What is emunas chachamim ("believing in the wise")? What does it mean? Do we believe everything every book? Can you not believe in a chacham? Is it bad not to? I know that we need to follow a rav but what about the rest? On which issues should we ask?
To follow your rav on halachic issues is obligatory. Regarding other issues, the idea of "emunas chachamim", asking rabbis also about secular issues, has become inflated over the past century, starting with hassidim towards their rebbe, and then in the yeshiva world, they began following suit. The 1st Lubavitcher rebbe (!) even castigates in one his letters "who ever heard of such a thing in all of history, and where do you find this custom in even one book of the scholars of Israel, rishonim or achronim, to ask advice regarding physical/secular topics, what a person should do in his physical/secular life!" In general, as opposed to other religions, Judaism precisely stresses the importance of our "free will", and doesn't intend for us to give that up, and have others decide for us. Similarly, other religions believe in the "infallibility" of their clergy, that they don't make mistakes, but we believe that every one, including the greatest rabbanim, including Moshe Rabbenu and David HaMelech, makes mistakes, "For their is no person in the world who only does good, and doesn't sin". God created the world in a way that each of us must take responsibility and are held responsible for our actions.