Thread: The ins and outs of the stock idle control system.

This thread should answer questions about what each idle control mechanism is called, what each mechanism does, how it does it, and how they are connected to the airways in relation to each other.

There are four main mechanisms, located in two different assemblies. They are confusingly labeled and often called the wrong thing by pretty much everyone, including me. You could go to the FSM to get all this info, but I'll try to spell it out correctly for once.



Assembly #1 - IACV/AAC (Idle Air Control Valve/Auxiliary Air Control):


Mechanism #1) IACV - Idle Screw. This is the first part of the general idle control assembly. The screw controls a small, independent air passageway that provides idle air at all times to the engine. Screwing it "out" provides more air, and is analogous to slightly cracking open the throttle plate some. Your engine doesn't stall or almost stall when coming to a stop because of this control.

Mechanism #2) IACV/ACC (Idle Air Control Valve/No Clue What ACC Stands For). This is the second part of the general idle control assembly. This is a PWM controlled valve (~160 Hz) that the ECU uses as the main idle control valve. It also provides the finest control of the idle air. Your engine holds a perfectly steady idle mainly because of this control.

Mechanism #3) IACV/FICD (Idle Air Control Valve/Fast Idle Control Device). This is the third part of the general idle control assembly. This valve is an on/off valve used by the ECU to raise the idle when the A/C is on and/or when the power steering pressure is high (during full lock).




Assembly #2 - AICV (Auxiliary Idle Control Valve):


Mechanism #4) IACV - Air Regulator also known as AICV (Auxiliary Idle Control Valve) or Cold Idle Valve. It is open when the valve is cold, and slowly closes as the valve warms up from the incoming air and from an internal electric heater (+5v from the ECU). It should close completely when warm. It accomplishes this with a bi-metal strip. This is a stand-alone assembly. It can be found by the firewall below or behind the intake manifold.
More: http://www.sr20-forum.com/600784-post18.html


This is very important and seems to be lost on a few people. All of these idle controls are in "parallel" and none of them are in series. None of them control the amount of air going to another control. They are all independent. It may not look like it when you first view the air passageways, but they are all independent.

*simplified idle air passageways diagram goes here*

This thread is a technical thread about how the idle system works.

This is not a thread for troubleshooting your current idle problems.

That thread is here: http://www.sr20-forum.com/general-maintenance/38-how-bad-idle-troubleshooting.html

The Valve you have labelled as "ACC" is actually "AAC" as you rightly described it in the green heading, it is called the "Auxiliary Air Control" . Listed in consult as the AAC. Bit of photoshop on the pic and you'll be set hehe.

Very nice.

From what I can gather from a couple different FSM sources is that the entire assembly is called the "IACV - AAC".
But the mechanism itself is possibly called the ACC or maybe the AAC. It's called either on different Nissan models. I think it might be a case of a typo in a bunch of the FSMs but I'm not ready to call it just yet.

For me the jury is still out.

If it is listed in the consult as AAC maybe we can lean toward that, but I've found too much info to the contrary to be entirely convinced. The entire assembly I know for sure is called the "IACV/AAC" assembly. It would be weird if they had a component of it called the "IACV - AAC" wouldn't you think?

Doing an active test in consult on the AAC allows you to raise and lower the idle in minute degrees depending on its percentage which matches with your description:

"It also provides the finest control of the idle air"

I even say "you need to clean the AAC" and when i say that i am implying removal of the whole idle control valve bit from the intake manifold and cleaning it.

As you say it could be a typo in the FSM and that's possible too. I guess the main thing is you've outlined the function of each part clearly with pictures.

Thanks for the info! Hands down the best idle system writeup I've found. Can we expand on the Auxiliary Idle Control Valve a bit?

I am interested in removing the Auxiliary Idle Control Valve, mainly because the hoses going to it are hard and rotted out. I can replace them...but removing the valve would also clean up the manifold a lot (96 US low port). I'm not sure if I will do this yet, because I am unclear on how much this valve adds to the idle. I have a Calum DB with TunerCode, so adjusting tuning (such as target idle speeds) and disabling the valve from diagnostics is easy.

My understanding is that this valve is open when cold, then closes as temp rises, never to be opened again until cold. Is this correct? Cold is a relative term, so a 'cold' start here in Phoenix, AZ is not the same as a 'cold' start in northern Michigan. Logic tells me that as long as my startups are not in sub-zero temps the IACV/ACC and idle timing adjust should be able to reach the target idle speeds. Once the engine is up to temp, everything operates like normal since the valve would be closed anyway.

Has anyone ever removed this valve?

Without getting into minutia, you've got the basics down, yes.

Plenty of people remove it and results vary. Typically the results are acceptable or better in warm climates.

hey guys this is a big help, it may b an old post but does anyone have a picture on which wire the cold start wire is on the ecu as i dont think its hooked up, do i just connect to a 12v source when i find it? thanks

If I had my paper work in front of me I'd tell you what ECU pin it is, but yes essentially it gets a switched +12v source and a ground to run the internal heater. If that is not hooked up, then your idle can be high when cold for longer than normal.
If you have trouble starting while cold, that thing not being powered is not your problem.