What are the common places for a vacuum leak after emissions delete?
 

I have an s4 Tii that has a surging idle that I think is due to a vacuum leak. The air pump, OMP, and rats nest have been deleted. I’d like to get this fixed but I’m unsure where to start, so I figured I would ask here for where vacuum leaks usually form when these things are deleted. If it can leak from anywhere I’ll just go through the entire intake and BAC system, but it’d be nice to know specific places to check.

I don't know if there's any common places, but any vacuum lines you even looked at while working on the car are very likely suspects.

basically everything you just touched is the most common

If you deleted the vacuum spider, you almost certainly had the upper intake manifold removed. Which means that it could be leaking basically from anywhere.

Did you delete the other vacuum lines as well, or just the vacuum spider? There is a chance that its not a leak, but hooked up wrong. Either way, even if you have an emissions delete (I do too), you should put your other vacuum systems back. The majority of the vacuum systems are not emissions items specifically. Most are to balance idle air flow.

I probably should have specified that I didn’t delete anything, I bought it with all that stuff removed. Unfortunately it makes tracking down the vacuum leak much harder.

I did remove the intake manifold and there was a ton of rtv or some sort of gasket paste around and on the intake manifold gasket, which doesnt give me a ton of confidence in the quality of the way he did the delete. Not to mention the big hacksawed lump of what I’m assuming used to be vacuum hard lines. I guess I’ll just be going through the whole intake system and replace all the lines.

i was going to say, mine answer sounds a little sarcastic and i don't mean it to be. if it was ok before you took the manifold off, and its not after though, it was something that you touched.

for what its worth i usually pull these apart to some known good point, and just go and check everything. usually the hoses are ok, and its the gaskets but ymmv!

It was surging before but im not sure if it was quite to this extent, so maybe i should add to the rtv pile (or just get a new gasket). I dont know any known good point, so I’ll be going through everything. Do you know of any place that has a diagram of the rats nest so I can see what should be capped off? or is that a search bar question

yeah, there are lots of nice color coded diagrams out there for the vac system. There are even simplified diagrams after emissions removal that are super handy.

If it were me, I'd replace all the intake gaskets, they are pretty cheap still, and replace all the vac lines with silicone hose. As j9fd3s said, this will get you to a point of "known good" and will simplify your trouble shooting in the even that you still have surging idle after replacing everything.

Thanks, i’ll be sure to do those things since its good maintenance anyway, but i think i found the source of my vacuum leak. One of the small bolts holding the BAC valve in place fell out. Once I replace that I’ll hopefully be in the clear and I can focus my efforts on replacing all these vacuum lines. Thank you so much for your help

If you happened to know how many feet of vacuum line I would need to redo the vacuum lines that would be great.

i use the factory one, but its not good enough, so i make sure that the hose actually connects where it should. the T2 has that bonus vacuum rack, and its easy to get a hose crossed back there.
so, for example; i make sure that the Relief hose actually goes from the Relief solenoid to the relief actuator on the ACV.

the Mazda hose 9935-10-4999, comes in a 1 Meter length, usually this is enough for me, but i only do the ones that are bad.

After we did a total sequential - emissions recommissioning on my car, I took the car for a "smoke test". Although we thought we had buttoned everything up, four leak points were evident.

Smoke test was $50 and took 30 minutes.

They plug up a number of intake pipes and use a smoke generator to pump smoke through the system.

Using a vacuum pump on one of the lines is super handy. You can get a pump for less than 30€ and it will tell you if your system is still closed. One time you don't need to spend to get the expensive system.