Or, if its water pump is a '62 type, the phenolic water pump impeller might have cracked and/or worked loose of its iron hub.

But really, the engine should overheat easily anywhere if the water pump's not doing its job, not just on the highway.

Pierre's suggestion is a good one; watch for water to start flowing obviously into the radiator via the top hose when the thermostat opens.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

After 30 minutes of idling, the lower hose will be cooler than the top, but not cool. It'll be pretty warm to the touch. So you have a lack of flow due to something. It's just a matter of taking things out and checking; pump would be last on my list.

Any chance your problem is just a loose fan belt? I have had a couple over the years that would hold at low RPM but slip at high RPM or at load. They looked perfectly fine and did not squeal, but would slip like crazy under load, even when tightened.

And is the thermostat installed with the button into the head? Should overheat all the time if reversed but be sure.

I failed to take notice of the year, those early phenolic deallies are suspect...

To be clear, I've never seen a radiator hose collapse, I've only heard tell of it. But a blockage of some sort, lack of proper flow does seem likely.

The low vs high rpm belt slippage is consistent, but I've not seen that either unless there was a telltale squeal when slipping at higher rpms.

brian

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https://tinyurl.com/yynpj4v2

Please specify: Brand? Type? New? Old? Did you pan-test it before installation, and are you sure it's right way round and opening fully (not blocked by gasket material or similar)?

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

Test the thermostat in boiling water and see at what temp it opens and when the water is boiling how far it has opened.

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Ed
64 Valiant 225 / 904 / 42:1 manual steering / 9" drum brakes

More specifically: Force thermostat valve plate open, insert string or fishing line and let plate close on it, tie other end of string to pencil or chopstick or similar, long enough to lie across both edges of cooking pot so you can submerge stat fully without it touching pot. Use candy thermometer to keep track of temp and note temp when stat falls off string and hits bottom of pot.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

As an FYI, the 2 badly slipping belts that did not squeal for me were both Dayco Top Cog types, with the belt serrations on the outside. Needless to say I never bought a 3rd one!

timing over 3000 is suspect.

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Yeah....Im the one who destroyed this rare, vintage automobile.....

Good point, Sandy. If the timing is advancing too much it'll get hotter for sure...

Did you play with/check the timing recently?

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Actually, if the timing is advancing too much it'll ping loudly and run poorly. If the timing is retarded at high engine speeds, the engine will overheat.

But we aren't even sure yet if the engine's actually overheating. All we know is the gauge reads high under certain conditions, then goes back to a normal reading "within seconds", which is suspicious with regard to the engine's actual temperature. Engines tend not to cool down from overheat to normal within seconds.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

Thanks for all the support and feedback! Hopefully I will get to work on the car later this week.

B

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These ones go to eleven...

Double check for air pockets also. I did a coolant flush and had a little bit of air in the system. This caused my gauge to spike under a load and fall back to normal at idle. But this would affect you at every speed i believe.

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~Nic

Fair enough, Dan, I seem to recall timing too far advanced causing overheating, but I've forgotten far more than I know about cars.

I agree with the suspect nature of how fast it cools down, as I said in my earlier post, is it really overheating or is the gauge just saying so..

brian

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Advanced ignition can overheat an engine eventually,but you will probably be more concerned with the torched pistons by then... A retarded ignition at higher power will certainly cause overheating,which leads me to ask is the distributor advance mechanisms even working? It certainly seems like a gage issue but it's really time to rule out a few possibilities.

Status update; I'm in the process of installing an after market temperature gauge.

The water pump is fine, but the 180 thermostat is actually a 195 labelled as 180. The radiator is clean and has no blockage.

Once running, I will play with the distributor and mixture.

B

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These ones go to eleven...