We have a sewer gas smell coming from the toilet. The smell is only present when the seat is up, hence it must be coming from inside the toilet bowl. There is a vent stack, and when other fixtures run -- the water level inside the toilet does not get sucked down so that leads me to believe venting is not the issue. The only thing I could think of is perhaps the bowl isn't filling with enough water to begin with? Any ideas? The bathroom has a washing machine, two sinks, and a bath-shower. Also, the vent stack is about 6 inches in diameter, possibly too large?
I had that problem too, we found it was the wax gasket under the toilet, the smell travels, we checked everything else and after I fixed it the smell lingered for a few days and went away the toilet was rocking that was the problem. Good luck