FOLLOWUP! In July 2012- I had sewer gas leaking into my bedroom, bathroom, and then up to the upstairs level of the house for five months. It was awful. I have an 18 inch crawlspace and a raised floor. Under the house is dirt and the pipes. I had FOUR plumbers and TWO handymen check it out. They found NOTHING! They were too big to fit around the pipes!!! We had put in a new toilet, and I had that pulled up and put down again, just in case. but that was fine. We checked all the traps, and now keep them full of water, so they are fine. The plumbers gave me estimates of $4500 for a sewer cleanout installation in front of the house, and $6800 to repipe the bad pipes, if they found any.... I did not have the money, and they really had no plan, just guesses. Last August 2012, I luckily found a neighbor friend who was able to crawl around under the house. He was a little guy, and knew how to do plumbing and other repairs!! He found leaking and large cracks in the main drain sewer pipe at the ground level, around the back of the pipe, hidden from general view and he used epoxy to repair it. This pipe was attached to other pipes, and if it needs to be replaced it will be a difficult job. That pipe was directly below the downstairs bathroom. That reduced about 70 % of the smell and seems to be holding tight with no more leaking there. But sewer gases were still leaking from somewhere else, but not as bad, or as often. it still kept coming back about every three days.They centered in my bedroom, and seemed to come out of the floor. Oddly, from the beginning, the gases only happened around 1am until 5am, then stopped all day long, so no one could smell them, unless they stayed all night. From the first repair, the gasses only occurred about every three days or so, and not as severe, but still stinky. My friend came back into town in November, and went back under the house, and found stress marks on the larger pipe that attached to the MAIN drain sewer pipe, and ran towards the front of the house. This pipe traveled directly under my BED, and the cracks were basically under my pillow area. (What LUCK!) He had to replace that whole pipe, and to get it inside we opened the ground vents and slid it through, and the old pipe out. After he replaced that and all the fittings, the gases were gone! THANK YOU GOD!!! It cost me $800 just to investigate all this, and get nothing for the repairing of it from that! Plumbers were just ready to get paid, with no idea what was actually wrong!!! My friend only charged me $550 plus parts for this! I suffered through it for five months, and hope the sewer gases didn't harm me in any way! I sure appreciate the clean smell now!!! By the way, it was an acrid stinky burning smell. It did NOT smell like METHANE, or anything described as general sewage smells. It was just hard to breathe it, and would cause eyes, throat and nose to burn. I had a runny nose right up to the day the last pipe was replaced, and it all stopped. GOOD LUCK TO ANYONE STILL HAVING THIS PROBLEM. I learned far more about plumbing than I ever thought I would need to know!

I am not a plumber, but I think I know what is happening. The "cap" is a vent to prevent a vacuum forming in the sink drain. It is spring-loaded and, without water flowing in the drain, does not allow air into the drain nor allow foul gasses out. My guess is the short horizontal run of pipe from the "T" to the vent is clogged. When the shower is running, the drain water forms a vacuum in the waste drain system. This vacuum pulls water out of the sink trap allowing the foul gasses to escape through the trap and sink drain. An unclogged vent would have opened and "killed" the vacuum. I believe your sink drain is improperly plumbed as far as venting. I suspect you have the infamous "S" trap rather than the desirable "P" trap and also have a problem with the bath drain venting

We also just had one bathroom with a "sewer smell". In our case, when the water was first turned on in the sink, there would be a blast of the foul odor. First I thought it was the water itself, but then I decided to catch the water in a pot - no smell then. But when I poured it down the drain, up came the smell. Aha, perhap ssome bacteria, etc down the drain. So I poured a bit of bleach down the drain and that took care of it. Hope this helps solve your problem. (BTW, if that wasn't the cause, my next step was to pour some bleach down the sink & tub overflow drains.)

Sounds like you have a leak in the sink trap or you are siphoning water out of the trap at the sink which compromises the water seal and allows sewer gases into the house. Do check your vent and check your trap for leaks