No furnace in the attic, Harold.

We went into the attic. The vent stack runs vertically from the roof down to the ceiling joists, where it makes a 90 degree turn, horizontally towards the outside wall of the house. We didn't dig through the fluffy insulation looking for t-connections. At the outside wall, turns right and travels along the corner where the wall and roof connect. We don't know where vent(s) attach.

Opened another hole in the bathroom wall. Behind the head of the shower, we see the copper water lines up to the faucet handle, and up to the shower head. Down, we can see the overflow join the tub drain, drop to the p-trap. After the P, the waste line turns toward the center of the house. It meets up somewhere below the floor level with the toilet. Past the toilet, the basin joins the liine -- we found the basin waste line, and where it connects to its vertical vent.

Is that the vertical vent to snake next?

In the attic above the toilet there should be a line coming up through the wall that connects to the horizontial line along the corner of the ceiling and the roof.

Gentlemen --
Your advice and instructions saved the plumber called in HOURS of investigation. He agreed that the bathtub trap was being siphoned dry by the adjacent toilet. It was easy for me to describe what you'd had me investigate, and demonstrate how and when the smell comes and goes.

Inside the wall, we could determine that the bathtub waste drain was tied into the toilet's waste line BEFORE joining the vent line. Therefore, the toilet used the easiest source of air -- the tub drain, instead of its own vent.

The solution (and it worked instantly!) was to add a vent line from the bathtub drain line (before it got to the toilet's line). We routed it up and joined it to the shared vent line, above where the toilet and vanity were attached.

No more smell, a plumber bill of less than $200 in all (including materials). I do have drywall repair to do now, but that was expected.

Thanks again!

Sounds like you are saying that the tub was not vented. Strange. And congrats on your tenacity on finding the problem.

No, no, NOOOOO!
After the teenage daughter took a shower, the SMELL IS BACK.

The plumber was 'sure' he felt fresh air coming down the vent when he cut the splice in above where the sink waste line joins the vertical stack, so he doesn't think there is a clog up that vent line.

Now what, fellas?

Did you ever find the toilet vent?

Harold - We didn't specifically find the toilet vent -- we found a vent in the wall immediately behind the toilet On that wall, the sink and tub are on either side of the toilet. While I did not personally see that the toilet was attached, I have to believe it is?

In the wall behind the toilet there should be a vent pipe going up to the attic. From previous description of the pipes in the attic , you should find a pipe coming up through the wall immediately above the toilet. This pipe should go horizontally over to the vent pipe at the eave of the house. Cut and snake the vent pipe going down the wall toward toilet. It's the toilet vent that is blocked not the tub vent.

Harold, where is the best place to make the cut(s)?

I can visualize that vertical meeting the horizontal with a Tee. Do I cut all three branches (removing the Tee completely, or just the vertical (and just shove the Tee over and out of the way).

Planning ahead, I'd want to make the cut a few inches below the Tee, so that I can get a collar/ring glued back on to restore the seal. Yes?

Is there any reason to go into the vent line where the wall is cut away? Go up and down from an access cut there?

What if the clogged vent is over the other nearby toilet? (there are two). The second toilet is 'around the corner' - that is, the tank is against the outside wall of the house, instead of against the shared wall. It's well over 24" from the vent, though -- so I think I remember that you all said that it is too far away to share a vent, but it would be tied in (somewhere) to the waste line. OK, just talked myself into snaking it too, while I'm up in that attic.

Cut the horizontial line close enough to the tee to start the snake down the vertical line and far enough away from the tee to get a coupling on the stub of pipe coming out of the tee, maybe 2 or 3" from the tee. Cut the vent pipe then check to see if the toilet is still pulling the water out of the tub trap. If it does the blockage is in the vertical line to the toilet. If the toilet does not pull the water out of the trap the blockage is in the horizontial line going towards vent pipe at the eve of the house.
When reconnecting the vent piping, if the pipes will move apart enough to get a regular coupling on fine. If the two pieces of pipe will not move apart 2 or 3" use a slip coupling.
A regular coupling has a ridge on the inside so that one piece of pipe goes half way inside the coupling and the other pipe goes in half way from the other side. To do this however you must be able to move the two pipes apart half the length of the coupling to get it in. If you can't move the pipes apart, use a slip coupling. It does not have the bead or ridge on the inside. Slip it on one piece of the pipe, slide it back out of the way, put a generous coat of glue on both pieces of pipe and slide the coupling over the joint.