it has a girl and dude crawling through a vent or something. Then, just for the point of throwing it it, she farts on his face. It was a weird horror/sci-fi flick
I'm afraid were gonna need a few more details ther chief.
it had relly cheesy effects and b-movie monsters. I think it was an 80's film.
Sounds possibly like the Lamberto Bava crapfest DEMONS from 1986. Were they trying to escape from a movie theater?
I keep thinking it's Demons , as well, but for some reason it just doesn't feel right.
I don't recall any farting in demons, maybe the crapfest the spookies you meant but theres no vent from what I remember.
Probably not it, but how about The Terror Within? Set in the future with ugly mutants invading an underground lab. I think there was a sequencing involving a scene with them crawling through ducts.