Had similar problem after I had already replaced the upper radiator hose. Turned out radiator was leaking on left (driver's) side of vehicle. It wasn't that obvious to look at, although there was always a good puddle of coolant under the car whenever my wife pulled back into the garage. It was leaking right where the plastic end cap mates up to the side of the aluminum radiator. Not so obvious when you raise the hood because it drains right down that vertical seam and onto the bottom plastic cover. Maybe I'm wrong, but I'm thinking if you had that kind of trouble with the thermostat, you'd be seeing some overheating (high reading on guage, trouble light, and/or some steam).

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2009 X5 3.0
2001 X5 4.4 - Sold