Leaking cylinder head gaskets often does not show up with a pressure test, especially after the engine has cooled down. The best way to identify them is with a "sniffer" test. That involves drawing air from the radiator through a glass cylinder with two chambers partially-filled with a special dark blue liquid. If combustion gases are present that liquid will turn bright yellow.
When combustion gases leak into the cooling system fast enough they can pool under the thermostat and prevent it from opening. Thermostats open in response to hot liquid, not hot air. That is likely what started all the trouble. Your radiator fan may indeed have been bad but it is not needed at highway speed. A fan that does not run will not cause overheating at higher speeds when you have natural air flow through the radiator. A leaking head gasket can cause overheating at any speed if the thermostat closes and blocks coolant flow to the radiator.
The two additional clues to a leaking head gasket are the temperature gauge going up and down as the thermostat repeatedly opens and closes as the air pools under it and it gets hit with hot coolant sloshing around, and the reduced circulation brings less hot coolant into the heater core in the dash so the air will not be very hot.
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Sunday, January 22nd, 2017 AT 10:16 AM
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