Overheating while cruising can be caused by insufficient flow through the radiator. Bad water pump, clogged radiator, collapsed hose, etc.

Do you have or know anyone with an infrared thermometer? That's the easiest way to tell if it's too hot. In any case, while cruising, the coolant temperature should not rise and many times will drop.

When I first got my truck, the radiator had a pinhole leak. When it got warm it would squirt across the engine. Good times.

 

Welcome. It's great to see young people getting involved in an old man's sport! We've all been through times of making do on the cheap. I think getting an actual on the engine temp when the gauge read high is important. The infrared themometer is quick, cheap and easy. Almost every garage, radiator shop and parts store will have at least one under the counter. I recommend you plead your case and get someone to give you a reading.
Another free diagnostic is to run the truck until the gauge gets into a high reading then open the heater valve (temp) and turn the heater fan on high. You are really adding another small radiator to the cooling system and the temp gauge should go down at least a little.
We usually think in terms of internal radiator restriction, but the fins can get clogged externally with dirt & debris. I like to clean the fins with compressed air but I would caution against going after an elderly rad. with 100 psi shop air. I would try a garden hose with a nozzle, spraying water through from both directions.
If this radiator is old & leaking, I don't think you will hurt anything with a dose of stop leak. I would keep the engine running with the heater valve open to prevent plugging the valve or heater core. Good luck!

 

It may also help to do a radiator system flush to remove old fluid and some rust scale. You can use the prestone kit to add the fresh water inlet to the heater hose line. Generally there are two types, 20 minute and 50 mile; read the labels. 20 minute type you pour in the radaitor and run it at idle for 20 minutes after it gets warm. 50 mile type you pour in a drive around with it for a few days. Then let the engine cool and pop the cap off the radiator and flush it out with a garden hose. If you are connected to a waste water treatment plant you can (call to verify this in your area) pour the old radiator fluid down the sink drain.

Here are some other things you can do to troubleshoot your radiator system. When the engine is first started how long does it take to warm up? A hole in the system prevents pressure build up which helps to increase the boiling point of the coolant. When the upper hose is warm, how does the lower hose compare? Can you squish the hoses easily? Is there a spring inserted in the lower hose? A spring prevents the hose from collasping at higher RPM's. How much paint and what type is there on the radiator fins? This affects the cooling capacity as much if not more then dirt and corrosion. The normal paint doesn't allow heat to transfer as well, it should be radiator specific. Was the engine rebuilt to include an overbore? As you overbore the engine more heat can transfer to the coolant faster then the stock radiator can dispose of it. What is your ignition timing set to?

I know these are a lot of questions but they all contribute to overheating. Oh and if you don't have a laser thermometer you can use a oven thermometer stuck between the fins near the top inlet of the radiator to see what the temp is. It's not calibrated but you get a pretty close idea.

 

What if the PO has used stop leak several times and the rad is plugged? Does any coolant come out of the petcock when open? I would pull the lower hose and drain it. Than flush it with water before I used radiator flush. All of this before fixing the leaks.

 

Sounds like a weak water pump/ or clogged radiator. I would start the truck and look to see when hot if have good cross flow running through the radiator. Also when after draining fill back up with water and pull bottom hose see if all the water comes out fairly quickly if takes a while prob stopped up. just a few things to look at before buying parts.

 

to find a leack cover all the holes andput smoke in the radiater thths what i did :P

 

I recently drove a newly purchased 65 Merc - with the same engine as yours Lucy - about 500 km from the interior of British Columbia to the coast, in 35+ Centigrade heat (about 100 degrees Farenheit more or less). The temperature needle was maxed out all the way, which initially freaked my freak, but after checking the rad and coolant a few times - no boiling over, nothing scary at all in fact - I figured it must be a faulty thermostat or gauge, and just kept driving, albeit with my fingers crossed and not too fast (not that too fast was really possible). Although it sounds like your rad needs some repairs, you might also need a new temperature gauge, or fix the one you've got. As jowilker says, monitor it. I'm getting everything checked out in the next week or two by qualified Ford mechanics, so if I find out anything useful I'll pass it on.

 

my radiator is original and i never have problems with high temps. is suppose its possible its been rodded out when i was little but not to my knowledge. if you dont overheat in town but do at highway speeds i would say t stat is stuck open. add to that an old radiator with less flow due to hard water deposits in the tube and that reduces cooling capacity even further. also bents fins dirt etc. use an infrared thermometer to check when the t stats open and youll know if it is opening. inlet and outlet temp of the radiator should never be over 20*f on an 18 wheeler radiator. not sure on these old trucks but maybe 10* i would think at a decent rpm with truck not moving. more than that and it signifies low flow which is usually plugged radiator tubes or possibly water pump although the impeller would have to be rotted or rusted away or possibly spinning on the shaft.