When you said in your original post that the expansion tank seemed dry, do you mean it was empty, or that it was simply dry on the outside (in which case I gather you mean the tank itself appears to not be leaking)? If it were a simple case of your overfilling the tank, sure, the excess would be pushed out of the system, but it wouldn't empty the tank. Often a vehicle gets the hottest just after it has been run a while and then shut off. Pressure builds, and if there is a weak point in the system (like a weak o-ring joint on the side of the radiator), coolant is pushed out until things stabilize. I think that was my radiator's story. I had to remove that bottom plastic shield so I could look up to see where the leak was coming from.

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2009 X5 3.0
2001 X5 4.4 - Sold