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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.
Let'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 As the plunger is withdrawn, it moves further and further away from this passage and allows more and more air through.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.
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.
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.
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.)
This image shows the IAC passage (triangular opening between throttle barrels) with plunger fully inserted (set at zero.)
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