Plumbing Help?

[NocalFJ60]

Last week I noticed when walking in the bathroom a sewer smell. I tracked it down to the bathtub drain. I filled the tub with water and took the cover off the overflow drain. I then held my hand over the overflow drain and let some water drain down the drain in the tub and got the plunger out and got a lot of black junk up the drain. I am thinking it was caught in the trap and cleaning the trap now the problem would be gone. A couple of days went back and the sewer smell came back.


What should I try next? I don't think I have a leak but it seems like the trap is going dry letting sewer smell back up the drain.

 

[PAToyota]

Check the air vent. Birds tend to get clogged in them - don't ask how I know...

When the air vent is clogged, the drain syphons the water out of the trap and allows sewer gas to flow back up through the drain.

 

[Toyo FJ40]

Check the air vent. Birds tend to get clogged in them - don't ask how I know...

When the air vent is clogged, the drain syphons the water out of the trap and allows sewer gas to flow back up through the drain.

You usually will hear the drain make a sucking noise when it drains or if you flushed the toilet in the same room if the vent is clogged. That or gurgling when other water is run.