Sewer gas smell

[Jimdamedic]

We have a persistent problem with sewer gas in our Cougar 5th wheel. We’ve flushed the tank and I’ve even run a camera into it and there’s no retention. It’s been flushed. We have used the packets that dissolve debris. We use deodorizer and cleaner. The issue only happens when the tank is over half full or so. Once it’s emptied it goes away completely. I can’t see any issues with the vent on the ceiling but I don’t know how to check it. The only issue That I can find is a very slow leak in the black tank valve. After a few days a couple of cups will accumulate behind the gate valve I’ve put on the hookup. Any ideas?

 

[Kirk]

I suspect that your problem is the plumbing vents under the sinks of the RV and that the odor is from the gray water tank. Notice the black item which is easily replaced and available from any home supply store like Lowe's.

The gray tank often smells worse than the black from decaying food particles and greases that coat the tank walls.

 

[Kirk]

The only issue That I can find is a very slow leak in the black tank valve. After a few days a couple of cups will accumulate behind the gate valve I’ve put on the hookup.

How do you know that the leak is from the black tank valve and not the gray? Those valves are not difficult to replace but the leak into the line would not cause any odor inside of the RV.

 

[Gary RV_Wizard]

Agree with Kirk - the most likely problem is the Air Admittance valves under the sink(s). It's a flapper-type valve that can get weak and not close off, allowing gray tank odors to seep into the bath or kitchen area. Gray tank odors are as bad or worse than black. The valves are easily replaced and are available wherever plumbing parts are sold, e.g. Lowes or Home Depot.

A leaky dump valve is a minor nuisance but doesn't cause odors inside. Or outside either.

 

[tlmgcamp]

OP has linked the odor with the black tank as it starts when the tank is more than 1/2 full and stops when the tank is empty. I am thinking that if the tank vent might come straight into the top of the tank, but is too long. As such, when the tank is 1/2 full the fluid blocks the vent and the odors find another way out

 

[Rene T]

I can’t see any issues with the vent on the ceiling but I don’t know how to check i

When you say vent on the ceiling I assume you mean the vent that goes through the roof.

 

[Jimdamedic]

Agree with Kirk - the most likely problem is the Air Admittance valves under the sink(s). It's a flapper-type valve that can get weak and not close off, allowing gray tank odors to seep into the bath or kitchen area. Gray tank odors are as bad or worse than black. The valves are easily replaced and are available wherever plumbing parts are sold, e.g. Lowes or Home Depot.

A leaky dump valve is a minor nuisance but doesn't cause odors inside. Or outside either.

The odor only shows up when you flush the toilet. Let me as another question. Should you smell sewer gas when you’re on the roof? I do not.

 

[Jimdamedic]

When you say vent on the ceiling I assume you mean the vent that goes through the roof.

Correct. I don’t know how to tell if that vent in the roof is operational.

 

[Jimdamedic]

How do you know that the leak is from the black tank valve and not the gray? Those valves are not difficult to replace but the leak into the line would not cause any odor inside of the RV.

The water smells like urine. I guess I don’t know that it is the black tank however.

 

[Rene T]

Correct. I don’t know how to tell if that vent in the roof is operational.

The easiest way is to open the tank drain then stick a water hose down the vent pipe just a few inches then open the nozzle. You should see water come out the drain piping. It would be a good idea to put a bucket at the drain pipe to catch the water

 

[Jimdamedic]

The easiest way is to open the tank drain then stick a water hose down the vent pipe just a few inches then open the nozzle. You should see water come out the drain piping. It would be a good idea to put a bucket at the drain pipe to catch the water

Hmmm. I’m going to get back up there. The vent has A fixed cap that is only vented on the sides. There is no way I saw to flush it with water. I thought of that.

 

[Rene T]

One style is you Look at the top of that cap. There may be a 1/2” impression that is filled with silicone. Dig it out and you’ll see a small screw at the bottom of the hole. Remove the screw and the cap will lift off. Like I said that is one style.
Another style it the cap has some fingers hanging down in the pile and they grab the inside of the pipe. That one you just pull up and it will come out.

 

[JoelP]

When I was camped last month at Laguna Seca there was quite a bit of wind. I was parked in a spot where I was exposed to that wind. I noticed a glug...glug...glug sound from time to time. My guess was that the vent was being pressurized by the wind and that odors were being sent back in, probably through the sink. I installed the $23 Lippert 360-degree antisiphon vent on the top of the vent pipe at the roof and have not noticed any odor since then, but it remains to be seen when I am parked in a very windy spot again.

 

[Rene T]

My guess was that the vent was being pressurized by the wind and that odors were being sent back in, probably through the sink.

The sinks have traps to stop that

 

[Onyrlef]

If the vent on the roof were plugged up the black tank would be extremely slow to drain, the vent serves two purposes. It's likely you've got an uncapped pipe somewhere in there or they've tied the grey and black vents together and you've got a malfunctioning anti-siphon device. I've also seen 1 1 /2" ABS joints not glued from the factory and they've worked loose. The anti-siphon devices would'nt ordinarily tie in with the black tank but try this. Tie a plastic bag tightly over the anti- siphon device under the bath sink, flush the toilet, if it expands there's your problem. It should let air in, but not out. Try that on each anti- siphon device to eliminate that possibility.

 

[John From Detroit]

When connected to the park sewer do you leave your gray valve open like many advise..... Close it.

 

[Kirk]

The odor only shows up when you flush the toilet. Let me as another question. Should you smell sewer gas when you’re on the roof?

That may mean that it isn't the plumbing vents but don't overlook them as they do need to be replaced from time to time. Another possibility would be if you have an exhaust fan running in the bathroom vent that will lower the room air pressure and so draw air from the black tank into the room. You should always turn the vent fan off before you flush.

The water smells like urine. I guess I don’t know that it is the black tank however.

You will discover that the water from your gray tank can often smell much worse than that from the black. But the water from the gray water tank will typically look gray, while that from your black tank will be brown and often have suspended solids. Visual is a much better way to tell them apart than by sniff test.

 

[Jimdamedic]

When connected to the park sewer do you leave your gray valve open like many advise..... Close it.

Never. It’s always closed unless dumping

 

[Ray-IN]

Got an old tape measure you don't mind getting dirty and wet? Remove the roof vent cap for black tank, extend tape measure down until it touches the bottom of the tank, jot down measurment. Then slowly pull it up until it catches on the bottom of the vent pipe, jot down measurement.
Now, if you can access your black tank, measure outside height, jot down measurement.
Now do some arithmetic to determine how far the vent pipe extends into the black tank.

Another thing to look for is the vent pipe into black tank seal, especially if the pipe has dropped into the black tank more than a couple inches.

The last thing is the toilet base to flange seal; is it fitting properly and not leaking?

 

[Rene T]

Look at the vent pipe up on the roof. If it’s somewhat even with the roof then it’s just like it was leaving the factory. If the end of the pipe is down in the hole 8” to 12”, then you know the answer.