tristanjm

I’ve had this S5 for a few years bought it running but needed a rebuild. Got all the plates machined, and new rotars. We’ve put it together 8 times and never got it to start, this time we finally got it to start but does not idle at all and have to keep it running with the throttle. It also backfires too. we’ve deleted all the rats nest we could while we had it apart so i’m assuming it’s to do from that, but not sure. we deleted the OMP, sub zero, and etc. I’ll attach videos if i can. Just really stuck and hate to take it all the way apart again.

professionalpyroman

So, the easy first answer is put your rats nest back. It routes quite a few functions that are related to, or operate during idle. Deleting the rats nest is bad advice if you are trying to keep the car a daily driver. Put the vacuum system and associated solenoids back to the stock configuration. The second thing you need to do is check for vacuum leaks. If you have messed with the throttle stop screws, you will need to return those to stock as well.

Deleting the sub-zero system is fine. Those caused idle issues by leaking, and there was a service bulletin put out to take them out back in the day. Just make sure the manifold isn't leaking at this location and you'll be fine here.

Put the OMP back or the ECU will go into limp mode and you will also have a whole other set of problems. The oil injectors that the OMP uses to pump oil for the apex seals also provide a significant amount of your idle air through their vacuum lines. Blocking these off is also bad advice for a daily driver. If you want to also premix, thats fine, gonna be a little stinky. But there is no reason to remove a perfectly good OMP.

Did you cap off the injector air bleeds at the manifold when you removed your rats nest? The idle air that you need is supplied by the 4 OMP oil injectors, and the fuel injector air bleed lines. The only adjustable idle air supply is at the BAC on the side. The throttle stop screws are not idle air adjustment. The throttle bodies are supposed to be a set datum. If you changed those, they need to be brought back to where they belong or the throttle position sensor will be off. There is also some air that comes in from the EGR. If it's working properly, great. If not, fix or delete it and adjustments can be made accordingly. Fixing it would be the ideal option.

TLDR The vacuum system is important and the car runs nice when it is in correct, working order.