X5chemist- thanks for your info, though I don't quite understand how to read them.

My prior understanding of ST & LT fuel trims is that when your engine is operating properly, the MAF is reporting the correctly measured airflow, and the O2 sensors are adjusting the trims to correct any imbalance, then the LT trims should become "normalized" (approaching zero added or subtracted), and the ST trims should be compensating for second-to-second changes. I haven't seen -/- on my scantools for ST/LT trims, ever (but when my old OEM MAF was setting codes, I did see +/+ everytime).
Foxwell freeze frame while using old Siemens MAF, prior to switch to cheap MAF

On my Foxwell (also on Torque Pro & Bimmertool Pro apps) I've never observed double negative -/- fuel trims on the X5, and as I said previously, my two Chevy trucks never vary from a zero LTFT. Does your scantool always report in that fashion? Do you have Torque Pro? and if so, what does it report? It would seem to me that consitently withdrawing fuel would indicate a rich condition in your engine. Did your old original MAF generate similar fuel trims to the present cheap MAF?

__________________
01 BMW X5 E53,3.0i-5L40E, 7/13/01
topas-blau,Leder-grau,"resto-project car"

Here:
14 Lexus ES350,3.5L-U660E
09 HHR Panel,2.2L-4T45E
04 Chevy 2500HD,6.0L-4L80E
98 GMC Sierra 1500,5.7L-4L60E

Gone:
66 Chevelle Malibu 2dr ht.,327>441c.i.-TH350>PGlide/transbrake
08 Cobalt Coupe,2.2L-4T45E
69 & 75 C10s,350c.i.-TH350
86 S10,2.8L-700R4
73 Volvo 142,2.0L-MT4
72 & 73 VW SuperBeetles,1.6l-MT4
64 VW,1.2l-MT4
67 Dodge Monaco 500 2dr ht.,383c.i.-A727
56 Chevy 210 4dr,265c.i.-PGlide