It's called thermosiphon when the coolant circulates unaided, common on old engines and less fancy stuff. Might work to some extent while the thermostat is open on a newer engine/vehicle.

I think part of the fan running is to cool the under hood/engine from the outside in addition to what might be cooled by the radiator/coolant.

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
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"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-