Does this illustration make sense?
Say, you just finished mowing on July 3rd and you want some liquid refreshment. If you want to cool down your "room temperature" drink in a hurry, would you use one large cube of ice or crushed ice? answer: crushed ice. why? because it has much more surface area to allow the thermal transfer and cool down your beverage faster, right?
It is the same with the radiator versus engine block. All the fins and thin passages of the radiator are way more efficient at heat transfer than the passages and thick walls in an engine block. So whatever heat is created in the block is quickly dissipated via the radiator passages when it passes thru the radiator. The thermostat slows the coolant down in the engine (or stops it?) so that the engine can warm up to correct op temp.
Now say you like your drink really cold, do you get more thermal transfer by letting it sit still with ice in it, or stirring the crushed ice in the drink rapidly, as you might do with iced tea?
Ever used a hand crank ice cream maker? Do you just add ice and wait for ice cream to form, or crank and spin the cream in the ice mixture til your turn is over and you pass it to someone else for a while?
Wouldn't coolant moving rapidly thru a radiator act similarly? Hot engine block >--> cool radiator. Transferring heat.
I used to have one of the old (1980?) Oldsmobiles Custom Cruiser wagons that GM put their sorry designed 350/v8 diesels in, and when it died an early death as often happened, the previous owner replaced with a bolt in gas engine swap. When the thermostat was out in the winter, you could not get it to warm up or produce any heat from the heater core. I'm guessing, purely guessing, that GM must have put a HD radiator in those cars. (* quick search of AZ shows it was a bigger rad in the 350 diesel vs 350 gasser)
Long story short, the radiator was much too efficient for the engine without thermostat. The engine did not get hot, needle stayed buried on the cold side, without thermostat.
Just trying to add another .02 for the thread.
ChiefRichard, I hope you find it is just something simple as others suggested like paint or oil that was being burned off. Get a temp gauge to know for certain.
