This is a good thread with valuable discussion.
I am late in reading it.
My take away:
in the future when giving advice
urge the original poster to use two thermometers to measure at the inlet and outlet of the radiator,
either by inserting a thin probe inside to touch coolant at the hose slip on joint,
or by touching two aluminum or brass radiator fins.
There have been hundreds of cooling discussions on Moparts
and this basic point about getting good engineering data has been seldom mentioned.
It is even more true in freon HVAC system where before and after temperatures and pressures are “industry norms.”
It is related to the joke:
Dentistry costs money,
lack of good Dentistry costs even more money.
(because the bacteria P. gingavalis that attacks the gums also attacks the lining of blood vessels throughout the body, heart valves, and maybe the brain itself
You are correct in gathering the data and analyzing it to devise a path to a solution. Due to my background in this area the in vs out difference tells you how much heat is being removed and the efficiency of the radiator. But if one does not know what the efficiency should be or is not able to calculate it leaves them with another unknown. The other variable(s) that enter in are the efficiency of the core itself, how clean it is, it's physical condition, air flow, speed, fans etc.
External probes will give you an indication, The same is true for IR heat guns.
I have always used a Thermocouple probe placed directly into the coolant as close to the thermostat (OUTLET side) as possible. The T/C probe is then plugged into a laboratory grade digital meter which can be placed in the car through a window if needed (usually is).
The probe is very sensitive and quick responding. so much so that if a semi should change into your lane you can watch the meter climb until a good distance is established at which time it will start dropping back.
Another interesting tidbit, with marginal cooling system, Going N on I-29 from the I-680 interchange the temps will climb. How fast depends on the cooling system (radiator usually)
Going S on the same road they drop or stabilize. This section of road appears to be level but obviously is not. The same variables can be duplicated on other highway's that do have noticeable grades.
One thing I learned from Glen Ray is that all cores are not created equal, even though the catalog may say they are a direct replacement, that is not always the case for the older cars. This is due to the bean counters using thinner tubes, fins and cheaper manufacturing methods.
There are also different cores to choose from, economy, mid, high. Then throw in the number of fins, tubes, whether the fins are louvered of not as well as a potful of other variables like staggering the tubes and the Manufacturing processes used.
Generally speaking, if the T/sat or restrictor is in place, pully ratios are correct, belts are tight and you have a heating issue at highway speeds. It is most likely the core or misdirected airflow.
If the problem is at low speed / idle you're not getting enough air across the core. Do keep in mind a less efficient core will require more air.
Another final tidbit, removing the hood will generally drop your temps ~ 15-20 degrees
