Septic gas coming in via b/rm sinks
I am new to this forum, but saw "gwilson2" talking about several complex plumbing issues. I have one that I believe is complex, because I simply cannot figure out what is happening. We purchased a "new" home that sat vacant for 5 years. Among other things, we are smelling a sewage gas smell from two separate bathroom sinks. ( The sole thing in common is the 3" main drain to the septic tank, which is flowing freely) I checked the venting on one of the two b/rms and found the toilet, lav sink and shower all have independent drains to a 3 inch main drain. The sink and shower have P traps installed and they are all vented, via a common 21/2" WDV. The "plumber" connected the fixtures, via a 2" horizontal pipe approx 16" above the sink drainpipe. He went vertical into the attic and placed a 90 elbow and ran a horizontal 2" pipe approx. 14 feet before he placed another 90 elbow and went vertical thru the roof.
My question is how can sewer gas be coming into the bathrm via the sink drain? I flushed the toilet and drained the tub and carefully removed the sink P trap. It was still full of water, so it is not being sucked out, as I initially believed.
I believe this problem on two separate DWV set-ups in the house is due to some "minor" plumbing error. I know the horizontal pipe in the attic should be sloped as sewer gas will condense in the colder months and I suspect the smell will be intolerable.
Any ideas that I can check into myself , without tearing walls open?