the control valve doesnt leak, but itt might fucked up the vaccuum or let air enter in the system, i dont know... because the last few year it leak from the water pump 3 winter in a row and then expansion tank , couple plastic tube etc...when heater at 32 .... now i put 16.5 no problem...at least for now ...

bottom line is as u say it might have something to do with heater valve, can it let air in ? and then psi go up and the system cant hold pressure and part start falling ???

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BMW X5 3.0i 2006