I think I know what the problem is, the plumbing was not done correctly. The toilet is flushed and I believe it is siphoning out the water from the sink. It doesn't seem to effect the shower though. I just need to confirm and try to find a solution. It is a vertical stack (2nd Floor Bath) with the toilet draining on top and the sink and shower draining below and there is no branch drain for any of the lines. My understanding is that the shower and sink should have a branch drain that comes out above the toilet. Or the toilet should be below the sink and shower. Any suggestions on how to fix this? I hate to tear the bathroom walls/ceiling apart. I do have the 1st floor ceiling tore into and thought I might be able to do something from underneath (like moving the toilet so it is below the other two drains.

This is a prime example of incorrect plumbing. If your drawing is correct. Then yes, your sink is most likely having sewer gas being pushed through the trap, causing your smell. If your sink and shower were tied in above the toilet, you would probably have no problem.

What if I were to move the toilet so it entered below the sink and shower? Would that help the situation at all? Like I said, I have access as the ceiling is opened up right now.

I believe it would cure your problem. However, your sink drain should be a straight shot into the vertical stack( no 90s) But I would start by tying the toilet drain in below the sink and shower, this should solve your problem, may not be to code, but will work.
Good luck and please let me know how it works out.

When you flush your toilet do you hear a gurgling noise in you sink?

Thanks for the help! I don't hear any gurgling at all. I do notice that if the sink is used after the toilet is flushed there is no smell. Oh the joys of buying and owning an old home! I'll let you know how it works out!

What if I were to move the toilet so it entered below the sink and shower? Would that help the situation at all? Like I said, I have access as the ceiling is opened up right now.

In your original drawing you are discharging a major fixture past unvented minor ones and that's a big no-no.
You ask,
What if I were to move the toilet so it entered below the sink and shower? Would that help the situation at all?
If you connected the sink and the shower back to the main that would still leave them unvented and if you connected the sink to the vertical stack like Dad suggests, ( providing there's nothing draining into the stack from above the sink) that would still leave you with a unvented shower.
To vent your group I would leave the configuration as it is with these changes. I would run a vent off the lavatory through the roof or revent in the attic back to a existing roof vent. I would then tie the shower drain back to the lavatory drain line where it would be wet vented.
Now, all ypour fixtures are vented. Good luck, Tom

Just to confirm: in my area of operation your installation would not pass City Inspection.

Thanks for the advice everyone!

Tom,

I have attached an amended sketch about what you recommended. Does this look right?

I thought I should mention this as well, I flushed the toilet and watched the shower drain. The water moved up and down a 1/4" or so. Also, I took the sink trap apart and it was full of water. I guess I am not sure where this odor is coming from if the trap is still full.

I still intend on fixing it correctly though...

Hey Jed,
Your last drawing correctly vents the lavatory and wet vents the shower. You're good to go. Although I would lose the dogleg on the lavatory drainline. Regards, Tom

Hey Tom,

Just curious, would it be possible to not run a vent from the lavatory and instead use an AAV on the lavatory drain line? Just trying to see if I can avoid opening up the drywall as little as possible.