Tracking down faint sewage odor in basement .
I have a semi-finished basement, and there is an odor of sewage in one room. That room has a sheetrocked ceiling and is below the guest bath. I saw no signs of water anywhere, so I made a couple of holes in the sheetrock (we're redoing the basement soon anyway) so I could get a look at the pipes. So far, I see one joint on the 3-4" cast pipe that may be the problem. The lead joint appears to be weak at the top, and there appears to be a dark, sticky residue from the top to bottom on the back side of the pipe which I can barely see, but no liquid on the ceiling underneath it. Does this sound like it could be the problem and if so, how would I temporarily seal it to see if it eliminates the odor? Thanks.