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I've had two Mazda's, a 97 323 and 98 626. On both these car's engines there was only a single IAC mounted on top of the throttle body, and no other device for adding air flow on the intake side.

However, on my 99 Nissan Almera with the GA16DE engine there are two IAC type devices mounted on one side of the throttle body, one called the Auxiliary air control and the other called the Fast idle control device:

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A system diagram in the engine control manual shows both these as part of the idle air control system ( pg 12 ):

enter image description here

In addition, there is also have a Fast Idle Cam ( pg 55 ) on the throttle body which seems to directly open the main throttle plate:

enter image description here

Why put so many idle air control devices instead of just one like on the Mazda's I've seen. What is the purpose of each one?

  • 1
    Great questions Robert. I didn't realize engines came with two separate valves, though I've heard of both (didn't put two & two together on this). I'm sure somebody will have an answer, but I bet it just comes down to vehicle specific design. – Pᴀᴜʟsᴛᴇʀ2 Jan 2 '16 at 18:32
  • Thinking about this, I'm wondering if one of the two is there as a preset for when the A/C turns on to idle the car up a little bit. I can imagine this would be a preset amount of air to allow for the higher idle. – Pᴀᴜʟsᴛᴇʀ2 Jan 2 '16 at 20:05
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    @Paulster2 I was thinking about something similar. In my question about the idle problems on this car I noticed that when the IACV-AAC was already maxed out at 100% open, and I turned on the AC, I still saw a jump of about 100rpm which made me think maybe the AC is hooked to the IACV-FICD: mechanics.stackexchange.com/questions/22851/… – Robert S. Barnes Jan 2 '16 at 21:15
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The manual you linked to explains what they're used for.

The AACV is the main idle speed control while the FICD is used to supply additional air to the engine when the AC is turned on.

AACV (Page EC-GA-117)

This system automatically controls engine idle speed to a specified level. Idle speed is controlled through fine adjustment of the amount of air which bypasses the throttle valve via IACV-AAC valve.

FICD (Page EC-GA-122)

When the air conditioner is on, the IACV-FICD solenoid valve supplies additional air to adjust to the increased load.

As for why two IAC's and not one, there could be several reasons for Nissan to set it up like this:

  • a bigger IACV wouldn't provide fine enough control (as Fred Wilson points out)
  • this is a relatively small engine (1.6 L), so the additional load coming from an AC would result in a large percentage change in air flow which the AACV cannot accommodate by itself.
  • parts availability

Regarding the "third" idle air control

As discovered empirically, the FIC mentioned in Page EC-GA-55 of the service manual is the cold-start enrichment mechanism that slightly opens up the throttle plate.

It is not the same as the FICD, which is activated exclusively when the air-conditioning is in use.

  • How did I miss that? Did it also say what the Fast Idle Cam on the throttle body is for? – Robert S. Barnes Jan 3 '16 at 8:13
  • @RobertS.Barnes the cam is used to crack open the throttle plate slightly. The FICD is actuating the cam. I can't find any reference to it in the manual but the diagnostic test on EC-GA-55 appears to suggest that (the last image in your question) – Zaid Jan 3 '16 at 8:24
  • Best thing is for me to take a look under the hood when it's not so darn cold and rainy out. – Robert S. Barnes Jan 3 '16 at 8:57
  • Actually, it seems like there really are three independent devices. The FIC is separate from the FICD it seems. I went out and did a cold start at 19*C and saw that my TPSv was at 0.56v, and slowly dropped off back to 0.46v at 90*C. Once the engine was warmed up, I tried turning on various devices, lights, defrost, cabin fan, rad fan, AC, PS pump and none of these affected the TPSv. So I think the FIC is only used during engine warm up, AC is connected to the FICD, and everything else is connected to the AACV. – Robert S. Barnes Jan 3 '16 at 13:45
  • @RobertS.Barnes in that case the FIC is for cold-start enrichment, the AACV is controlling base level idle and the FICD accommodates the air-conditioning. I take it the TPS is working as it should? – Zaid Jan 3 '16 at 15:05
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The reason to use two is a matter of scale. Fast idle requires a high volume of air but the exact amount is not critical. So a big bore with a cheap slow moving plunger is all that is needed. Idle requires a much smaller amount of air but a much higher degree of control than fast idle. A more expensive fast cycling shutter valve is the most common design.

For years now the OEM's have all settled on using a computer controlled throttle for idle control and many other speed and load controls.

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