New place, there’s a funky smell in one of the bathroom sinks coming from the overflow hole. No problem, I use the drain blaster to clear out some gunk and pour boiling water down the hole, problem solved.
Only the smell keeps coming back, sometimes after a week or so, sometimes after just a couple days. I’ve never had a mold/mildew problem so persistent. Anyone have a remedy?
Pour some bleach down it.
Make sure that your trap is working. It’s possible that the water is being sucked out of it due to a clogged stack pipe.
Or that water is leaking out of the trap.
Vinegar and baking soda daily for a week or more. A plumber may need to check your traps or vents.
The trap isn’t leaking and I don’t think it’s the stack pipe, because there are twin sinks and only one has the odor problem.
Bleach might do it, but it’s just so damn smelly and I know the family will complain. Maybe I’ll do it on a weekday morning and leave the windows open all day.
I tried vinegar, which didn’t do anything. Not sure how I would get baking soda down the overflow hole.
That’s what did it for me when I had this issue a few months back, and it worked for my set of circumstances. The chlorine scent didn’t linger all that long, but I’m not particularly sensitive to that smell. Plus it smelled better than what was coming out of my overflow hole.
Are you certain that the smell is coming only from the overflow hole and not the drain? That seems strange to me, since they are connected and there is only a few inches between the overflow hole and its junction with the drain. If it’s really true that there is no smell from the drain, the suggestions in this thread implicating a faulty trap or clogged stack don’t seem valid, as the odor from rising sewer gas would be at least as strong at the drain as at the overflow, I would think.
Well, I posted after putting some boiling water down, so there’s no smell to test at the moment. When it comes back I’ll double check that it’s not coming from the drain as well. But it isn’t a sewer gas smell, much more like mildew.
If you pour a copious amount of baking soda and then pour vinegar it will boil up. It really helps with sink and drain smelly issues. YMMV, but it works for me.
Don’t tell my wife I said this, but every so often I have to clean out a glob of her hair that’s caked with conditioner/shampoo and who knows what else. It gets stuck around the stopper, particularly where the pivot rod grabs it.
Maybe see if its smelly after flushing a nearby toilet.
See the water from the toilet fills the pipes, and drives the air down, which has the effect of pulling water out of the S bend (its actually atmospheric air pressure pushing the water in… due to lower pressure inside… ) and/or back up … there’s meant to be a vent in the pipes, going to outside, to ensure the pressure of a big flush, toilet, laundry tub, bath tub, doesn’t empty or bubble the S bends… check vents are clear, install vent where needed ?
My first guess would be that the sink drain in not installed properly partially blocking the drain on the over flow leaving a little bit of water sitting in there.
That certainly fits the facts of the case. How could I, a non-plumber, determine if that’s the problem?
There might be a simpler way, but here’s what I’d do:
Put a bucket under the trap.
Underneath the sink, there’s a nut that holds the pivot bar into the drain and through the stopper; if your drain-pipe is plastic, the nut should be plastic, also–probably a wing-nut. Twist the nut off the D-P. Pull the pivot bar out of the stopper/D-P (there’s a plastic ball on the pivot bar that seats into the D-P). Remove the stopper. You might be able to see gasket material blocking the overflow exit. Pour water into the overflow and see if it drains at about the same rate. If so, reassemble in reverse order; your problem is somewhere else. If the water doesn’t drain through the overflow sufficiently, and/or you can see the gasket material blocking the overflow drain, I’d call a plumber (that’s just me; a novice plumber can remove the upper drain pipe and fix all that, but I wouldn’t, especially if, like me, you’re renting.) Replacing the pivot bar into the stopper can be a real bastard, just have patience and persevere.
IANAPlumber; any/all comments encouraged.
*While you have the stopper out, shine a flashlight down the drain and see how junked up it is. If it’s grotty, get a bottle brush and scrub. Flush with very hot water.
Thanks for the comprehensive advice. However, based on previous home plumbing experiences, rather not try it.
I am renting; but this seems like one of those “save the call to landlord for bigger problems” type things.
My amateur plumbing knowledge agrees with this. The boiling water kills the mold, but the standing water later lets it grow back.
Preaching to the choir. 
If this is sewer gas leaking into the house, that IS a bigger problem type thing. Plus, catching it now could make it easier for the landlord down the road. Calling can’t really hurt and s/he might appreciate your foresight. Landlords hate when tenants let things go and become bigger probs for someone else.
Dave Barry has the best advice: The only way to keep a plumbing part from leaking is to tighten it until it breaks (then throw it away). 
get some of those green sticks advertised to keep the smell down in your sink. i had the same problem, and those cleared it up with no more effort than stuffing one under the drain plug.
green gobbler or sani-sticks.
I was going to say take out the stopper because it’s going to be covered with foul black slime and hair and have a stench worse than death, but you beat me to it.
Every once in a while we have to take ours out and scrub. The smell is God awful.