Tuesday, September 26, 2023


Ask Dave: My RV’s black water flush valve doesn’t work! Why not?

Dear Dave,
My RV’s black water clean-out valve has quit allowing water to go into the tank. Why doesn’t it work? —Ronald, 2002 Newmar Mountain Aire

Dear Ronald,
The first thing I would check is the filter or screen that is in the actual service center connection. Most of the time you will be using hard water from a campground dump station to connect and flush the tank and it can have rust, calcium, and lime that can plug the screen and reduce or stop the flow of water.

Hose can get kinked

Typically, the valve on the service center has a hose connection that supplies the flow of water to the actual black water tank flush valve that is installed on the side of the tanks so it can flush the monitor panel probes. This hose can get kinked and restrict flow as it sometimes is exposed in a compartment. You can shove something inside the compartment and not realize it pushed the supply hose, too.

We just had a similar situation with a local campground host, Peggy, and her Tiffin motorhome (read about Peggy here). We could see the supply line behind the service center valve, which had bent up over the chassis rail and twisted back into the compartment. I opened the adjacent compartment on the other side and we could not see where the supply hose went. She was going to get an electrician’s “fish tape” and see if she could push it through and find the obstruction. I’m hoping she can update us on how this went—she is a daily RVtravel.com reader! (Hi, Peggy!)

The last thing that could be clogging it is the actual black water flush valve itself. Again, rust, calcium, and lime can clog the small openings of the “spinner.” You might want to try some Thetford tank blaster or multi-use CLR.

Otherwise, you will need to remove the valve.

 You might also enjoy this from Dave 

Ask Dave: My RV’s hidden black tank valve is stuck open. How do I find it?

Dear Dave,
How can I reach my RV’s hidden black tank valve? It’s out of view. The valve is stuck open and I need to lubricate it, clean it or replace it. 2018 Winnebago Vista 31BE (Class A 32′). I have the Winnebago diagrams, so I know where it is—I just don’t know how to get to it. Thanks! —Tony

Read Dave’s response.

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

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Dave Solberg
Dave Solberghttp://www.rv-seminars.com/
Dave Solberg is a leading expert in the RV industry and author of the “RV Handbook” as well as the Managing Editor of the RV Repair Club. He has been in the RV Industry since 1983 and conducts over 15 seminars at RV shows throughout the country.


  1. I wonder if the black tank vent pipe has slipped down or is too long,maybe even from factory. If pipe is in liquid it cant vent. I really dont think even the dumbest person would use a cheater vent for black water. And don’t forget, gray tanks stink also, just a different liquid.

  2. I had the issue with my black tank flush at one state park. Didn’t seem like the parks water had much pressure. Used it at another state park on another camping trip and it worked fine.

  3. Hi, Dave! Well miraculously after we discussed this, the flush worked! I believe it began to work after I re-leveled of my coach. You may recall the significant grade of my site. Thanks again for all the great advice!

  4. Per Larry’s comment, I did the same, but there was pressure in the line (must be a check valve on the inlet) when I unclogged the poop from the flusher. Take care.

  5. When the tank was empty I pulled the flusher from the side of the tank (3 screws and it popped off. I then washed it and took a fine drill and enlarged the water holes slightly. Works much better now, hasn’t reclogged since, and my sensor always reads correctly. I replaced it using butyl tape as the sealant and it does not leak.


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