smoke test won't show booster leak. Both my X5 and X3 had a similar problem. They both lost booster vacuum overnight and replacing the vacuum valve and grommet was the fix. Both boosters themselves where ok. The X5 was the worst (miss and codes) and also needed the aux air valve and pump relay which cured some of it, but took the vacuum valve and grommet to totally fix it. I would have a hard time believing some of it if I didn't go through it myself. Mind you I am talking about M54's and I ruled out the more common stuff first.

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1988 325is (purchased new) sold
2004 X5 3.0 2005 X3 2.5
2008 X5 3.0 (new to me)