Plumbing Issue - Sewer Gas Smell when running water

[Tidybuoy]

Wondering if anyone would know the cause of my plumbing issue:

I've lived in this house for 25 yrs. and never had this issue until recently. When I run water in the master bathroom sink, I get a sewer gas smell for about 30 seconds.

The plumbing is 99% original. All fixtures work (i.e., bathtub, toilet, sink) and all drains work properly. I'm thinking that a couple of years ago, the sink trap was leaking/corroded and I replaced the metal trap with ABS. The plumbing connection at the wall is 1.5" and the sink drain is 1.25". I replaced with a 1.5" trap and converted to 1.25" just below the sink drain. I'm thinking that the trap should be 1.25" and convert at the wall.

Any other thoughts? I want to correct this.

Thanks!

[wdfifteen]

What is the source of your water? Well, city?
A lot of wells around here have significant sulfur in the water and produce a rotten egg smell.

[Tidybuoy]

Originally Posted by wdfifteen

What is the source of your water? Well, city?
A lot of wells around here have significant sulfur in the water and produce a rotten egg smell.

City water from wells. However, the master bath is the only area with the smell and only the sink.

[oldE]

Dump some bleach into the overflow of that sink. Bacteria has likely built up in there.

Best
Les

[Tidybuoy]

Originally Posted by oldE

Dump some bleach into the overflow of that sink. Bacteria has likely built up in there.

Best
Les

I will try that. It seems like this all started when I replaced the faucet that came with a new sink drain that is plastic. Every now & then, I have to pull out the plug and it is covered in slime.

[john70t]

Call Roto-rooter or another company and get a main sewage line camera scope from the house. Or buy/rent and DIY.

If the blockage is in the street pipe, it's not your problem to fix.

Effluence gas being pushed back into the house means there is back pressure somehow.
These are normally blocked by "p-trap" loops under every drain filled with water.

If a shower/toilet/tub hasn't been used in a while....add a little bit to the drain until the gurgle sound is gone.

[Evans, Marv]

Here is a possibility. Water flowiing down the trap could be letting sewer gas come back up the drain pipe. The trap keeps sewer gas out by trapping water in the U-shaped tube at the elbow. If the water draining through is a laminar flow, it opens up a spacer in the pipe and lets gas come up. I get this in the sink I use in my bathroom, because I took out the drain stopper because I thought it made the sink water drain too slow. So when I turn on the water, it flows down the drain in a laminar flow through the trap and creates an air space for the sewer gas to come up. If the water isn't running, there is no smell. I jsut put up with it. Investigate that.

[look 171]

Dead rat stuck in the vent up above? Its a long shot, only because the P-trap stops the sewer smell including vent

Open up the p-trap and see if its full of water. If it is then its coming from elsewhere

[red 928]

Originally Posted by oldE

Dump some bleach into the overflow of that sink. Bacteria has likely built up in there.

Best
Les

My bet also.
The bottom of the rectangular holes in the side of the drain pipe
(pictured below)
can sometimes be above the bottom on the overflow channel,
creating a small dam for stinky stuff to accumulate.

Not hard to fix, usually a file will do the trick
but you have to take it all apart first.
Think of it as "port-matching"