How To: IAC Rev Mod (Fixing the hanging RPMs) -

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Old 01-28-2007, 09:07 PM   #1
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Default How To: IAC Rev Mod (Fixing the hanging RPMs)

DISCLAIMER: Myself ([email protected]) and will not be held responsible for damage to your car, engine or components if you decide to attempt this modification using this guide.
This is a super simple modification that you can do to your 99-04 Mustang GT 4.6 2v. With this mod you will create a new IAC gasket to fix the annoying RPM "hang" that stock GTs have. Have you ever reved up your engine and had it take forever to get back down to idle? This is what this mod will address and fix.

RPMs will return back down to idle faster. Makes for cleaner sounding revs without the annoying "hang".
- Engine braking effect will be present. Meaning; when you let off the gas, the car will actually start to slow itself back down instead of coasting.
- Makes the exhaust have the "gargle and pop" sound like an old hot rod when its on the down scale of a rev.
- A "hissing" sound may be heard from under your hood when your standing near it. Sounds like a large vaccum leak. I believe this occurs because of the large amount of air restricted to the IAC by this mod.
- The vehicle may not be able to pass certain local emmisions tests. The reason our cars hang the RPMs in the first place is because of some emmisions crap Ford did. (Just keep your old gasket if you have to smog)
- You may have to re-learn the car a bit as far as shifting (manual trans. cars). The RPMs will fall faster and require you to shift a little sooner then normal.
- If you drill the hole too small, the car will not idle correctly. (Just make the hole bigger!)

As you can see, the Pros far outway the Cons, so go get your materials and tools and get to work!
- Gasket Material: You can get this at any parts store like NAPA, Auto Zone...etc... It comes in a sheet, just tell 'em you need about 12' x 12' of it. Or, you can even use tin or thin rubber, I used gasket material to insure no air leaks.
- Socket Wrench (A socket extension is recomended)
- 8mm socket
- Ball point pen
- Drill
- 3/16' drill bit
(or even a 3/16' punch may work if no drill or bit is available)
- A cardboard box
- Common household scissors

-1. Remove the PCV hose so you can access the lower IAC bolt, and remove the (2) 8mm bolts holding the IAC to the upper plenum. Remove the gasket. IMPORTANT: Keep the gasket somewhere incase you ever want to reverse this mod and/or you have to get a smog check!

-2. Place the stock IAC gasket on the sheet of gasket material, and use a ball point pen to trace the stock gasket onto the sheet of new gasket material. Make sure to trace out the bolt holes on the sides of the gasket as well. Mark an "X" in the center of one of the large holes.

-3. Using common household scissors, carefully cut out the new gasket following the lines you traced.

-4. Set the new cut-out gasket on top of a cardboard box. This will make it easier to drill (or punch) your bolt holes and center hole through the gasket without it ripping in your hands. Drill out your bolt holes using a 3/16' drill bit, you may need to make a circualr motion with the drill in which it will "round" out the holes a little bit more. Test the sise of the hole by trying to push one of your 8mm bolts through the hole. If it doesn't go through, round out the hole a little more until it goes through with a snug and tight fit.

-5. Using the 3/16' bit (or punch), drill a hole straight through the little "X" you made in the center of one of the large holes. The size of this hole will determine how hard your revs fall or if your car will stay running at all and is the whole purpose of the mod. I've heard of people using 5/32' bits for the IAC hole, I tried a 5/32' bit and my car would not hold an idle and would die as soon as I let my foot off the gas, so I stepped up to 3/16' and all was well. If this happends for you with the 3/16' bit, then keep stepping up the size until your car runs properly and holds a nice idle.

-6. Now put the IAC back on with the new gasket (make sure the hole you drilled is on the side of the large hole in the plenum). Re-connect the PCV hose. IMPORTANT: Do not over-tighten the 8mm bolts back into the plenum!

-7. Fire up the car and check the idle. Make sure it holds an idle around 750-950 RPM or so, basically checking if it is idleing ok. Wait for oil pressure to build...etc... and give it a nice rev. Check to see if the car revs up clean and falls back down to idle with no problems. I've heard a few people talk about how this may affect a cold start, but I did this mod out in 18' F weather and the car fired right up. It won't go into a high idle, but it will idle never the less. ENJOY!!!!

Thanks to and the members of for the ideas and inspirations for this write-up.

P.S.: Disposable cameras FTL!!!
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Old 01-28-2007, 09:18 PM   #2
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Default RE: How To: IAC Rev Mod (Fixing the hanging RPMs)

This Post FTW. Try It guys this man has put a hell of a lot of time into this! hes been at it all day just for the guys at MF! Thanks Nate!
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Old 01-28-2007, 09:20 PM   #3
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Default RE: How To: IAC Rev Mod (Fixing the hanging RPMs)


Thanks alot. Hey, will this affect my A/F ratio at idle? Will it hurt anything on a high compression engine? My car hangs BAD. ALOT worse that a bolt on car.
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Old 01-28-2007, 09:23 PM   #4
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Default RE: How To: IAC Rev Mod (Fixing the hanging RPMs)

or you could just buy the $1 thing on ebay that fixxes the IAC and save yourself the trouble... but not the less good write up. time for a sticky and a lock
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Old 01-28-2007, 09:24 PM   #5
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Default RE: How To: IAC Rev Mod (Fixing the hanging RPMs)

I need to do this someday when I have some spare time I guess. Thanks for the write up.
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Old 08-23-2008, 09:18 PM   #6
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Is this only for a 99-04?
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Old 08-23-2008, 10:19 PM   #7
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I am curious about the A/F at idle too.
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Old 09-29-2008, 02:09 AM   #8
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^me2, creating gargling/mini backfiring is an excessive amt of unburnt fuel being burnt near or infront of the cats, not good for your motor and can eventually lead to blowing rings and frying valves, if you have a piggyback ecu or plug n play that you can map on your computer for fine tuning then this would seem reasonable, anyone feel the same? just my opinion, this is a super annoying characteristic in the mustangs, however.
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Old 09-29-2008, 04:43 AM   #9
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Hmm... My mach gargles quite a bit but I do feel like the revs could drop quicker. I'll have to see if the previous owner ever did this.
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Old 11-19-2008, 01:50 AM   #10
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I believe the AFR at Idle is around ~14.7:1, It would take loosing alot of air to cause a supremely rich condition enough for the gas to ignite in the headers.

You can always change it with a tuner however
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