The Walking Dead (2010)

Answer: Walkers are attracted to noise, they can tell who's dead and who isn't dead, in S1 when Rick and Glenn cover themselves in walkers' guts they're able to get far by walking through the herd of walkers because they smell like they're dead, but as soon as the guts get washed off by the rain the walkers immediately attack them, if you draw attention to yourself they will attack you which we've seen that many times throughout the series that it's not always successful. Whether you're covered in guts hiding under dead corpses or under a car.

Answer: Walkers use sight and sound to find their prey, with sound being their first choice. When the characters cover themselves in guts, they aren't attacking because they can't "see" them and they're not making any noise. The same goes for when they're hiding under cars - walkers can't hear or see them. They'll go for anything that makes a noise, regardless of what it is. It's also very likely that walkers don't have the ability to smell, since they can't find people hiding under cars. Nor can they smell the Whispers who walk amongst them.