The viscous fan coupling is a unique design, when the engine is cold the fluid is very think and the fan turns slowly so not to cool the car too much and waste horsepower. As the engine heats increases the fluid thickens up so the fan is engaged to cool the engine more. But if the clutch fails, the unit will not pull air to keep the engine cool at slow speeds. If left too long the entire unit can fail and can come apart which would lead to a damaged hood or radiator. So if your car is running hot at low speeds the fan clutch is the most likely problem.