My thoughts would be intake leak. I'd have the intake system smoke tested to see if you have some bad intake or injector o-rings. From there it could also be the MAF or the rear crank case breather hoses that are notorious for turning to mush.

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2013 X5 xDrive3.5i Premium
2002 530I Sport Package
2001 X5 4.4i
2001 M3