[olddog]

What is the fuel pressure doing and how much do you have? You should have 39 psi with the vacuum disconnected and it should drop 1 psi for every 2 "Hg of vacuum.

Try disconnecting the PCV valve and plugging the line. See if that makes a difference. The PCV is sucking air in and it is not measured by the mass flow meter as the system comes. That can cause it to run lean at idle.

Does your ECU have a chip in it? The mass meter, engine displacement, and fuel injector size either have to be correctly entered into the chip or the mass meter must be scaled in the correct ratio to injector size verses the original Mustangs 19 lb/hr injector size. As stated give Chris Richards a call. He is fairly knowledgeable.