Well I have an S4 TII with S5 manifolds & turbo without any emissions and when tune the ecu and have a good a/f ration (13-13.5) at the desired RPM for my idle my car is fine. Then when my e-fan is turned on or my headlight go on the idle and vaccum drops then I have to re adjust everything but when I turn everyting off the idle goes back up. I upgraded the alternator to an FD so my volts are always at 14. Is there anything I can do about the RPM, Vac drop.

 

I had similar problems. Turned out to be some gounds not grounded properly.