Whenever we run our washing machine, there is a strong smell of sewage in the house. The washer and toilet outlet separately. However, the vent for the toilet is connected to the vent for the washer.

Do you hear any gurgling when the toilet flushes or the washer discharges? If you have a basement, check the floor drain. The water seal in the trap evaporates over time if not used and the discharge from the washer forces sewer gas out of the drain. Any fixture that drains and is not used regularly is suppect. If you smell sewer gas you have a opening into your sewer. Sewer gas is dangerous to your health and the methane in it is explosive. Look into this immediately. If you can't locate the problem click on back with all the information you have and we'll go into it in depth. Regards, Tom

Thanks for the quick reply. There is no gurgling. Our plumbing is in a crawl space. I went down there and ran the washer. There is no drain water leaking anywhere. The traps are all working. I checked the vent pipe for clogs, but it was clear. We have copper pipe connected to old cast iron for a drain as it goes underground in the crawl space. Today I smelled the odor and we had not been running any water. The crawl space has a small opening into the basement and that is where the smell is the worst. Could there be a leak at the junction of the copper pipe and cast iron? How would I test for that? Also, do atmospheric conditions (e.g. temperature or wind) effect the operation of the vent. Our vent is on the northeast side of the house with the prevailing wind coming from the south and west.

There has to be a opening somewhere for sewer gas to escape. However, if there was a leak in the line some where and raw sewage spilled out it would produce a sewer gas smell.
Where the smell's the strongest is the place to start looking. If you find such a leak it must be repaired and lime spread on the spill.
Wind blowing over a vent produces a vacume of sorts in the system. How that can be checked is to watch the level in your toilet bowl. If you see it fluctuate then you know you have a breeze blowing. But I've never heard of it sucking water out of a trap.
Your best bet is to track it down using the "sniff test". Let me know what you find. Regards, Tom