Well, the first thing that's wrong is that the word "car" is too short for the search function. It will only work on words four characters long or longer.
The second thing that's wrong is that cars are kinda the focus of the site. There's eight million posts and most of them will be car-related. The term "cars" would thus be too common for the search function.
Try to refine what you're looking for. Put up in this thread what it is you're actually trying to find and people will tell you what you should put in to maximise your chances of finding it. Knowing how to use any search engine - even Google - is the best way to get the most out of it.