Thursday, August 11, 2022


Ask Dave: Black water tank is clean but there are still smelly odors. Why?

Dear Dave,
After flushing the black water tank, there are still smelly odors like a sewage smell in the bathroom that seems to come from under the sink in the cabinet. I flush the waste tank until the water comes out clean and there is still a strong smell. Is there a one-way valve or something like that that may be stuck open? Thanks. —Luis, 2020 Winnebago Micro Minnie 1706FB

Dear Luis,
The first thing I would do is clean the tank out with Thetford Tank Blaster a few times to really clean the tank and sidewalls. You will need to drain it a few times, as well.

After you do this, look under the cabinet to see if there is what we call a “cheater vent” that comes up from the black water tank through the cabinet and has a cap on it. The tanks need a vent pipe to keep the system flowing when dumping so there is not a vacuum created. Typically, the pipe comes off the tank and goes through an interior wall such as the bathroom or full-length wardrobe, hidden between the walls. Sometimes the location of the tank in the undercarriage needs to be placed away from a wall, so the manufacturer brings the vent pipe up under a cabinet such as in the kitchen or, in this case, the bathroom sink cabinet.

Siphon caps help get rid of smelly black tank odors

Since the pipe cannot go all the way to the roof due to the open space above the counter, they use a siphon cap that is designed to allow air to be drawn in to eliminate the vacuum. It has a rubber gasket that is supposed to prevent odors from going into the rig.

These only last for a short period of time, from what I have encountered, and need to be replaced. You can get them at any RV parts supplier. I was able to find one at a local home improvement store.

If this does not eliminate the odor, then you will need to search for a loose connection in the black water plumbing. Sometimes the pipe from the toilet to the black water tank can loosen or even disconnect, allowing odors to come up into the bathroom. If the sink in the bathroom drains into the black water tank, check all fittings and connections from the “P” trap down, and especially the pipe to the black water tank. Also, if there is no water in the “P” trap, odors from the tank will come up through the pipes. If you put water in the trap and it eliminates the odors, then it tells me the tank is not clean and needs to be sanitized.

Dave Solberg is a leading expert in the RV industry and the author of the “RV Handbook.”

Read more from Dave here


We have started a new forum link for Ask Dave. Please be as brief as possible. Attach a photo or two if it might help Dave with his response. Click to visit Dave’s forum. Or send your inquiries to him using the form below.

Click or drag a file to this area to upload.



Notify of

This site uses Akismet to reduce spam. Learn how your comment data is processed.

Inline Feedbacks
View all comments
1 month ago

The poster says he is getting odors from the black tank. The sink in the bathroom should be plumbed to the gray water tank along with the shower and kitchen sink. It is possible that odor is actually coming from the gray water tank. The air admitance valve may be stuck open and even though the trap has water in it, that valve is placed past the trap.
Since he is talking about a specific Winnebago model, a call to Winnebago will tell him how the tanks are plumbed. A good cleaning of the gray water tank and replacing the air valve may resolve the problem.

1 month ago
Reply to  Bob

My Winnebago Class A has the bathroom sink plumbed to the black tank. (according to the diagrams from Winnebago). This is done to insure sufficient fluid into the black tank. Another possible source is a dry trap in any sink or the bath shower/tub drain, or no cap on the outlet end of the tank drain hose (slinky) – collecting odor in the utility bay and the odor being sucked into the living area from the utility bay or dry traps by the ceiling exhaust fan(s).

Pierre Woody
1 month ago
Reply to  Bob

Bob, you’re right, the black tank vents at the roof, once again Dave dropped the ball.

Subscribe to our newsletter

Every Saturday and Sunday morning. Serving RVers for more than 20 years.