I bought a high efficiency washer in the house I recently purchased, which was built in 1998. The first 3 months, I had no problems with the washing machine. Then, suddenly, there was an extremely strong sewer gas smell coming from the drain pipe that the washing machine was emptying water into. Keep in mind I'm on city water as well. A plumber came over, scratched his head, and didn't know what to do about my sewer gas problem and gave no advice. He simply pulled the drain hose further out of the drain pipe. I read on a forum that perhaps the high efficiency washers flush the traps too much and dry them out so after each wash, I was pouring water down the drain pipe. Now, I cannot tell if there is a trap on the drain pipe because it is in the wall. I'm assuming there is one but I can't be sure. Pouring water down the drain pipe certainly helped but the sewer gas smell is still there. It is strongest at night and gets worse the day after I use the washing machine and it is now gradually worsening again. I use the washing machine about once a week so the trap should not be drying out. Also note that I do not have a basement in my house. The washer is located on the first floor. There is no floor drain. So my question is, what do you think is going on and how do I fix this embarrassing and frustrating sewer gas problem? Any help would be appreciated. Thanks.