My guesses;
1) a restriction in the coolant supply to pump OR,
2) percentage straight water in the coolant is too low. and/or
3) a big ol' air-bubble trapped somewhere OR rad just not circulating properly
Easy test;
1) Drain the coolant, remove the rad, and cap the lower spigot. close the valve
2) check the bottom tank for a floating restriction, like a rag.
3) Place the rad into a tub with the tub-drain plugged, and fill the rad with tap-water.
4) rip the spigot cap off and make a note of how quickly the rad drains. and; make a note of the color of the exiting fluid. and make a note of how long it continues to drip. Pick it up and shake it, listening for water slosh.
Ideally; ALL the water will gush out in seconds, all be as clear as it went in, and the drip time is short, with NO slosh, and; the quantity of water that came out is about the same as what you poured in.
The tells;
If the water is greenish; obviously you flushed a pocket out.
If you hear water-slosh; obviously, water is trapped inside.
If the bulk of the water does not gush out, why?
If the drip time is long, why?
If the collected water that exited is not the same as what you poured in, why?
My guess for all three whys above, would be that the tubes are severely restricted.
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Cool story;
About 35/40 years ago, I got drunk a party and had to spend the night. The temp overnight dropped like the proverbial stone, and the coolant in my rad gelled.
Next morning, this produced the same symptoms as you described. I had to take the rad out, and bring it inside to warm it up. The lady of the party-house, my younger brother's new wife, was NOT impressed.
After the rad thawed , I plugged it back in, filled it, drove home, with a stinking hangover, two baby girls in the back, and a sulking wife in the front. and once home, I upped the freeze-protection.
BTW-1
We were never invited back to that house again. Which was OK with us, cuz that women, altho gorgeous, was intolerable. They got divorced about a year later, and they did not procreate. That house was eventually bulldozed, and my brother died about 5 years ago. The girl? No idea.
BTW-2
The Swinger rusted out in the mid 80s, but that same 318 is still powering up my sons D100; and that same rad is still in use today, cooling my HotRod 367; and the 904 got built up as a clone of a trans I pulled out of a 71Demon-318, that I thought and still do, was the best shifting 904 that I ever had. Course I put a kit in it and a 2800 stall. Mu son got that trans too, and he loved it.
BTW-3
I warned little brother, never marry a barfly. I was right.
Little brother married again, to another barfly. I was still right.
They divorced, the wife taking two baby girls with her.
Finally, after Two divorces, he met a nice girl from a distant land and NOT a barfly. They never married. and never had kids. but they stayed together until my brother died of brain-cancer at age 60.
The end