oreo123 wrote:

There has never been any garbage run ito this. Since we tried it a week ago
there has been a total of maybe 8 gallons of fresh water run into it.

My thinking is its either the slope of the pipe under the finished floor. Or
its something vent related.

I need someone who really knows venting and plumbing to make an educated
call on this. I want to go the least destruction way possible.

Can't tell from your sketch the venting for sure but looks like a good
possibility would be that they didn't get the slope in the 12' run from
the sink to the stack.

What's the orientation of the cross-over shown?