sounds like a bad intake manifold runner control actuator to me. happened to me a few months ago. check the cable that runs from the little silver box screwed to the front of your engine to a spring loaded lever under your throtle body. mine snaped so the computer was giving a 1518, secondaries stuck open when in truth they are actually stuck closed. it's worth a look, as it is much cheaper than paying the dealer to remove your intake. do a forum search on IMRC or 1518 for more info as to potential problems.
