On the radiator that was in the overheating truck. I've been the good boy and flushed my radiators and changed the coolant and did all the things you did, and my core still got clogged up, mostly out of sight of the filler neck. When I did the above test, my outer core areas were dead cold, even tho the tank was a constant temp all the way across. The lack of cooling are led to a chronic overheat under load in warm ambients.

Not responsible for advice not taken...