Have you check the fuel pressure with a fuel pressure gauge and see if your fuel pressure is too high?
If fuel pressure regulator is bad, it leads high fuel pressure, which sure causes very bad gas mileage.
Hope this helps.
99 BMW 540i 6-speed 110K Km
03 SAAB 9-5 wagon 80K Km
92 400E (Sold) 245K km
Still missing the days with the Benz, it kept me busy.