North American Turbocoupe Organization

Tuning the IAC
Paulie Offline
Gentleman over the years of working on Fords i have trouble shot many IAC issues. (Idle problems)So i normally do a tune up on the IAC. The IAC is set from the factory with a plunger that regulates bypass air for proper idle speed. But one fault of the early units is at rest it does not close completely and is also spring biased. This can be a problem at times with a modified engine or for that matter a stock engine. So here is how i address it. I remove the electric part and shim the coil back with a select shim of different thickness to get my desire air bypass with the IAC disconnected from the ECM (plug removed). I then shim it to allow the desired bypass need. Usually i use very little bypass from the IAC. By moving the coil back from the plunger it allows the throttle plate to be advanced more and have better idle control. If you do not want to mess with the older unit the black plastic unit on the far right is from a late model ford and is already setup for minimum air bypass. I have been doing this for many years with great success and wanted to pass it on. Along with a restrictor plate that replaces the stock gasket this setup seems to be in my opinion the best way i know how to idle a ford engine. The upper left unit is a completed modified unit that is a spare for my cars. Just wanted to pass on this info. Also the valve must be very clean and cleaned on a regular basis at tune up time. Have a great day gentlemen

Note naturally TPS must be adjusted to app .90-.97 after proper min idle is set

Pete D Offline
Interesting. What range of thickness of shim(s) do you use?
Pete Dunham


Paulie Offline
Peter Sir i use shims to control the bleed rate. This can be anywhere from a few thousandths to a thick sheet of aluminum. You can also trim the pintle down. But the shims work better. I start out with app .070 and work from their. It depends on how much bypass you need past the IAC. Basically its a trial and error situation. Thanks for asking Peter

gmaibach Offline
Senior Member
i have an IAC problem but it really dont bother me... i keep it plugged in when its cold and when it gets warm i unplug it.
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jmthunderbirdturbo Offline
Posting Freak
any reason you cant just thread the holes in the intake and put a manual bleed valve in there, then adjust idle with that? then youd never have to move the idle screw, i.e never move the TPS...?

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Paulie Offline
Yes sir that can be done and its something i tried. But then you still have the IAC that is FINIKIE. Modding the valve the way i do it limits its response only when the car is cold and it takes allot of the function out of the valve. Ford realized this and redesigned the valve to the black plastic unit. Now if you are familiar with the valves that have the little plastic KNOB on them that is what ford did also. They did what you are talking about and made a bleed hole. But the unit with the bleed hole also does not have a buffer pintle and is a good replacement for the early units we have. The problem with our early units is its buffered.(basically it floats between the 2 springs). The mod is just something i came up with as a fine tuning thing to stabilize the idle a bit without buying a new valve. Thanks gentlemen and have a good morning

OU812MACH1 Offline
Senior Member
there is a IAC plate that the 5.0 l stangs use with ''bleed off '' screw to help compensate for a small hunting of idle...the IAC motors carbon up...u can clean them up and re install them.. sometimes u need to drive the vehicle over 35 mph for 2 min for it to reset completely....
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