I was looking through a Mustang site and saw something stating that K&Ns may contribute to unusually fast failure of IACs. Considering I went through...hmm, I was on my third IAC in three years on my '95, which I'd put a K&N on soon after I got it, I'd say that theory has some merit. I have yet to replace the IAC on my '97, which has had paper filters all along.
