Originally Posted by SublimeMach (Post 730531)

on the auto's.. how do i make the hole bigger? and why?

and another thing.. is there any pics of how to install it, which side the small hole goes on? thanks

Originally Posted by FordMan (Post 730643)

I screwed mine to a piece of scrap wood and drilled it. That will keep the plate from spinning when the drill bit breaks through, because we all know that hands don't like holding spinning pieces of metal.

Originally Posted by NC Mach 1 (Post 731015)

Their 5/16". Another way to open them up is with a dremel.

Fordman is right. A sharp drill bit will grab and either slice your fingers or bend the plate. So be careful !!!

Better yet find some short wood screws and screw the plate to a board.

Originally Posted by CompBCurrie (Post 733081)

Could anybody send me a CAD or SolidWorks drawing of one of these plates? my car is in storage now, 2 hours away, but i have access to a shop...might as well make good use of my time. Thanks for the help.

Originally Posted by geoffav (Post 733103)

An IAC gasket is a great template, just make one hole smaller. See above posts for hole sizes to start with. I use a second gasket on mine but it's not required. Good luck.

Originally Posted by Yeller04Mach01 (Post 747597)

My 04 Mach 1 started idling about 1500 RPM and then suddenly droping down to around 800 RPM when ever I stopped. After awhile it started to die when I stopped .

Car is under warranty so I took it to the dealer. They found one of these plates on my car. It was causing the IAC valve to carbon up inside. They replaced the IAC valve, removed the plate and charged me $200. since they said the plate caused the problem. :Flame:

I just wanted to let everyone know this plate can cause the valve to carbon up inside and affect the idle. Something to check on. :27:

Originally Posted by FieroMan (Post 751172)

Thanks for all the good info, Everyone. I will be installing mine on Sunday if all goes well.

I am also installing a JLT RAI, so I wonder if I should install both at the same time or just the RAI. I'm thinking just RAI to make sure all is working correctly, then adding the restrictor plate to the IAC.

I assume I need to disconnect the battery for a half an hour after each install?

I don't know the computer re-programming process (since I have been away from the Mustang scene for over 10 years now), so do I wait a while to see if the computer is learning to deal with the changes or should I assume something is wrong if the car sounds bad after the very first start? (on both the RAI and the restrictor)

Does that make sense? The kids woke me up too **** early this morning...or was it that I stayed up too **** late last night...

Originally Posted by Yeller04Mach01 (Post 747597)

My 04 Mach 1 started idling about 1500 RPM and then suddenly droping down to around 800 RPM when ever I stopped. After awhile it started to die when I stopped .

Car is under warranty so I took it to the dealer. They found one of these plates on my car. It was causing the IAC valve to carbon up inside. They replaced the IAC valve, removed the plate and charged me $200. since they said the plate caused the problem. :Flame:

I just wanted to let everyone know this plate can cause the valve to carbon up inside and affect the idle. Something to check on. :27:

Originally Posted by FordMan (Post 751227)

Install the JLT first, and then the plate. You may have to enlarge the smaller hole in 1/32" stages to get the desired idle drop, and by doing so separately you will know that it's not the RAI having any influence on your idle.

I would disconnect the battery for the RAI install, but not for the IAC plate mod.

Originally Posted by weaver (Post 751437)

Sounds alot like the problems I had when I installed one of these. I tried at least 6 diff. hole sizes and not one acted good enough to keep on the car. I have since removed it and now enjoy being able to not worry about it having a negative effect on the car. IMO these plates aren't worth the trouble.

Originally Posted by Yeller04Mach01 (Post 747597)

My 04 Mach 1 started idling about 1500 RPM and then suddenly droping down to around 800 RPM when ever I stopped. After awhile it started to die when I stopped .

Car is under warranty so I took it to the dealer. They found one of these plates on my car. It was causing the IAC valve to carbon up inside. They replaced the IAC valve, removed the plate and charged me $200. since they said the plate caused the problem. :Flame:

I just wanted to let everyone know this plate can cause the valve to carbon up inside and affect the idle. Something to check on. :27:

Originally Posted by Mach1Canuck (Post 192828)

Difficulty (1-10): 3
Time Required: 1 hour
Equipment: Socket set, .5square feet of 1/32" aluminum sheetmetal, tin snips



This is for anyone that doesn't like when the RPM's that are slow to drop or even rise slightly when you push the clutch in to shift. Another symptom, is that when cruising in a higher gear and you let your foot off the gas, the car almost cruises on its own for a bit.

First, understand the problem. The engine is recieving too much air through the IAC valve. From my knowledge Ford did this for emissions reasons.

As a side effect of this fix, when you downshift, the car burbles a lot more, sounds like a carburated engine. You get a little more engine braking effect.

The FIX:
Most older mustangs have an external IAC valve. Meaning that it is easy to find, follow a small air hose (1/2" ID) to a silver cylinder thing on your intake (the IAC solenoid). Once you found it, the older fix was to buy a 1/2" copper pipe cap, drill out a small hole, insert this into the line the restrict air flow through the IAC, and your done.
The Mach has a slightly different setup.
We have an INTERNAL IAC air supply. Meaning that there is no hose leading to it.

To find your IAC valve, first remove your entire shaker assembly from your car.

Next, look between your intake and the firewall, there should be a silver cylinder shaped object with 2 bolts leading to your intake.

Remove the two bolts and move this assembly.

You should see the following, the IAC assembly has two "ports" with the solenoid controlling the air flow between these two ports, what we want to accomplish is to restrict the airflow leading through this.

To do this, there should be a small gasket on your intake, or it could be stuck to the IAC valve.

Remove this (carefully)

Trace out the shape of this gasket as well as the holes for mounting bolts, and the two airholes, onto a piece of 1/32" aluminum or something similar.

Cut out the outline of this "restrictor plate" from the aluminum.

Cut out the holes for your bolts to go through.

Cut out a single one of the Air holes.
Now the tricky part. You have to decide how big to make the other hole because this determines the amount of air you are restricting.

The smallest you should go would be around 5/32", the worst case if you go to small is your car will stall when fix is complete, if this happens just take the plate out, make the hole bigger, and check that.
The hole I used is around 9/32" which is about the medium you would want to go.

After your "restricter plate" is cut out, mount it with the small hole on the passenger side of the IAC, and make sure to replace the gasket against the intake.
Start your car, you should notice the RPMs drop when you start back to normal very quickly.
If it idles smoothly, try putting turning on the AC, this will load the engine down.
If your car stalls out, remove the plate, make the hole bigger, try again.
If still idling smoothly, bolt your shaker back on, take it for a spin. You might need to adjust your driving/clutching style after being used to the crappy RPM hang, so don't be discouraged.
Also, you might want to disconnect your battery during the procedure to make your computer relearn it tweaks after you finish.

If, after you start the car you notice a "whistling" sound, then take a look at your plate, make sure the BIG hole is at least as big as the hole on the IAC valve. ALso make sure edges of holes are rounded (sharp edges are bad), and the holes are round. This happened on mine, and I fixed it.


Any questions, please post here.



Please don't yell at me if you break your car, you accept that you are trying this at your own risk. It works on mine, but your mileage may vary.

Originally Posted by STREET BULLY (Post 810261)

So how and what do you
use to make the :what: hole bigger?

Originally Posted by G (Post 751215)

The IAC can and will carbon up and have OTHER issues REGARDLESS of whether an IAC restrictor plate is installed. When my GT was 100% stock but a year old it had a bad one. My 03 Cobra had one that was less than smooth so I had to clean it a couple of times. If this was an item that rarely screwed up I might agree with the dealer --- I think they used the plate as an excuse.

Glad you got it fixed.

Originally Posted by bris09 (Post 816715)

I spent the last week tweaking the voltage and stop on the Accufab just to find out today that the less than two month old IAC is sticking. They're junk.:Flame: I'm going to make a plate and try it before I buy the third IAC. Yes, the car is still under warranty but it is not worth fighting with the dealer. They refused to replace the MAF sensor because of the K&N filter.

Originally Posted by G (Post 816729)

Yes they do suck. I have a spare that I cleaned and stuck in a baggy. I wish there was a "quality" replacement for them. Some dealers are a PITA.

Hope you get it sorted out. :D