[ibjeff]

OK... I want to start by saying I have done my homework over the past several weeks and several times thought I had this problem licked. I consider myself a handyman but this problem has got me stumped.
We moved into this house in June 2005. In late October 2005 my wife and I noticed a foul septic odor mainly in smallest room in the basement (which is located in the corner of the basement). The left side of the basement is completely under ground where the right side is at ground level. The room that smells is on the far left. The smell comes and goes depending on the usage of the plumbing, most of the time the smell is there or it's not.
Now let me give you the details. We live in a split level home 30 years old at the end of our street downhill. We have a septic system and city water. We have no water or odors in the back yard were the septic is located. The sewer vent pipe on the roof is clear and there is only one pipe. The room in the basement that houses the odor is our computer room with no plumbing or pipes in the ceiling. The basement bathroom is located next to this room but never has odor. The laundry room is next to the bathroom but also has no odor. The laundry room houses the plumbing into the floor. The shower in the basement does not get used very often but I did pour water in the trap. All the sinks and toilets flush fine with no slowness or bubbles. A few of the rooms in the basement are carpeted but there seems to be no floor drains anywere.

[CliffARobinson]

I know how annoying this problem can be, hunting down the reason for this horrible problem. I found a primer on Troubleshooting Sewage Smell in the Basement. I hope this helps you in your quest to solve this mystery.