Francis, your fan clutch has gone south. If it is still spinning after you shut down the engine, it has lost it's ability to "clutch". This device operates on a viscous coupling which, when it reaches a preset temp engages the fan blades to the fan pully, thereby increasing air flow thru the radiator. the clutch is there to prevent wasting horsepower driving the fan when the extra load (heat) is not there. This is a precision device (why they are soo $$$). When the coupling loses its' silicone oil (most frequent cause of failure) it is no longer able to engage when the ambient temp rises. It should get harder to turn as it gets hotter.