i've come across a few cars that the fuel pump has gotten real weak and is only mantaining 20 odd psi, if it's running smooth look for common shared systems, had a 95 740il with one plugged cat and a blown pcv plate that was a pisser to figure out, it wouldn't get out of it's own way the computer was trimming the mixture 50% lean on one bank and 50% rich on the other among other weird readings (damn i am getting to hate the later cars, they get pissed off and then over correct and snowball) the early cars are easyer to figure out when things get weird