head gasket???
 

have a 2000 x5 with M62 4.4i, water pump grenaded on my wife on the freeway and overheated, don't really know the exact details of how long it was overheating, no more than a couple minutes, she pulled over right after the low coolant level beep on the dash display, but it was steaming. Replaced the water pump, thermostat, and belts, filled back up with coolant and drove but after a couple mins it was steaming again. Filled back up with water, made sure thermostat was opening, once it warms up coolant just flows out of expansion tank, but not with a bunch of bubbles, and there is no white smoke out of exhaust. Possibly drove with a little less power, but couldn't tell if I was just being paranoid, if I were to guess I would say 90% power. Does this sound like a blown head gasket?? Will check oil today, and hopefully be able to check compression.

did you spill any coolant when putting it back together? steam could be from spilled fluids?

I had a similar issue with my 2000 X5 4.4. It was steaming lightly after having driven on highway for a little while and I had to add coolant pretty often. The leak was not obvious but after some research I finally found where it was coming from. It came from small cracks in two cheap plastic hoses. Since BMW was only selling the entire hosing circuit I replaced these pieces myself and 10k miles later and after a very hot summer not one drop of coolant had to be added. Here is the related thread.
http://www.xoutpost.com/x5-e53-forum/...oken-hose.html