I had a similar problem with a truck. Three years and I could not find a coolant leak. One day, boom, head gasket failure. Bore scope the #1 cylinder. Does the piston top look cleaner than others? Does the plug look different from others? Even is the leak is small, steam cleaning will occur over time.

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'06 X5 3.0i - bought @143,123 miles (12/26/20)