The stock viscous fan uses a center viscous coupling that is supposed to stiffen as the engine temperature goes up.  The viscous fluid is supposed to solidify when temperatures attain over 230°F.  Spinning the fan by hand when the engine is cold will cause it to spin freely with slight friction.  If the fan spins freely with no friction, the viscous coupling is bad.  Sometimes, the fan can past this quick test but still fail under operating conditions - the engine will overheat, especially at idle or low vehicle speeds.  Remember, the fan is supposed to get stiffer (the viscous coupling gets stiffer) as engine temperatures get close to 230°F.

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