long time ago I had a similar situation but with another brand of car - the engine was iron block and alu head - the problem was a slightly warped head - I shaved it myself on a sheet of glass and 400 paper.

after that job of removal, decking and reinstall with genuine gasket didnt have that problem.

You might want to start with the cap on the expansion tank, it may have a bad seal.

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2012 BMW X5 (Beef + Granite suspension model)

1995 E300D - The original humming machine (consumed by Flood 2017)
2000 E320 - The evolution (consumed by flood 2017)