There are two belts. The belt closest to the radiator drives the A/C compressor. The other belt drives the fan, alternator, water pump and steering pump. Without the water pump turning, the coolant is not circulating. No circulation of coolant means the engine will overheat the coolant until it becomes steam. The cooling system is not capable of handling the pressure steam will create. The coolant will exit the system at the weakest point. The Inner belt, closest to the engine needs to be replaced. There is a reason the belt came off. It could be that it is worn out. It could also be that a pulley or tensioner or one of the accessories driven by the pulley is bad. You will need to find out why the belt came off. You will need to be sure the belt is routed properly. I would certainly not run the engine until the belt is replaced.

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X5 4.6 2002 Black Sap, Black interior. 2013 X5M Melbourne Red, Bamboo interior
Dallas