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Old 06-13-2018, 09:57 PM   #1
Vincenthdfan
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Default Does the Idle Air Control (IAC) Valve totally close when unplugged?

Quick question here...trying to determine if my brand new Idle Air Control (IAC) valve is defective right out of the box or not.

I know about the unplugging and engine idle dropping to 500 rpm, etc...

BUT, when you have the brand new out of the box IAC in front of you and you blow on one of the ports with your mouth, you shouldn't get airflow past the shuttle valve and out of the other port, correct??

Both of the IAC's I've been swapping back and forth (one original and the other brand new from Auto Zone) will pass air past the shuttle valve from one port to the other port and straight out the opposite open hole.

Normal, or not?

Thanks!
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Old 06-13-2018, 10:44 PM   #2
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I know it’s not exactly the answer you’re looking for, but another member dropped this bit of knowledge on me:

"Take it off the intake and plug its wires in
Turn on the key
Computer will open IAC Valve all the way for starting, key on engine RPMs at 0
So IAC Valve should be open
Unplug the wires, IAC Valve should close all the way
Plug wires back in and it should open all the way
Repeat as you see fit

Volt meter can tell you if the 2 wires have Ground and 12volt key on
The computer Pulses the GROUND wire to vary the voltage at the IAC Valve(open and close) when engine is running, 12volt wire is always 12v with key on"


When I tried the above test, the valve opened and closed as indicated, both on my old stock Motorcraft and new Hitachi IACs. I gather from this that neither were defective. Hope it’s some help to you.
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Old 06-14-2018, 12:49 AM   #3
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I guess what I'm trying to ask here is....

Is there an engineered (and planned for) amount of air flow built into a brand new IAC when its in the closed/de-energized position?

Just wondering if blowing into one is a good pre-installation bench test, or if that is a total waste of time due to an engineered amount of air bleed in closed position?

Has anyone else out there tried blowing into one to see if it flows freely in the closed position?

I can visibly see the shuttle valve is closed, yet air is flowing freely through both of mine...normal indication perhaps?
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Old 06-14-2018, 05:45 AM   #4
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When the engine is warm the computer keeps the iac open around 17-20%. It will say 0% open on a scanner but it is 17-20%.
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Old 06-14-2018, 07:58 AM   #5
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Mine always let air through when they were unplugged.
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Old 06-14-2018, 06:31 PM   #6
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OK,
Just trying to understand the system better so that I can do a better job of troubleshooting.

Thanks all.
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Old 06-15-2018, 11:07 AM   #7
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IAC Valve gets 12volt with key on
The computer controls the Ground to open and close it, this is just safer that running 12volt wires, can't short, but could open IAC all the way if it did, lol.

Anyway, the computer "Learns" how to control the IAC Valve, if you change to a new IAC Valve then you can force computer to relearn it by unhooking the battery for 5 minutes or so, this clears computers "learned" memory, like it does the Radios "pre-sets", same thing, both have KAM(keep alive memory) that has power all the time, unless you unhook the battery

So after unhooking the battery, for any reason, your idle may wander a bit, even with original IAC Valve, until computer learns that 7.5v, for example, sets 1,000rpm when throttle is closed, i.e. TPS under 1 volt

The Ground is Pulsed, which in essence, lowers the voltage, same as if you pulsed 12v side.
But the voltage in a vehicle is not stable..........12v engine off and then close to 15v engine running, then it drops down to 13.5v after a bit, WTF!
So computer uses a Reference voltage, its Power wires, for IACs learned RPMs, it does a voltage difference not a set voltage.
So, for example: 12v - 7.5v = 4.5v
Computer uses 4.5v difference to set 1,000rpm, so doesn't matter what vehicle voltage is(up to a point, lol) it just lowers it 4.5v in this example.
Pretty much all sensors or controls use reference voltage in vehicles
This method is very very precise.

The feedback for computer learning IAC Valves RPM/voltage is, of course the RPM of the engine, so pretty straight forward, but....1994 and earlier computers(EEC-IV) didn't have direct RPM feedback.
1994 and earlier used external self contain spark systems, these sent computer the Pulse for RPM, so it could time fuel injection and also act as Vacuum advance(for TFI and EDIS)

Computer has "target" RPMs programmed into hard memory, my 1994 manual trans idles at 625 warm, only varies by 5rpm either way, so tight control
Temp sensor(not sender) causes computer to increase idle RPMs when engine/coolant is cold, so 1,100rpm on cold startup is normal
As engine/coolant warms up computer will close IAC Valve more until it gets to Target idle for your vehicle.

Computer also opens IAC Valve all the way for startup, so key on = full voltage at IAC Valve.
When you start any fuel injected engine, without touching gas pedal, it should REV to 1,500+ RPM, then drop down to temp controlled idle level
That's because IAC Valve was open all the way

IAC Valve without power will close all the way but that doesn't mean 0 air flow thru that passage in most cases, which is why there is an "anti-diesel" screw on the throttle linkage.
With engine warmed up and idling you unplug the IAC Valve and then set the anti-diesel screw so idle is 500 or so, barely running.
You need to be careful because this adjustment also effects TPS(throttle position sensor) voltage, which needs to be under 1volt when throttle is closed, .69-.99v is spec

Last edited by RonD; 06-15-2018 at 12:37 PM.
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Old 06-15-2018, 11:11 AM   #8
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I just throw comments in there to get you thinking, hope you don't feel offended.

Maybe you have some kind of vacuum leak that only happens when you're moving? that's why I jumped on the brake booster idea because my brakes are great even though I have a loud groan in my booster.
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