Sewer Gas Smell When Running Water into Drain

[SAS]

I'm struggling trying to figure out why the sink in our main floor powder room smells like sewer gas - but only when you run water into the drain. It doesn't seem like the problem would be the trap running dry for several reasons. First of all if the trap was dry or too low, then I should smell the sewer gas before I start running the water. I've tried, but it only starts to smell once I run water into the drain. Secondly, the sink is used several times a day, so it wouldn't be likely that the water is evaporating out of the drain. Finally, it doesn't seem likely that it's being siphoned due to a lack of adequate venting since I can see water in the trap when I look down the drain. Also, there is no reason that the vent would suddenly stop working as I believe it is tied into the same vent as the bathroom directly above it. If debris (or an animal) fell into the vent the upstairs bathroom wouldn't it also slow the drain from the upstairs bathroom?

Here's what the trap looks like:

 

[Reach4]

1. Suppose the smell is from the sink itself. Maybe plug the vent holes with fingers etc, and see if you get that same smell. If so, a bleach treatment could deal with that.
2. Suppose the smell is not coming from the sink, but elsewhere, such as from a floor drain or tub. Try covering the sink basin with a plastic sheet, and see if you still get the smell.
3. Is the smell stronger or weaker from inside of the vanity cabinet?

 

[Sponsor]

 

[SAS]

1. Suppose the smell is from the sink itself. Maybe plug the vent holes with fingers etc, and see if you get that same smell. If so, a bleach treatment could deal with that.
2. Suppose the smell is not coming from the sink, but elsewhere, such as from a floor drain or tub. Try covering the sink basin with a plastic sheet, and see if you still get the smell.
3. Is the smell stronger or weaker from inside of the vanity cabinet?

My mistake was assuming it was "sewer gas" and not simply coming from the drain itself. I was focusing on what could cause the trap to be low enough to allow in gas from the waste line when in fact i needed to focus on odors coming from the trap itself. The sink does not have an overflow, so if it wasn't a lack of water in the trap, the problem had to be in the drain and trap itself. So I took it off, thoroughly cleaned it, and so far so good.