Mystery pipe under the sink and an intermittent sour smell

bathroombathroom-fixtures

First question, what is the black pipe in the picture above where the drain pipr goes into the wall?

Additionally, we have an intermittent sour smell coming from this bathroom. I replaced the wax seal of the toilet thinking that might be the problem. However it was not. Last night, the smell occurred again. After some investigation and wafting, i think I narrowed the smell to underneath the sink (hence the picture and post).

I guess the second question would be how do i get rid of the sour smell?

https://imgur.com/gallery/RLDoQ

Best Answer

I seemed to have fixed this issue with a 90 degree elbow to fit over the pipe that comes out of the wall, and an Air Admittance Valve that I put on the other side of the elbow pipe. No glue or tape was necessary.