I'm still having problems with a sewage odor in my house. I bought this house last year, it was actually built in the 50's. I noticed when I do laundry or take showers, I would get an odor coming into the house. I've went under the house in the crawl space when we smell the odor in the house and checked all the plumbing and there isn't a smell anywhere. I also added a second p-trap on the washer line and another vent pipe to the plumbing and all of this has not helped at all. Although, there is no longer a loud gargle sound when we run the washer.

This smell is really intense and is a burden! I thought about adding a wider drain pipe from the washer down to the p-trap thinking that maybe the washer is blowing the water out of the trap causing the odor, but since I have two traps on this line I wouldn't think it would blow water out of both traps? I'm running out of thoughts though and really need some help.

Another thing confusing to this situation. This only happens when it's cold out and I have most of the vents for the crawl space shut. When the smell occurs, I go open them and the smell leaves quicker than if I kept them closed. But again, when I went under the house when the smell was in the house, there wasn't a smell to be found. So I do not know why this helps the smell leave or why it only happens when they are closed.

Please help... I'm very frustrated about this entire situation!!

Sounds to me like your plumbing vent pipes are clogged up... The vents should exit through your roof. If they are clogged, water cannot flow through the pipes so air has to escape somewhere, usually up through your traps and into your house along with sewer gas. As a side note, I have 2 floor drains in my unfinished basement that get a nasty sewer smell in them if the traps are allowed to dry out or the water gets stagnant. I just have to pour a few gallons of water in them once in awhile to keep the water fresh and the traps filled. The fact you only notice the smell when the vents are closed means the gas is just building up and with the vents open, its escaping. I would have a plumber check your plumbing vents for clogs.

I've flushed both vent pipes out so I know they are clear.

Sewer gas can come from plumbing fixtures whose traps have gone dry or have lost enough water that the water seal within the trap has room for air to pass through it. Cracks in plumbing drain lines or the vent pipes are the only other source of sewer gas leaks I can think of. If the crack is in a drain line, you should have a water leak as well. (if the pipe is cracked under ground, you'd need a camera run through the pipe to detect that). If the vent pipe has any cracks they can leak a lot of sewer gas in. You need a speciality cool smoke machine to test for that. It involves blocking off pipes and all plumbing vents. I would say call around to some plumbers and see if any of them have the ability to test plumbing vent pipes for cracks and leaks using a cool smoke machine.

One more thing I just thought of, since the odors evacuate with crawl space vents: how is the seal between the foundation wall and the floor? If there are sewer gasses that have leaked in the ground from a broken or cracked pipe they could be leaking in through this "broken seal" as well... In which case, running a thick bead of caulking to seal the floor would most likely fix it.