Sewage smell coming from toilet

[tomanderson312]

I have a basement bath (shower, sink & toilet) that drains into a secondary small septic tank. Recently, we began getting a sewage smell from the toilet bowl and cannot figure out the problem. Here's what I've tried:

1. To make sure the smell was coming from inside the bowl, I covered the bowl with saran wrap and checked the next day. The smell had stopped verifying it was coming from inside the bowl. (A lesson I did learn here is to make sure you tell your wife about the saran wrap!)
2. I checked the shower and sink drains and no smell. It only comes from the toilet bowl.
3. I replaced the wax ring on the toilet, but the smell continued.
4. I checked the vent pipe to make sure it was not clogged...all clear.
5. I had the septic tank pumped and checked for any problems...all OK.

Any suggestions? I'm out of ideas here.

 

[Matt30]

Have you noticed any change in the water level in the bowl? It would be getting siphoned.

Take the lid of the toilet tank and confirm that the refill tube is going down the overflow tube. This is what keeps the bowl filled after a flush. Flush the toilet and confirm you see water flowing down there.

If the bowl is full, put a little mark on the inside of the bowl where the water level is, then check and see where the water level is when you notice the smell.