Otto, it's been a couple of days since your last post. This happened suddenly, if I followed the posts correctly, right after you installed your pump after a rebuild. Why did you send the pump off for repair? Leaking? Bad Bearing? or was the engine overheating then. That wasn't clear to me.

In case you haven't found the problem, here's some things to check: It's clear that there's inadequate water flow based on your temperature readings. Years ago, I ran a thermocouple down to the lower radiator outlet and another one to the t'stat housing -190 top and 140 bottom. I was looking for a way to increase the temperature delta. Why do you have too little flow? not hoses, not radiator, not t'stat, No bubbles from cracked head or gasket. What's left? Pump not pumping enough water. Why? impeller came off shaft or is slipping? Not likely as it's a press fit, likewise the pulley flange. So for some reason the pump isn't turning or not turning fast enough. Belt slipping? I'm sure you checked that. What about the rubber bonded crank pulley? I've seen these begin to slip and not drive the belt. They get oil soaked and deteriorate. When you look at the running engine the fan will be turning but is it turning fast enough? Pulley will turn it and the generator at idle but not at 2500 RPM.

1951 3800 1-ton
"Earning its keep from the get-go"
In the DITY Gallery
1962 261 (w/cam, Fenton headers, 2 carbs, MSD ign.), SM420 & Brown-Lipe 6231A 3spd aux. trans, stock axles & brakes. Owned since 1971.