I have a 2000 gt with a mac o/r h pipe and 40 series flowmasters.(best set up ever!), but the car will not idle right. I took it to a garage and they said their diagnostic showed that the IAC was messed up. I put another one off of another gt that worked perfectly but didnt solve the problem. When im driving at lower rpms you can hear a ticking from under the car and i think it is an exhaust leak. If it is at the connection to the manifold, that would throw off of the air/fuel ratio since it is before the first set of o2 sensors, wouldnt it? I have steeda mils.

 

ahhh, let me answer the question for you. Most likely you have a warped header, which will give you that ticking sound.

 

Ok, let me give you my experience on the tap i had on my car. Before I installed my exhaust, I had a slight tap driver side fender well. when i installed it got louder( noticable to other people). It happen to be the head it self. There was a TSB out for it. I think it was only for 2002 to 2004 not sure. I thought mine was a exhaust leak too.