What would cause a sewer gas smell in the spare bathroom only when the washer drains?
Without knowing more of your situation, let me speculate: The draining of the washer happens quickly. The draining water causes a vacuum in the line which then sucks the water out of the trap(s) in the other fixtures. This lack of water seal allows sewer gas to enter the house. This could be caused by an improperly installed fixture, one perhaps installed by a homeowner or Mr. FixIt that did not appreciate the necessity for properly venting a fixture. A quick fix might be a Studor Vent. These are not approved in all locations-check your codes.
