As a result of a remodeling job, the workers capped off a sewer pipe that was once properly vented out the roof. They must not have realized that I had a basement bar sink that is a part of same set of drain pipes.
My symptom was that the bar sink would no longer drain properly. This made sense to me: there needs to be a vent so that air can be drawn in to help the system drain.
The OTHER symptom was sewage getting sucked through the bar sink P- trap and belching up a foul sludge into the sink (I've remodeled my bathrooms before and have had my head nearly in the toilet sewer hole and yet I've NEVER smelled a smell like this). This was strange, this part of the sewer circuit is not active and should not have material in it.
Does the vent also serve other purposes other that proper drainage? Without it, does it create some sort of, I don't know, vacuum that pulls sewage towards pipes that would otherwise be clear?
Connecting this pipe back up with the vent in the roof would be a considerable effort. So I put an indoor vent cap on the end of the pipe. It supposedly lets air in but won't allow are to come out. I don't get sludge backups anymore but I do get sewer smell. I take it that these indoor vent caps don't work very well.