You can try MAF cleaner, sometimes that is all that is needed. To test MAF--remove the connection from the sensor if the MAF is good the idling RPMs should increase.

Not sure what lines you are referencing and don't have a 3.0 to take a look. I do know the have nothing to do with the gas tank.

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X5 4.6 2002 Black Sap, Black interior. 2013 X5M Melbourne Red, Bamboo interior
Dallas