Do some reading here as I can see the path you're headed down, it will give you some understanding of fuel trim parameters: FUEL TRIM(MING) DIAGNOSTIC TIME | Motor | Find Articles at BNET

I'm currently troubleshooting an issue of my own similar to yours. No real success yet though.

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2006 X5 3.0L - Schwarz Black/Truffle Brown
Stock
But I keep it clean