I was replacing my secondary Air relay on my 2001 Mercedes e320. While replacing I was removing what appeared to be a plug, connector on the bottom that was extremely dry rotted. I replaced all the vacuum lines routing out of the top as they were dry rotted but I cannot figure if this bottom plug is supposed to be just a plug, if it is supposed to be open, etc. I cannot find this item on line. I see it in drawings but never see a part number. Can anyone tell me if this is just a normal line plug or if it is something special? I placed a line plug on it but got a P0410 code again after replacing the relay, pump, both valves, the 4 prong electrical connector in the fuse box and using Sea foam to clean out the ports under the valves. I am wondering if this has something to do with my issue. My pump runs when it is cold so I cant figure this out.

I'm trying to picture what you're talking about, but I can't. Can you post a picture?

Just a guess at this point, but I'm thinking it may be the filter, it's just a little screen. And yeah, if you plug it, it will code.

Good luck.

Some vacuum relays / switches / valves have a vent port to dump vacuum. This cap might be a small filter. A piece of open cell foam works here. ( For an example see a small engine green foam air filter. )