[bmw90r]

E90 M47 2007 with cooling system pressure issues. "Cooling low" error while driving, coolant leaked from cap. Opening cap caused extreme pressure, like boiling. Engine 90C entire time. After cooling down, the cap could be opened, no oil in coolant.

First mechanic diagnosed blown head gasket (positive CO test, coolant leakage from the cap).

Second mechanic closer to home disagreed: coolant release when opening cap normal for BMWs due to higher system pressure. No positive CO test; pressure-tested cap and reservoir deemed fine. "Most likely trapped air escaped, when the issue happened."

Home testing:
Cold engine: Low/no pressure.
At 90°C: 1.4 bars normally, 2.2 bars when revving, back to 1.4 bars after.
Upper hose very firm.
0.6 bars retained 10 hours post-shutdown.

Is this pressure behavior normal?