We have a 2 story home with a bathroom that has a shower and toilet upstairs. The upstairs bathroom is not used very much or sporadically at best. Every 3-4 months the water in the toilet will disappear and the whole upstairs will fill with a very foul odor. (The tank still has water in it.) When this occurs even flushing the toilet every few hours doesn't seem to help. This has occurred since we put the house up 18 years ago, back then it only occurred in the spring and fall, now it occurs about 4 times a year. I'd really appreciate any help figuring out why the bowl doesn't hold water itermittently and why it fills the upstairs with the foul odor. Since our daughters have grown and moved out we use the upstairs for quests and return visits.

Thank you!

It sounds like the venting system going to the roof is plugged.

When the water disappears from the bowl, but the tank is full this means a "negative vacuum" has been created. Check the roof first, if you don't correct the problem there then go through the gas traps on the whole line until you hit the basement.