I know this may reach you a bit late but after I did my custom single turbo upgrade on my N55, I to was getting a constant misfire on cylinder #5. I tried swapping injectors, plugs coils and everything else to try to get it to move around but nothing worked. I knew that before I touched the car it worked perfectly fine, so it had to be something that I did, eventually I went back to the ignition /injection harness and found that when I re-installed the harness into the DME, the ignition output pin in the DME for cylinder #5 was bent and not inserted into the connector! once this was fixed all the errors went away, these cars are smart! you just have to be able to retrace your steps.
