Did you run the auxiliary water pump and turned the heat up to 32C when you added coolant? I wonder if you got air in the system and just adding in the expansion tank wasn't enough.
Doru, a user here just changed the pipe and he didn't find the job too bad but let's hope is not the pipe.

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4.8is 2004 imola red
4.8is 2005 Black on Black - DINAN/2 sold
4.4 2004 X5
2003 330xi
2008 328xi - sold
4.4 2000 X5 - sold