Hi guys, I've removed emissions on a S4 NA, because of a massive vacuum leak that I couldn't pinpoint. I've left the BAC valve, OMP spider vacuum lines, and haven't yet got around to removing the ACV.

The question is, when I removed the EGR valve, I'm not sure whether the plate I made properly blocked it off, because there is a hissing sound coming from it seems underneath the UIM.

If there was a leak there, would it be intermittent at idle.. like.. 'hisss... gap... hisss... gap', with the hisses and gaps all being even? Its either that, the OMP vacuum lines or the nipple I had to block off under the UIM.

Sorry for the long thread

 

the vac spider is a common source for vac leaks, did you keep the old one or replace it with a new one. You should redo any remaining vac lines with new ones if your removing the rats nest.

 

Yep, I kept the vacuum splitter (spider) and replaced all the lines on it. I doubt the splittler would leak? Would the OMP vacuum lines make an intermittent hiss like I described?

 

I don't know exactly how the NA UIM is bolted, I only know my TII, but my TII has 5 bolts from the UIM to the LIM and 2 or 3 bolts at the other end where it's held up by a rack. Anyway, if those UIM to LIM bolts aren't the FIRST bolts you tighten you may have set the UIM at such an angle that it won't seal right. So try loosening all the UIM bolts then set the UIM to LIM bolts first.

 

Thanks SpeedOfLife, I think the UIM is secured well to the LIM though. I'm fairly certain its okay. And if the leak was there I'm assuming the noise would be a constant sucking, not just an intermittent one.

 

Vacuum leaks can make the car do weird things though, including causing random or intermittent noises. Get one of these:
http://www.boostpro.net/prodtester.html

I bought one and it's a huge help finding leaks, I must have killed 4 or 5 leaks with it on my TII even after I'd tried other methods. You don't have to fight the noise of a running motor and you can feel around for leaking air.

 

I'd buy one, but by the time it arrives in Australia, it will probably be weeks away. I can just pull the UIM back off and have a look, but if I find out whether the OMP vacuum lines have constant vacuum, or intermittent, it would make things alot easier. Someone must know?

 

ok, question 2! Can an air leak from the primaries be that big that it will make an audible hiss?

 

A good rule of thumb is simply that if they (and this goes for basically ANY vac lines on an FC) are hard or older than 5 years you should replace them.

I'm pretty sure it can, the sealing rings and the diffusers themselves can seal poorly, and to boot I think I've read of people having that problem before.