Sewer gas problem downstairs after installing upstairs sink

[Old Southern Rock Guy]

I replaced the upstairs kitchen sink. Had to use new p-traps, etc, since the new sink was deeper than the old one. This is a two basin sink, one side has a disposal. Since this replacement, which -did not- involve me doing anything to the vent pipe, I'm getting sewer gas coming thru the downstairs traps on my bar sink and washing machine. This only happens when I use the upstairs sink. If I put the stopper in the bar sink, and fill the sink with water, I can actually see the gas pushing past the stopper, and bubbling up thru the bar sink. Help! (and thanks in advance).

[Mike Grooms]

Old Southern Rock Guy,

If you see air bubbles being forced back through, then it's not a matter of siphoning. I think the drain that serves these fixtures is partially clogged, or undersized. Now that you shoot water down with a garbage disposal, it can't drain quite fast enough, forcing trapped air up through the downstairs sink.

Perhaps the vents were never right, or perhaps the bar sink was installed at a later time, without the proper vent. It may have never been noticed before, but now the added disposal is causing the trouble.

If you drain water without the disposal running, and there's no problem, then the culprit is isolated. The solution might be more difficult to figure out, though.