Have you ever tried to run a Hot Radiator Flush with your radiator?
In my early days of owning Chauncey, my Series 80, I had cooling problems that I could not solve.
I ran Radiator Flush through it, idled for an hour or so, drained it and refilled with water.
It was better, but not superb.
Before the next driving season, I removed the radiator (not a really big job) and bought 6-gallons of 10-Mol Acetic Acid (vinegar), laid the radiator across two sawhorses, boiled a large pot of water, poured some of it into the bottom radiator pipe (with the top pipe plugged with a rubber bung), added one or two gallons of the Vinegar and let it sit for 30-minutes.
I then bunged the bottom pipe, stood it in end (top up) and pulled the bottom bung and flushed it with warm water.
I repeated the process two more times.
Finally, I flushed it with Cold hose water.
Of course, I was wearing expendable clothes and industrial grade rubber gloves.
That did the trick.
Over the years, the radiator apparently accumulated serious globs of water pump grease at the bottom that wouldn’t completely dissolve through normal running.
Normal hot flushing of the radiator while on the car wouldn’t even touch the grease, let alone dissolve it.
After that intervention, Chauncey ran very Cool and then I acquired a Greaseless Bering water pump, so no more grease accumulation at the bottom of the radiator.
Now, that water pump is out for a rebuild (after 8-years of run time) because it was weeping ever so slightly, and I have a Grease Bearing type holding the water in place. I haven’t driven the car much since I removed the other pump, but I will in the next few weeks.
Just a few thoughts on the matter, even if when you drain your radiator the water “appears” to be clean, you may have accumulated grease in the bottom of your radiator.
Keep us posted!