IAC Testing Procedures

Posted by Chris Myer 09/14/18 4 Comment(s) Sniper and Terminator X,

This article was originally written to describe how to test the IAC functionality the Sniper EFI System. However, most of the information is directly applicable to Terminator X so we invite those owners to use this as well.

It is not uncommon to question whether your Idle Air Control Motor (or IAC) on your EFI System is fully operational.  At times you might find that your idle is not behaving as you expect--too high, too low, or erratic.  The purpose of this instruction is not to provide a complete guide to troubleshooting idle conditions.  There is another instruction that is dedicated to Troubleshooting High Idle Conditions in the Sniper EFI System.  The purpose of this instruction is to help you confirm that your IAC is working correctly.  This instruction is aimed primarily at the Sniper EFI System but you might find that you can adapt these concepts to any aftermarket EFI System.

How Your IAC Works

Image Of Idle Air Control Motor Showing Key Components and MotionLet's start by giving a simple introduction to how the IAC works.  It is essentially a very simple linear actuator that moves a shaft in and out based on a series of pulses to a pair of coils.  One coil (energized by one pair of wires) moves the shaft in.  Another coil (energized by a second pair of wires) moves the shaft out.  On the end of this shaft is an angled plunger.  When fully extended, it completely blocks air from passing through the IAC passage and into the intake manifold.  As the plunger is withdrawn, it moves further and further away from this passage and allows more and more air through.  By controlling the airflow through the IAC passage this allows the ECU to control the RPM of the engine at idle.

It is just as important to know what the IAC does not do.  It does not provide any feedback to the ECU on its position.  It does not reset itself to a known position when the Sniper EFI System is powered down.  Therefore, for the ECU to know exactly where the plunger is located relative to the IAC passage it must issue a series of pulses to completely close the IAC (the "zero" IAC position) before it can then use that as a reference from which it positions the plunger as desired.

What could go wrong with an IAC?  Three things come to mind.  First, In high-mileage vehicles the shaft can become jammed up due to a build-up of oil and debris.  This can often be cleared with a blast of aerosol carb cleaner.  Second, In addition to wiring or other external connectivity issues, the coils themselves can fail.  That would require that the IAC be replaced. If you happen to know the pin-out for your IAC then you can confirm this by testing the continuity across the two coils.  Finally, if electromagnetic interference (EMI) is able to induce enough voltage on the control wires it could actually move the IAC shaft without the ECU being involved, which would cause the IAC to become "lost".  At this point it would not work correctly again until the IAC was re-zero'd by the ECU.  See my instruction on How to Troubleshoot RFI/EMI in Sniper EFI Systems.

Best Practices

It is a good idea to get in the habit of listening to your Sniper EFI System. Yes, it will talk to you--but not with words.  When you key on, you will hear three distinct sounds:  The ECU initializing the IAC, the fuel pump priming, and the injectors firing their prime shot.  If you get into the practice of hearing all three of these sounds you will know if you have a problem even before you turn the key to crank.

The ECU initiating the IAC is the first sound you hear when  you key the system on.  It is virtually simultaneous with the fuel pump prime.  It is not very loud and may, in fact, be masked by the fuel pump noise.  The ECU extends the IAC plunger to its limit, and assumes it is fully seated in the IAC passage and then takes it back out to the park position (normally around 50-80%, depending on the temperature of the engine.)  It is difficult to describe the sound but it sort of sounds like a plastic hair comb and pulled it across a hard edge.  Those are the pulses that send the shaft out to the zero position and then back to the parked position for that temperature.

Testing Procedures

If you hear the sound I describe then you should also confirm that the shaft is actually moving.  You can do that by looking down into the IAC opening with a flashlight while having someone key the system on.  You should see the plunger come into view (as it closes the IAC airway) and then back partially or entirely out of view again.  If you see that happening then the IAC is probably working.  But on the Sniper EFI System you can actually test this a little bit more precisely using the handheld touchscreen controller and these steps:

Note:  This process will involve you changing the IAC parked position for testing purposes.  You should make a note of the position before you change it so you can return it to the current configuration or, alternatively, save a copy of your configuration so that you can upload it back after you complete the test.

  1. With the engine off, remove the air cleaner.  Power on the system but do not start the engine.
  2. Navigate to Home > Tuning > Advanced > Advanced Idle > IAC Startup and click on "IAC Parked".  Here you will see a graph of IAC park positions for various coolant temperatures.  The purple vertical line shows the current coolant temperature reading.

    Typical IAC Parked Position Curve

    In our example, above, the coolant temperature sensor finds the engine at 88 degrees. The IAC parked position at 88 degrees is approximately 80% (plunger is pulled 80% of the way out of the IAC passage.)

  3. Go to the handle (little square box) for the temperature immediately to the left of the current CTS reading and move it all the way down to zero.  Then do the same thing for the handle immediately to the right of the current CTS reading.  In our example, since the CTS says the engine is 88 degrees, the IAC Parked settings for the 80-degrees and 100-degree should be pulled down to zero.

    IAC Parked Position Reset To Zero at Current Engine Temperature

  4. At this point, If you click "Save" you should hear the IAC respond, and when you look down into the IAC opening you should see the end of the plunger protruding about 3/8-inch into the IAC opening, fully blocking the air flow.  If you are so inclined, you can perform mouth-to-IAC resuscitation and no air should be able to pass.  I'm mostly kidding, but if you try this make sure nobody standing nearby has a camera and a social media account.

    IAC Passage with IAC Plunger Fully Inserted (Zeroed)

    This image shows the IAC passage (triangular opening between throttle barrels) with plunger fully inserted (set at zero.)

  5. You can move the IAC setting for the bordering temperatures up and down and watch the IAC plunger respond each time you hit "Save".  Note that you are REALLY going to wish you had that stylus that came with your Sniper to adjust these handles--they are almost impossible to do accurately with your finger.  The end of the plunger is flush with opening into the IAC passge when the IAC is somewhere around 75% on the unit I used for this article, but that's not to say that your IAC is going to be exactly the same.  (The Sniper in the image is a bit older and Holley is prone to change components from time to time.)
  6. Don't forget to reset your IAC park position back to where they were before you started the test.


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Join the conversation by commenting below or click here to read more articles from Chris about the Sniper EFI System!

4 Comment(s)

Don Siegler:
09/15/18, 06:54:47 AM

Thank you Chris love it

Daniel Smith:
10/25/18, 10:03:19 AM

WOW! great information. Wish I had discovered this site before I purchased my unit. Please keep up the information.

Chris Myer:
11/5/18, 06:57:31 PM


09/29/20, 08:11:26 AM

I wished I had known about EFI pro when I started my project 3 yrs ago Chris you put out good information/visuals....will definitely be making my purchases with you from here out......You are very helpful I can only imagine if had purchased my sniper unit from you in the beginning

Chris Myer:
10/8/20, 04:33:32 PM

Thank you for your kind words

Dan Long:
05/6/21, 04:30:40 PM

Is it possible that a weak car battery (hold charge but might dip below 10V on start up) would cause IAC issues? I think my IAC is working correctly, but is my voltage drops too much on start up the ECU resets momentarily and may think the IAC is at a different position. My IAC appears to move pre-start, but it is still pulling air when the readout says it is at 0%.

Chris Myer:
05/11/21, 06:39:16 PM

I think it is entirely possible that you are somehow throwing your IAC for a loop but the question becomes moot once you solve what is either a weak battery issue or RFI resetting the ECU. You can address the IAC after you solve that much more pressing question. Remember to never try to start or even power up your Sniper or Terminator X-equipped vehicle with a charger connected.

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