2003 BMW X5 3.0, this past Saturday I replaced the waterpump, thermostat, upper radiator hose and lower radiator hose with new coolant sensor and o-ring, ac and waterpump belts. Parts cost, $308.00. Flushed and bled system with BMW coolant. Ran vehicle for 1/2 hr. Checked for leaks, none. Let cool down. Checked level, ok. Let vehicle run at idle for 20 minutes, no leaks. Drove home, 40 miles, no problems. Next day, Sunday morning, before I got ready for church, went out and checked level again. Ok. Let vehicle run for 1/2 hr. No leaks. Went back inside to get dressed for church, come out 2hrs later, moved wife's car to other side of the driveway, backed the X out and let it run to heat up the interior. Nice and toasty for the wife. Come back out and I see a stream of coolant from underneath. popped the hood, it's coming from the darn expansion tank. Pinhole on the side. parked the X back in her spot and we took the 2012 Mazda 6 to church. I was furious. Just spent another $200.00 for parts. Tank, cap, clip, tank thermostat, level sensor and water connecting assembly from tank to trans cooler. Paid $45.00 for 1 day shipping, from Pelican Parts in CA. Guess I should have done tank along with everything else the first time. So I will be taking my wife's car to work tomorrow, (she's off), drop her off to work Tuesday, (I will be late for work), and take off Wednesday to do the repair in 30 degree weather. Gotta love the X. Can't complain. 182,000 miles
Any info on expansion tank replacement.

Parts replaced since bought in 2007 with 42,000 miles
Dealer installed thermostat. (Warranty did not cover)
Heater control head (warranty covered)
Front door handle carriers
Front axles
2 sets of tires. (1st set, 18's yokohamas with Beyern wheels, now riding back on original 17's with Bf Goodrich.
Ac and water pump belts (2nd set)
2 sets of brake pads (front and rear). My second set are evolution racing pads with cross drilled rotors
Valve cover gasket. ( about 2 years old. Still have an oil leak. Oil splatter right front part of engine. Washed down when I did waterpump. Will keep an eye on where it is exactly coming from.
Bilstein touring struts and shocks.
Disa valve
CCV valve and tubing ( cold weather)
Alternator and battery (left stranded after install, $212.00 to tow to my house, had to reinstall with a Bosch unit. So far so good....
Updated to projector headlights with hids and halos and led tails.
Change the engine oil every 15-17,000 miles with Mobil 1 0w40, previously using Castrol Syntec 5w30.
Like I said previously, not a bad X with 182,000 miles. All highway driven.