Thermostat
 

This Saturday my X5 broke the water pump pulley. I replace it after dealing for a while (quite a while) with the fan. Then I put every thing back in place, put some coolant, and I started the car, to notice a leak coming apparently from the lower hose. A closer inspection show me that the coolant was coming out down the hose from the upper parts, that I think it is the thermostat, correct me if I'm wrong. My question is, the thermostat is the whole plastic pipes parts, or goes inside like the conventional cars. I noticed also like a kinda valve or lever with a sensor or connector and a hole beneath that and I think the coolant is coming out from there. Maybe
I have to replace all of this? Any advice on that?

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Replace both
 

The water pump bolts to the block, the thermostat is a housing that
bolts onto the pump. You should replace both. Not too hard, just a pain
to get to. I did it twice, first at 100,000 miles as a preventative maintenance,
next at 190,000 or so when I had other radiator problems.

Cheap insurance that you don't blow your motor and get stranded!

Depending on mileage I'd do the upper and lower radiator hoses, and the
overflow tank. I'm at 253,000 miles now.......ask me how I know!

The thermostat comes as one unit, thermostat, plastic housing and sensor combined. Easy to change, just a few bolts and hoses.