Well, just to clear something up, the use of "idler" as a modifier in the phrase "idler pulley" does not make the word an adjective. Nouns can be used to add information to another noun, just like an adjective, but they're just modifying nouns. A modifier doesn't have to be an adjective.
For example, if I can think of any good ones:
family reunion
company bank account
angler fish
Unless, for some weird reason, you're trying to convey that one pulley is more idle than another. Then it's an adjective. "My pulley is idler than yours." Man, I didn't even know that was a word. I always thought it was "more idle".
"You're the idlest bunch of bums I've ever laid eyes on!"
So I guess what I just said is that "idler" can be a noun or an adjective, but it can be a modifier without necessarily being an adjective, if that makes any sense.
Anyway, I'm confused now as to whether "idler pulley" is a valid term. Dictionary.com has stuff about an "idle pulley", but it says that "idler" is a shorter term for one of those.