Thursday, February 2, 2023


Black tank still smells after trying EVERYTHING

Dear Dave, 
Not sure of the year of the RV. I am having very strong odors after flushing and I have cleaned the black tank with EVERYTHING. I have had it flushed and all seems well, but in a day or two it is back to being foul. I’m thinking it is the filter or siphon but I do not know where to locate these and pinpoint which it is. Why does my black tank always smell? —Heather, Wilderness Advantage Extreme Edition

I would suggest trying Thetford’s “Tank Blaster” product as it will get rid of most of what I call the “Bad Bugs,” which are anaerobic bacteria that create those rotten egg smells.

Next, I would suggest looking for your vent. The black water tank has a vent pipe that comes up through the coach and is designed to allow air in so there isn’t a vacuum when dumping the tank. Most go up through an interior wall and are seen on the top of the unit. However, if the pipe comes up in a kitchen area with a countertop, it can’t make it all the way to the top so they cut it off under the counter and use a siphon cap, which we call a cheater vent. These are designed with a gasket that allows air to draw in but does not let the smell seep out. However, they go bad all the time! Find out where your vent is and also make sure it is installed on the tank. I have found them to break off or come loose and the odor comes up through the coach. publisher Chuck Woodbury swears by the Siphon 360 vent, which draws air from the black tank out the top of the RV. He tells me that since he replaced his cheap factory installed vent with the Siphon 360 he has no problems with odors.

Good luck!

 You might also enjoy this from Dave 

I want to clean my black tank valve but I can’t find it

Dear Dave,
I am living in my motorhome because Hurricane Ian flooded my house. I can’t find the black water tank valve. However, I know where the handle is to open and close the valve, but I want to find the valve itself so I can either clean it or replace it. I have looked all over the place for it. I can’t find it in the service center like the other valves. On another topic, I wonder if I can refill the propane tank without taking it to a dealer. Thanks for your help. —Thomas, 2017 Winnebago Vista 29E

Read Dave’s answer



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14 days ago

Just as we do at home with our septic tank we use RID-X in both tanks. After flushing each tank we add a little and let it do it’s job. Never any smell, occasionally using an inspection scope in the toilet I have never seen any big build up after emptying.

Roger Marble
14 days ago

I have been using Camco “Cyclone” vent # 40595 for 6 years and it has always worked great. Never any odor inside the RV. When parked I see that it swivels with the prevailing wind.

14 days ago

I chased a similar issue. It turned out being a bad plastic Dometic toilet. There is a seal between the base and the bowl that goes bad. Urine ends up inside of bowl (the air space between the inner and outer plastic of bowl) and then starts growing a bad smell. If you pull the toilet you can hear it swishing around inside the bowl. I replaced the toilet with a Dometic porcelain base toilet that does not have any seals.

15 days ago

I had one rig that had a terrible tank design. Instead of dropping straight down, ‘stuff’ would hit an angled surface and then drop down. Or cling to the angled surface. Worth a look. If that’s the case you might have to use a small spray hose/tool to keep that clear.

And just to review the basics. Do use some kind of tank treatment if you aren’t already. And, be sure you are not using putting any, or much, toilet paper in the tank. Use a small lidded trash can with liners instead. Because otherwise, it’s easy to create a ‘toilet paper tower’ which rises above the liquid, and can create lots of problems, including smell.

15 days ago

The black tank uses a different vent than the grey tank. The air admittance valve on the sinks has nothing to do with black tank odors, maybe. It is there to allow the sink to drain quickly and not gurgle or bubble when emptying.
While grey tanks odors will come from a defective AAV, the odor is different.
In some instances, a sink may be connected to the black tank if the sink is far from the grey tank, like on a second bathroom, to eliminate excess plumbing.

15 days ago
Reply to  Bob

Hey Bob, Dave didn’t say anything about a gray water vent. He only mentioned that if the black tank vent came up by the kitchen counter they used a siphon cap!

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