Monday, September 25, 2023


Ask Dave: The toilet leaks from underneath when cleaning black tank. Why?

Dear Dave,
Our RV’s black tank has cleaning jets. While cleaning out the tank this past weekend, my wife noticed that the toilet had water coming out from under it. What could be the reason and how would I fix this? If there is a video that I could watch, I would appreciate it. Thank you. —Timothy, 2018 Rockwood Ultra Lite 2606WS

Dear Timothy,
The cleaning jets you are referring to are part of the black tank flush valve. They are designed to clean the tank and the monitor probes that are on the side of the tank. When you connect the garden hose it is supposed to spray around the side and clean off any junk that has clung to the side so the monitor does not have a false reading. Typically, the jets are not supposed to shoot to the top of the tank, so maybe the valve got turned slightly. It should be mounted with four screws so that it can’t, but weirder things have happened.

The issue is probably that your toilet flange is not sealing. There is a grommet or gasket at the main tank opening and a plastic stand pipe is fitted into that. Then, there is a flange mounted to the floor that slides over the standpipe. This should all keep water from seeping out the sides.

Remove the toilet to inspect

I would remove the toilet and inspect the grommet and pipe, then see how well the flange fits in the pipe. I doubt this is the issue as you would not have water on the bathroom floor but rather underneath in the compartment or underbelly. With the toilet off, you might want to put clear plastic over the pipe and run the black tank flush valve at a very low flow rate to see what is actually happening. With the flange settled into the pipe and the toilet gasket on that, it’s hard to tell where water could come into the bathroom. It may be that it’s coming from somewhere else. Here is an exploded view of the components that might help.

There should also be a seal between the base assembly and flange. I would use a little plumber’s putty on the base to help seal as well.

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

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Dave Solberg
Dave Solberg
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. Hey Dave, I’ll bet that if the water is coming in under the toilet, then for sure so are the fumes from the black tank, especially when you turn on the bathroom exhaust fan!

  2. We had a leak with a cracked water valve on the back of the toilet, very hard to get to without removing toilet but we did it. It only leaked when the toilet was flushed, it wasn’t expensive, but it took an hour to get it out and an hour and a half to reinstall.


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