As
@Communicator mentions, although long retired I work on the early to bed, early to rise theory.
and as he mentions re fans kicking in when ignition keys out etc. manufacturers realised once stopped after a fast run, engines still had a lot of heat to get rid of and with water pump and belt driven fans not working the engines were getting localised hot spots causing head gasket failures etc. so then had to put signs under the bonnet warning of fans moving etc. when engine off. Many engines now have auxilary electricly driven water pumps to assist.
I agree older diesels often ran fairly cool, but in general engines run more efficiently when all the metals have expanded and settled down, which is why often diesels in particular sound quieter once correct engine temp reached.
Another point which is important is within reason it is not the coolant liquid running hot which causes the problem , more when it is allowed to boil and become a gas, as then it can reach much higher temp. in localised spots. Think of superheated steam!
This is the reason for a pressure cap on the coollant tank. From memory for every 1lb of pressure above the coolant water, the temperature it starts to boil raises by 1.5 degress Centigrade, so a 10 pound cap will stop water from boiling until 115 degrees Centigrade as I understand it. So that is the reason pressure cookers cook your food quicker, but also more important why never to release the radiator pressure cap on a hot engine, apart from the obvious danger of burning yourself, the water inside can turn to gas and very quickly damage/warp the cylinder head/gaskets etc.
Many radiator pressure caps these days have 1.2 Bar written on them, this equates to just under roughjly 18psi.
The older design car thermostats were fairly easy to test, looking like photo below left making them easy to test, however later ones are often like this and much more expensive to buy and replace.
As you may see the older design on left as it reaches correct temp. the wax stat allows the circular valve/flap to open at the top of the picture progressively allowing more water to flow through it.
I would contest the idea that engines use more oil (assuming the correct grade) at higher running temp. as the large alloy piston by then has expanded to provide a more efficient piston to bore seal than when running cool, this also explains why cold engines use more fuel.
In the old days thermostats were supplied by Smiths Industries and apart from temp. ranges were mostly interchangeable. It was common to buy a a "Winter" thermostat which opened at 88degrees Centigrade meaning the car heater and demister worked better.
Another point about radiator fans, in the mid 1970s I worked as foreman at a Mazda Dealership , a customer boght a brand new rwd 323 and wanted a Kenlowe electric fan fitted, which we did but had a problem even with the original water pump mounted fan removed to get the engine hot enough to test the fan relay settings, so in the end I put a board across the radiator and held the engine revs up a little whilst in the garage and eventually temp rose enough to test the new Kenlowe fan operation.
Subsequently I removed the metal water pump mounted fan on an old Singer Gazelle I was running, it never overheated under normal fast driving conditions, but the main benefit was improved fuel economy as no power lose from driving the fan. Interestingly I learnt that some car manufacturers declared engine horse power figures were done on a test bench minus all auxilaries, fan, generator, gearbox, etc. so were more "generous" than in real normal running cionditions.
