If the measured value is less than the nominal value - 0.2 bar:

WARNING: This page is about a different car, the 2002 BMW M Roadster, 2002 BMW M Coupe, 2001 BMW M Roadster, and 2001 BMW M Coupe. However, it is still accessible from the selected car via links, so may be relevant.

The cross-sections in the fuel feed line are contracted or the fuel filter is clogged,
or
the fuel pump voltage supply is not O.K., e.g. due to high contact resistance (corrosion) in the plug connection between wiring harness and fuel pump