Can anyone please ID this weird pipe configuration?

[manintrees]

I recently bought a house and the basement is unfinished. I was checking out the plumbing and cannot for the life of me figure out what is going on here. The 2 inch pipe is connected to what I assume is a vent and is not capped. Is this some kind of 'cheater vent' thing done by a hack or what? If I cap it (which I want to do) what might happen? Please help!

[Terry]

We need to see more of the pipe to know where that is connected.
But for starters, that is just wrong.
That is not a vent, that's unfinished plumbing.

Please cap that.

[hj]

Take a picture from the other side so we can see the actual connections. There is NOTHING that elbow could legally be used for. In fact, legality aside, about the only thing it would make sense for would be as an elongated "P" trap for a shower, and if the 2" connects where you indicate it does, it could not even be used for that.

[manintrees]

Thanks for the quick reply guys. There is no room for me to photograph the other side so I will attempt an explanation.
The big pipe in the back is my main stack. It goes up through the subfloor and out of sight. Here is a diagram. Thanks a ton for any response.

[Peter Griffin]

Put a gym cap on it and forget it. It's been working this long, why mess with it.

[manintrees]

Put a gym cap on it and forget it. It's been working this long, why mess with it.

That's what I'll do, thanks. I am 99% sure it's a vent as no water has ever come out of it. My only concern is that it is a functional vent (albeit in a very stupid place). If it is functioning as a vent I guess I have some drywall work ahead of me.

BTW, is a gym cap just a permanent ABS cap? If so, that's great because I just picked one up.

[hj]

It is a GEM cap, but a permanent cap will do the same thing.