Help!!! We moved into our 5-year-old house in July 2001. Last March (2002) we started getting a very bad sewage smell in the house during heavy rains. Subsequently, we looked in our crawlspace and discovered -lots- of standing water. A foundation drainage system and sump pump solved this problem. For a while, it seemed to solve our smell problem as well. Not quite. The problem is back. We get more than a couple of inches of rain, and the smell comes back! It seems to come from the A/C vents running through the crawlspace. The crawlspace itself smells of sewage. No water under the house now, that I can see, though.
The toilets, tubs, and sinks all drain fine, and no dry traps.
Do your vents leading to the roof have any leaks in them. The odors coming from those pipes can contaminate the crawl space or ac duct. A possible solution would be a charcoal filter in the vent system.