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Sewer odor while driving

We are having a problem with sewer odor from the bathroom area that occurs only when we are traveling. I have searched but found no definite help.

Anyone have any ideas about the cause of this?
Royce & Denise, MC #17410
'01 U320 4220 ISM450 Build #5895 SOLD
Toad: '10 Honda CR-V

Travel makes one modest. You see what a tiny place you occupy in the world. - Gustave Flaubert

Re: Sewer odor while driving

Reply #1 –
Vent needs replaced, under sink, available at Home Depot. Search vent odor on Forum
Tim Fiedler    2025 LTV Unity MBL on Order
2000 Chevy Tracker 2 Door Convertible 4WD Now lifted 4.5"
1985 Citroen 2CV6 Charleston
Murphy Rebel on wheels with 175HP Titan
Cessna P337
1980 48' Westport MY (!/4 Share)

Re: Sewer odor while driving

Reply #2 –
Vacuum (lower air pressure) in the bathroom than in the gray and/or black tank.

One of these (or similar) on the roof tank vent will lower air pressure in the tank:
Rotating Plumbing Vent Cap - Cyclone by Camco - PPL Motor Homes

Sure, there could be other, more involved issues-- could be a mud dubber next blocking the roof vent, could be that the PVC pipe to the roof vent has fallen into the tank, failure of an air inlet check valve Shop Keeney Mfg. Co. Plastic Mechanical Plumbing Air Admittance Vent at, etc.

BTW, allowing outside air through the dash HVAC raises air pressure in the coach and will help minimize odor.
Brett Wolfe
EX: 1993 U240
Moderator, ForeForum 2001-
Moderator Diesel RV Club 2002-
Moderator, FMCA Forum 2009-2020
Chairman FMCA Technical Advisory Committee 2011-2020

Re: Sewer odor while driving

Reply #3 –
As always Brett is the definitive source of information but I would start with the air admittance vent
Tim Fiedler    2025 LTV Unity MBL on Order
2000 Chevy Tracker 2 Door Convertible 4WD Now lifted 4.5"
1985 Citroen 2CV6 Charleston
Murphy Rebel on wheels with 175HP Titan
Cessna P337
1980 48' Westport MY (!/4 Share)

Re: Sewer odor while driving

Reply #4 –
Tim-- you are correct. 

Very easy to do the "sniff test".  If odor is worse in the cabinet, sure enough, that is your source.
Brett Wolfe
EX: 1993 U240
Moderator, ForeForum 2001-
Moderator Diesel RV Club 2002-
Moderator, FMCA Forum 2009-2020
Chairman FMCA Technical Advisory Committee 2011-2020

Re: Sewer odor while driving

Reply #5 –
There may be a second vent behind the toilet bowl in the cabinet. Mine has one.
The selected media item is not currently available.Kent Speers
Locust Grove, OK
1993 U300 SSE 40' (Restored at FOT 2009) Build 4323
720 watts Solar
6V92TA DDEC Silver Engine
2014 Subaru Outback

Re: Sewer odor while driving

Reply #6 –
I can find no vent in any of the places suggested, so I guess I need the more expensive solution that Brett suggested. Thanks.
Royce & Denise, MC #17410
'01 U320 4220 ISM450 Build #5895 SOLD
Toad: '10 Honda CR-V

Travel makes one modest. You see what a tiny place you occupy in the world. - Gustave Flaubert

Re: Sewer odor while driving

Reply #7 –
Could also be behind the washer if you have one.  Both our shower one and our washing machine one are there.
Learn every day, but especially from the experiences of others. It's cheaper!  - John C. Bogle

2000 U320 36' non-slide / WildEBeest Rescue
2003 U320

Re: Sewer odor while driving

Reply #8 –
Royce,

Can you locate the SOURCE of the smell-- where is it strongest?
Brett Wolfe
EX: 1993 U240
Moderator, ForeForum 2001-
Moderator Diesel RV Club 2002-
Moderator, FMCA Forum 2009-2020
Chairman FMCA Technical Advisory Committee 2011-2020

Re: Sewer odor while driving

Reply #9 –
These vents don't look like vents. They are like a black plastic canister atop some black PVC type pipes that are vertically mounted. It doesn't look like it but they are threaded and just screw on the top of the pipe. Inside is a one way flapper valve which allows air into the tanks. Once the flapper ages and becomes stiff and deformed it can also allow sewer gas to get up into the cabinets and eventually the living area.

These valves can be located in the cabinets behind the toilet, under the bathroom sink, in the closet designed for the washer/dryer and under the kitchen sink. It all depends on where the tanks are located.  I guess they could be anywhere they can be concealed. They are usually under $10 so once you take one off just replace it with a new one. They are not worth trying to fix.
The selected media item is not currently available.Kent Speers
Locust Grove, OK
1993 U300 SSE 40' (Restored at FOT 2009) Build 4323
720 watts Solar
6V92TA DDEC Silver Engine
2014 Subaru Outback

Re: Sewer odor while driving

Reply #10 –
Makes a difference if rear windows are open or closed, as well as the fantastic fan vent.  The wrong combo will suck sewer fumes, far be it from me to explain why. 
"Not so  long ago we were a nation of risk takers, riding five million pounds of  thrust straight into space."  Joe Gresh
Chuck Pearson
1996 U295
2018 Can Am X3 TurboRS

Re: Sewer odor while driving

Reply #11 –
Quote from: Kent Speers –
These vents don't look like vents. They are like a black plastic canister atop some black PVC type pipes that are vertically mounted.

ABS, actually.  If you search the forum for "air admittance valve" you'll find pictures.

Air Admittance Valves, aka "Stinky Coach Syndrome"
Learn every day, but especially from the experiences of others. It's cheaper!  - John C. Bogle

2000 U320 36' non-slide / WildEBeest Rescue
2003 U320

Re: Sewer odor while driving

Reply #12 –
The coach is parked at home and there is no sewer odor. It occurs only when we are traveling. No smell at all now.
Royce & Denise, MC #17410
'01 U320 4220 ISM450 Build #5895 SOLD
Toad: '10 Honda CR-V

Travel makes one modest. You see what a tiny place you occupy in the world. - Gustave Flaubert

Re: Sewer odor while driving

Reply #13 –
That's because it's a dynamic air pressure issue.  I suspect lower air pressure in the aft part of the coach is allowing noxious fumes to flow from a higher pressure (the grey/black tank) to a lower pressure area (the back half of the coach while you're going down the road.)  Try adjusting venting, it's the only thing I've found to work. 
"Not so  long ago we were a nation of risk takers, riding five million pounds of  thrust straight into space."  Joe Gresh
Chuck Pearson
1996 U295
2018 Can Am X3 TurboRS

Re: Sewer odor while driving

Reply #14 –
Yes, picture is the second link in my first post. Under $6 at the box stores.  And the rubber gaskets DO get stiff and fail.  Easy to unscrew and replace.  But, it the odor is not stronger under the cabinet, unlikely that is the source of the odor.
Brett Wolfe
EX: 1993 U240
Moderator, ForeForum 2001-
Moderator Diesel RV Club 2002-
Moderator, FMCA Forum 2009-2020
Chairman FMCA Technical Advisory Committee 2011-2020

Re: Sewer odor while driving

Reply #15 –
There will be a vent - all sinks have a vent - all the FT's I have owned have a vent (admittance vale at the top of a large diameter PVC. It will be in the rear, maybe corner of the cabinet the sink is mounted in. The air admittance valve will screw into the top. They are intended to be one way, but when they fan the odors can come back into the coach. When you drive down the highway, a small vacuum differential is created inside your coach vs. outside air pressure. If you valve fails, this differential caused the air in the tanks to flow back into the coach, with the resulting odor.
Tim Fiedler    2025 LTV Unity MBL on Order
2000 Chevy Tracker 2 Door Convertible 4WD Now lifted 4.5"
1985 Citroen 2CV6 Charleston
Murphy Rebel on wheels with 175HP Titan
Cessna P337
1980 48' Westport MY (!/4 Share)

Re: Sewer odor while driving

Reply #16 –
We all seem to have different experiences with the stink factor, when I first noticed the issue, thought sure it was  from the black tank, WRONG, was from the grey tank with old soay water, I found it by dumping a bottle of KenKleen or something like that down sink & shower, BINGO bas order went away. No more problems, before I store coach, I treat each empty tank and never any stink, I du use a little water making coffee & clean up, but never use the toilet until am on the road. ;D

Re: Sewer odor while driving

Reply #17 –
Everyone may experience this sooner or later.  Had this same problem.  Replaced both vents under sinks//they are cheap.  Santitized both tanks/put in Dawn into holding tank.  add Ice and drive for 5 to 10 miles start and stopping best.  If still there add country time lemonade powder to holding tank.  You should have no issues after that. 
Dan & Shirley Stansel
2002 U295 4020 AGDS Build#6054
Towing Buick Enclave & M &  G Braking
Emerald Bay, Lake Palestine, Texas
MC# 16650