If you were able to blow air into one side of the IAB and get air out the other side, with no power connected to it, I'd say it's bad. The stepper motor on the end of it opens the valve by the EEC giving it a certain 'duty cycle'. If it's stuck open, it's like a huge vacuum leak into the engine. I'd replace it, check for any other vacuum leaks, and reset the EEC (Click
here to see how to reset idle correctly). The check engine light usually comes on when an on-demand code is set, like if there's a hard fault. I pull my codes for the hell of it every couple weeks. Like said above, check the EGR valve, too. There should be no vacuum (or barely detectable) at the EGR valve at idle. This is why it's pointless to disable EGR: it only opens (controlled by computer again) at cruising speeds. Never at idle, never at WOT, and never while cold. Oh well, hope I gave you some ideas. Good luck.
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Capri306, Moderator, The Mustang Works Online
1979 Mercury Capri, 5.0L -- C4 -- 2.73
1987 Mustang LX Notch 