03 760Li - Random missfire diagnostic ideas?

[Appliance 328]

We are an independent shop working on a 2003 760Li and we have a random missfire with codes for cylinders 5 & 6 that has us stumped. The car originally came to our shop with failed catalytic converters and a slight miss at idle and light load. We have replaced the converters, the spark plugs, and the coil packs for 5 & 6. The converter efficiency code problem has been cured but the stumble at idle remains.
We seem to have very high vacuum in the crankcase (No Spec) and it was suggested that we should replace the pressure regulating valves (11157563505 & 6) which has now been done. This did not provide any change in the crankcase vacuum or the stumble. An experienced BMW shop has suggested that we replace the Oil Separator valve, but we have not been able to find this component in the parts manual nor has our local dealer been able to locate this component for this model.
Does anyone have any suggestions? Is anyone familiar with the oil separator valve for this model?
Thank you!

[Keif]

I'm not terribly familiar with the V12/N73 engine, but the CCVs (crankcase vent valves) should take the place of / be roughly synonymous with the OSV, as the older E38's M73 engine had, shown here (http://www.realoem.com/bmw/showparts.do?model=GJ03&mospid=48546&btnr=11_1432&hg=11&fg=15) (part #1 on the diagram, 11157501564 & 11157501565). So, it seems like you would likely be correct in not finding a separate OSV somewhere else on the engine.

Not trying to insult, just asking... have you read any of the training material on bmwtis.com / bmwtechinfo.com (just buy a day pass, poke around, and download the materials you need) on the N73 engine? I haven't read the one on the N73, but I read through the one about the N62 engine and it - as well as other nearby material on the site - provided some very helpful insight into how the various systems should ideally work, especially with respect to vacuum control, vanos, valvetronic, secondary air flow/purpose, etc. It's well worth the $30 to poke around on the site if you haven't yet.

Is the stumble any more aggravated during cold start or warm idle? Done any compression or leak down tests?

Best of luck! Please keep us posted with any new developments!