimately fixed the problem.

I would definitely validate that you *really* have a temperature problem first.

very true. good info is mandatory for good results.

Easy way to test your thermostat is to put it in a pot of water and boil it on the stove. You should see it open at the correct temperature with a cooking thermometer in the water and make sure it's the temp you want. Easy way to check new ones out of the box before installing too. I'm probably telling you something you already know.

Also, I would think that the heater hoses would be a great way to test coolant flow through the block. Just loosen them from the ports on the firewall and pump distilled water through one hose and see what comes out of the other hose. That's how I flushed my 460 block.

Have you confirmed that the water pump is working?