Sunday, January 29, 2023


Ask Dave: Stinky black tank smells coming into the bathroom. Help!

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. Today he discusses a stinky black tank.

Dear Dave,
I have a new ATC 5th wheel toy hauler and am having black tank smells in the bathroom. I treated the tank with deodorizers and flushed completely, but as soon as it warms up outside, the smell is back. The water is staying in the toilet and I checked the vents and they seem okay. —Gregg

Dear Gregg,
The first thing I would check is the venting system. Take a look at where the black tank is located. There should be a vent tube that either runs up to the roof or, if it is located directly under the kitchen counter, it will have a “cheater vent” with a siphon cap.

Let’s start with the roof vent

I would start by looking at the cap to make sure it is on firmly. Make sure that the pipe is not loose. I also would take the old vent cap off and install a Cyclone that will turn with the wind and siphon air from the tank out. If you get a good strong wind with the old-style cap, it could force the air back down into the rig.

When you remove the old vent cap, also remove the sealant and check to see if the pipe is loose or will pull up. It is not uncommon for the vent pipe to separate from the black water tank, which means the fumes/smell from the tank can come inside the rig. Typically, it’s difficult to get to the top of the tank and vent tube, so if it’s loose, you will probably need to tear out some cabinetry. Since it is new, I would make sure to document it with the manufacturer or dealer.

“Cheater vent”

If you have a “cheater vent,” there will be a pipe that comes up through the floor in the underneath cabinetry and has a cap over it. This is designed to pull in air to eliminate a vacuum in the pipes when dumping. It is supposed to be designed to only allow airflow in, but they go bad and I’ve had to replace a few. If you have one of these I would not test it—rather, just replace it.

Another issue could be the flange for the toilet on the floor. It could be cracked or not set properly or the toilet could be cracked. It’s easy to remove and inspect. Just take off the plastic base cover and you’ll see the mounting bolts/screws.

These should be the only ways for fumes to get into the rig. I would also suggest using Thetford Tank Blaster along with a deodorizer. And don’t run your roof vent outwards with the toilet lid open!

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1 year ago

When we first got our Winnebago Class A we had a bad sewer odor problem, especially when running the exhaust fan(s). I tried various deodorizers to no avail. Then one day the light lit as I was changing the water filter under the kitchen sink – I could see the ground outside – I had left the utility bay door open! Walla – further KISS principle application revealed a large opening under the sink to the utility bay and the slinky drain hose had no cap on the end of it! A rapid, temp fix with a peanut butter jar cap solved the problem! Soon after that was replaced with a genuine sealing slinky cap!

Royce Hershberger
1 year ago

Yes! Make sure it’s not the Grey tanks, they can be worse than the black tank, they don’t get rinsed like the black does.

1 year ago

I use a 1/8 cup of Rid-X in both tanks, on moving day I do a real good job of flushing the black tank and then leave a little of water, Dawn soap and Borax to slosh around and empty it after hooking up and then add the Rid-X. Also be careful using anything with bleach in both tanks. In the kitchen I always use a napkin to wipe off everything off before washing with Dawn soap and also use a fine mesh screen (purchased at box store in plumbing section) in the drain to catch any food I may have missed. We never have a smell issue and tank sensors work 90% of the time.

Dave Telenko
1 year ago

I had a stinky problem in my last motor home. only it wasn’t the black, but the grey tank! Seems we hadn’t used the shower for a while & the “P” trap had gone dry & let the stink in. From then on we made sure to pour some water in all the drains to keep the “P” trap full of water to make sure there was a seal!

Ron Seidl P.E.
1 year ago

Don’t forget about the Dometic 300 toilet issue that traps stinky stuff in the toilet. There is a recall, but they denied it for me.

1 year ago

You mentioned not to run exhaust fan with lid open in the bathroom. When workcamping we found the main source of oder was when we ran the fan when flushing. We ran fan when showering and when setting down but always shut fan off to flush then back on and found we had a lot less of a odor problem. The fan is so powerful it pulls air out of the black tank with flush valve open.

1 year ago
Reply to  Terry

This works for us, YMMV – we leave exhaust fan on but do not empty the bowl of water when flushing. It’s tricky timing but if you don’t completely empty the bowl of water you keep a water seal preventing tank odors from coming up like a P trap works.

Jesse Crouse
1 year ago

We use a Bio-Method that uses Dawn soap and Borax. Borax can be generic if you can’t find the name brand. No odors in the 2 years we have been using it.

1 year ago
Reply to  Jesse Crouse

Similar method – cheap liquid laundry detergent and Calgon water softener liquid.

Ron Lane
1 year ago
Reply to  steve

That’s the method I’ve used on my last four rv’s. No chemicals of any kind, no deodorizers, no borax, no rid-x, no Dawn. Results are no clogs, no drain problems, no odors, no sensor problems. Liquid laundry soap is designed to break down oils, grease, dirt, blood, and many other things all in “COLD” water and very little suds. It just amazes me how many people fall for the totally unnecessary black tank chemical requirement, rv toilet paper, and other “rv” required products.

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