One is a poop smell, which sometimes gets so bad that other people can smell it, and less common
is a gas fume smell, which you mention. The poop smell
is caused by an anaerobic bacteria, and it can be removed with a saline flush (I've done it myself). I don't have enough feedback on the gas smell, but it seems to have a similar source, reference your other sinus problems. You may possibly clear it out with a Neilmed Sinus Rinse, but that will only reach the lower sinuses.