My first reaction, what was the temp drop across the radiator, or the outlet near the thermostat to the water pump inlet? and are you reading the block temp directly? or the dash gauge? I'd expect even with a mostly plugged radiator, or a fan on wrong, or some other serious issue, that if you can work for hours without losing coolant then idling at medium speed for five or ten minutes should drop the temp back very close to the opening temp of the thermostat. The fact that it doesn't drop back, and the very slow temp climb, makes me suspect the gauge. Otherwise everything else you say makes sense.
"Normally, the design of a cooling system will permit operation at a temperature that is high enough for a good heat transfer," That is an admission that cooling systems will operate in marginal conditions, in my opinion. It's marginal to operate bumping against the overheat warning, but not harmful without overheating (losing coolant or boiling in the system). I would feel much more comfortable with the highest pressure cap recommended, 15# instead of 7# would add about 20*F to the boiling point.
"Normally, the design of a cooling system will permit operation at a temperature that is high enough for a good heat transfer," That is an admission that cooling systems will operate in marginal conditions, in my opinion. It's marginal to operate bumping against the overheat warning, but not harmful without overheating (losing coolant or boiling in the system). I would feel much more comfortable with the highest pressure cap recommended, 15# instead of 7# would add about 20*F to the boiling point.
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