Sewer smell only when driving

[RV Vagabond]

I rarely smell anything from the black tank in my motorhome, but as soon as I drive off, I can smell the stink from the drivers seat. What is happening there

[BPoland]

It could be the grey tank cheater valve is sticking open and allowing the fumes into the coach. You didn't say if the tanks were empty or not.

[Skip426]

Roof vents open ?
Dash air conditioning or heater on ?
Drivers window open ?
Toilet dump valve holding water?

[kalynzoo]

Do you have a vortex type exhaust on the tank vent? A straight exhaust often causes a downward pressure while in motion, sort of like playing the flute by blowing over the mouthpiece. The vortex type extension, sold at most RV stores and on line, will create an upward pressure when wind passes through, lifting the tank gasses away from the RV.

[trimacon]

Have had this issue. I found double checking that my p traps all had water stopped it, including shower.

[MisterT]

it's happened to all of us and it's usually the GRAY tank, not the black. Most motorhome toilets are 'sealed' with water when not in use, but the GRAY tank smells can be pulled back into the motorhome by backpressure from open air vents and windows when traveling. This usually happens when there is no water in the sink(s) or shower p-trap, which is caused sometimes when the tanks are emptied and the air pressure pulls any water from the trap.
We usually run enough water into each sink and the shower to fill back the p-traps before we leave.

GRAY water also stinks more than you might think - food goes in there.

[Waywards]

Originally Posted by kalynzoo

Do you have a vortex type exhaust on the tank vent? A straight exhaust often causes a downward pressure while in motion, sort of like playing the flute by blowing over the mouthpiece. The vortex type extension, sold at most RV stores and on line, will create an upward pressure when wind passes through, lifting the tank gasses away from the RV.

The RV Geeks have a video on this:

How To Keep Your RV Odor-Free - TheRVgeeks

[EdInArk]

Someone posted this thread, which I found interesting. There's a video that suggests we could build up a powerful smell in our gray tanks by leaving them open. So ... do you leave the gray tank valves open?

[CC Driver]

Check the vacuum break/check valve typically installed under the bathroom sink If you have a "saniflush" or similar black tank wash connection. Ours did the same. No smell until you drove down the road.

[twogypsies]

Do you have a ceiling exhaust fan running? Windows open?

[Nana25K]

Ours did that until we replaced all of the vacuum breakers ( look for one under the kitchen sink)

Also clean them once in awhile. I do this by first getting the drains clean. Dump some baking soda followed by vinegar. Then follow with really hot water. Then after dumping before travel we fill gray at least 1/2 full and add a generous glug of Dawn.
You'd be amazed how yucky the gray water can be when you dump on arrival.

[spyderRV]

I close all vents, run the dash AC or heater in the outside air mode. Added a

Cyclone Sewer Vent - Camco 40595

to the rooftop black tank vent.

[Acker1]

Also make sure you have water in the washer p-trap. Easily overlooked.

DICK

[CC Driver]

Originally Posted by RV Vagabond

I rarely smell anything from the black tank in my motorhome, but as soon as I drive off, I can smell the stink from the drivers seat. What is happening there

So what I'm reading is your driving stinks?

Sorry, couldn't refuse a perfect straight line.