Exactly, any problem with fuel delivery would show up more under load.

Most likely the off idle tip-in was on the lean side from the factory and even a small amount of carbon build up and engine wear turned out to be a problem which is why there was an update.

You still shouldn't have the pressure drop like you are saying but maybe that has something to with with how you are looking at the gauge ?? And still no codes ??

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