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Message: <blockquote data-quote="MustangChris" data-source="post: 1270995" data-attributes="member: 10830">Hi Adam!To step in before any "weird tech" pops up: #1)  The 99-04 coils are prone to failure.  In fact, 4v-COP guys (99-04 Cobras/Mach 1s) regularly DOWNGRADE to the traditional coil packs for reliability purposes (usually seen above 1,000 RWHP, but common on set ups 800+).  Your SCT should be able to tell you what cylinder is missing.  If so, move the COP to a new cylinder and see if there's a change.#2) you can not "service" an IAC.  You can only replace it.  people claim to clean them with brake-cleaner, and such... and that's just fine and dandy for them... but in reality they are ONLY replaceable units, NOT serviceable. (i know you said that you just replaced the gasket, but I just dont want another member thinking this is an open door to giving you advice to clean it. lol)#3) the intake manifolds are not usually prone to cracking (unless they are being hit with meth or n2o) -- did the previous owner work on the car at all?  TQing the manifold down inproperly may have torqued in a funny way leading to the crack (just shooting in the dark.)  If you your working on a red-necked rigged car, we could be looking at something else all together...If you have a buddy with a mustang thats running properly, my first suggestion is to swap their IAC onto your car.  They are pricey little bastards, but getting a working one is the only way to properly diagnose it.  I think this is your problem, but man, I'd hate to see ya throw $60.00 at an IAC for it not to be... lol.a few other options: dirty MAF, leaking gasket in the intake (either at the IAT location, or a coupler not tightened, etc.)If i really thoguht it was a bad coil, it'd probably give you more problems than just at idle, just my $0.02.</blockquote>