Inexpensive Adjustable IAC

[koda2000]

So the IAC valve on my 2001 Expl 5.0L started howling at idle about month ago. I removed it and cleaned it, but it didn't help. I had replaced the IAC valve on my 2000 XLT 5.0L (the spare truck) about 5 years ago when it started humming with $22 Chinese valve off eBay. It worked fine during the time I had it. This month I had a lot of unexpected expenses and I had some Amazon points to burn, so I bought another cheap $24.52 IAC valve off Amazon, which cost me $10 out of pocket.

After installing the new valve the howling was gone, but the idle was too high, hovering around 900-1000 RPM once the engine warmed. Today I reinstalled my old valve and was going to return the new valve for a refund. After removing the new valve I noticed there was a black plastic square on the back of it (if you look closely at the below image you can just make it out to the right of the electrical socket). I wondered if this was an adjustment screw. I took an appropriately sized screwdriver and tried turning the square in and out and it turned. I reinstalled the new valve, started the engine and waited until the engine warmed up. Then I turned the screw in 1/2 turn and sure enough my idle dropped from around 1000 RPM to around 800 RPM. I turned it in another 1/2 turn and the idle dropped from 800 RPM to around 650 RPM (perfect).

I've never seen an adjustment screw on an IAC so I'm happy this brand had an adjustment. It would have been nice to know this before I pulled it off and was about to return the part, but there was no installation literature included in the box it came in. I imagine turning the screw in/out moves the seat for the pindle rod allowing more or less air to get sucked in. All-in-all I'm now very happy with my $10 purchase.

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[J_C]

Interesting. Hopefully that screw has precisely cut threads and stays where you adjust it. I got a generic chinese 2 cycle carb a while back and the idle screw threads were so bad that the screw wobbles around while the engine runs.

 

[koda2000]

Interesting. Hopefully that screw has precisely cut threads and stays where you adjust it. I got a generic chinese 2 cycle carb a while back and the idle screw threads where so bad that the screw wobbles around while the engine runs.

I think it will be okay. The screw did not turn too easily, there was enough resistance to hold it in place. I suppose the manufacturer was supposed to use the screw to set the IAC to an OE spec, but obviously this one was not adjusted properly.

 

[1998Exp]

Hmm... This is a closed loop system: the IAC does not "know" the engine rpm, and the speed is controlled by the PCM. Actually, there is no one idle speed, and the PCM sets it, depending on a bunch of conditions. Seems to me that the screw you mentioned is a limit stop of some sort - something akin to the screw on a carburetor, and not what you wanted. If the engine idles OK with another IAC, I'd just return this thing, because it's faulty and does not respond to the PCM input properly.

 

[koda2000]

Well here's what I think. I agree that the PCM trys to set the idle speed. It does so by sending a signal to the IAC. The IAC responds to the signal by moving a needle/pindle in/out which then allows more or less air to get sucked in through the little cap/filter on top of the IAC. The amount of air coming in causes the engine speed at idle to increase or decrease (a bit like a vacuum leak in the case of an increase). The PCM can only send what it thinks is the appropriate signal to the IAC (in the form of voltage) and it assumes the IAC will respond appropriately. I've seen that in the case of a vacuum leak when the idle speed is too high the PCM may throw a code saying the that IAC is not responding appropriately and needs to be replaced. IDK what the max idle speed is before a DTC will be set but I think it's around 1200 RPM. I saw this happen on my 5.0L XLT multiple times (even after swapping the IAC with one that worked correctly on another vehicle). Idle speed can also be effected by turning the throttle plate stop screw slightly and the PCM does nothing to change the new idle speed afterwards. This leads me to think that the PCM does not monitor engine RPM at idle speed beyond varying the signal voltage to the IAC and setting a DTC if the RPM is too high beyond a certain RPM.

I realize my thoughts on this subject are not based in science, but they are based on what I've seen happen on multiple Gen II Explorers. I've increase the idle speed slightly on multiple SOHC 4.0L's by turning the throttle place screw slightly with no ill effect and the PCM did not try to turn the idle speed back down afterwards. Note that decreasing the idle speed using the throttle plate screw should not be attempted as the throttle plate can stick in the closed position.

@donalds - I don't think the adjustment on the IAC should be done at cold start, unless you know what the RPM should be at cold start (I've never noticed and with temps in the mid-high 90's the cold start RPM idle doesn't stay up for very long). I know it's higher than it is once the engine begins to warm, but I don't know what signals the PCM to increase the idle. I suspect It might be the O2 sensors or the ECT sensor. At any rate, if I experience a problem with is once the outside temps are lower I can easily fiddle with the adjustment screw again. For $10 I don't feel like going through the hassle of returning the IAC and ordering another one if I don't have to.

Edit: I just went outside and cold started our 5.0L Mountaineer (temps round 70). The idle was around 1100 RPM. I then cold started my 5.0L Explorer and the idle was around 1100, so that leads me to believe both IAC's are now in sync.

 

[J_C]

I assumed there is only so much adjustment the engine can make, that it sees a lean condition and increases fuel trim and RPM goes up.

The IAC isn't really a stepper motor because that requires multiple-positional control, more than two wires. Instead it does have to rapidly modulate the binary on/off position, perhaps so quickly that it can almost do it in steps but not that precisely.

I figured the vent on them was to equalize air pressure at the plunger so it can move a little faster, versus needing to use a larger coil or more current.

 

[410Fortune]

The mustang guys used to make a small spacer you install between intake and IAC with a little set screw in it allowing you to adjust the amount of airflow that bypasses the IAC when closed. I have used those before to get a proper idle on the pushrod 4.0 and 5.0

Also I have had to drill a tiny hole in the butterfly on a 4.0 OHV engine to get enough air to bypass the TB and IAC to get a good idle (650 rpm)

It seems that when we change intakes, port and polish, add headers, etc etc we can mess up the idle and no amount of idle set screw adjustments and cleaning or new IAC do not do the trick, there are still ways to get a good base idle.

 

[koda2000]

I'm not trying to arguing with you @donalds, or anyone else (and I don't believe everything I see on YouTube). I'm just stating what I have personally experienced. As far as I'm concerned what works, works. If the little vent on top isn't where the IAC draws air from, where does it get it from?

 

[J_C]

A stepper motor necessarily has more than two wires, unless it is something very advanced (on the space shuttle?) where it uses a serialized data feed to a controller on the motor, or where the power source switches the polarity which can achieve only one more step. Either method is more advanced than these IACs on ours.

See on the video, theirs has 4 wires, and at 10:45 the guy states that Ford is different. Ford doesn't use a motor, rather we have a solenoid that does not spin or step, it just moves in and out axially to open or close the valve, one step.

As mentioned above they could achieve two steps with a 2 wire power connection if it has source side polarity reversal, but that is not how ours is designed. It has fixed polarity that is common to the fuel injectors, so it is just a commonplace one step solenoid. You can also tell this by mechanically manipulating it, that it is just a rod on a plunger that pushes in and out.

I think koda is correct that the factory merely failed to adjust these properly and it's a DIY requirement. It probably lets too much air through if you don't limit the plunger travel, or vice versa if the factory happened to screw it in a different amount the next day. This situation would be a lot more acceptable if there were proper, well written English (lol) instructions, but Koda figured it out anyway.

... and yes, in KY we have two completely different kinds of bluegrass, though the type not on my lawn is mostly found in the hills to the southeast, like a scene from Deliverance.

 

[donalds]

Ok I must be an idiot then
I guess it's so game of whom is correct well you are
I'll remove my incorrect nonsense posting
Guess I need to work for NASA or space x
unsubscribed
Btw I was born in Pikeville ky

pikeville ky stamper lane - Google Search

 

[fast_dave]

@donalds

Love your new avatar - gotta' get the coffee mug for Christmas!

 

[J_C]

I guess it's so game of whom is correct

Btw I was born in Pikeville ky

Just want the right info out there, no matter where it comes from.

I apologize if my remark hit home about SE KY, but from what I hear it is now even worse, drugs have turned a beautiful area into a tragedy.

 

[imp]

Just want the right info out there, no matter where it comes from.

I apologize if my remark hit home about SE KY, but from what I hear it is now even worse, drugs have turned a beautiful area into a tragedy.

@J_C

Understood and agreed! Here's my two cents: I have never seen an IAC which allowed air to enter from the outside; all the pics above are similar confirming that: the IAC mounts in such a way to the throttle body that it's two holes "straddle" the throttle plate. IOW, the IAC allows air to bypass the throttle plate and the amount of air is controlled by the solenoid pintle via the PCM. Thus, idle speed can be held as various electrical and mechanical loads on the engine change, alternator load and say A/C compressor. It also allows idle speed-up when cold, like the steps on the throttle plates of the old dinosaurs.

Actually, if you could see it, upon cranking, the IAC opens to it's maximum to allow a big gulp of air into the engine while starting, then closes down as needed. It is not a "stepper" type device. The amount the solenoid plunger moves depends on the amount of voltage applied to it's coil, "fighting" against the return spring which closes it when off. Yes, it has 2 wires.

Note, though, that "drive by wire" throttle bodies have NO IAC, but rather a Servomotor which opens and closes the throttle plate, so closely controlled, that plate opening is used to establish idle speed. It also has ONLY 2 WIRES.

If anyone likes, ask, and I'll describe how the DBW system self-checks itself each and every time the key is turned to ON. imp

 

[1998Exp]

So can we summarize this discussion by stating that the $10 "adjustable" IAC does not allow setting the idle speed by rotating a screw, because that's the job of the PCM, and that screw is probably some factory adjustment for spring tension or rod length on an inferior product with poor tolerances?
Also, if one looks at J_C's diagrams, it appears that the purpose of that little cap on the top is to equalize the pressure inside the damper (that probably exists to smooth the rapid "clicks" of the solenoid) - obviously nothing to do with combustion air, which is always supplied filtered and metered.

And yes, do explain how the DBW self-checks -- just in case the "fly by wire" throttle in my significant other's Kia misbehaves and needs me to fix it. Thanks!

 

[donalds]

apologize if my remark hit home about SE KY, but from what I hear it is now even worse, drugs have turned a beautiful area into a tragedy.

My whole family is from ky ....on my mom's side... for generations we were coal mining hell my grandpa Jack owned a few ...he left them to the town as a last f... you to my grandma......
Now since the recession a very old family business is no more so all my family is now cancer ridden drunks that beat their wife's and kids which is why I now have medical conditions that well suck
man I miss that place it's beautiful and the smell of morning dew is like nowhere else

Do you like bluegrass ...music... May want to check out my uncle art stamper he is a legend in ky one of the best fiddle player there is
Look up stamper blues you may injoy @J_C

 

[koda2000]

So can we summarize this discussion by stating that the $10 "adjustable" IAC does not allow setting the idle speed by rotating a screw, because that's the job of the PCM, and that screw is probably some factory adjustment for spring tension or rod length on an inferior product with poor tolerances?
Also, if one looks at J_C's diagrams, it appears that the purpose of that little cap on the top is to equalize the pressure inside the damper (that probably exists to smooth the rapid "clicks" of the solenoid) - obviously nothing to do with combustion air, which is always supplied filtered and metered.

And yes, do explain how the DBW self-checks -- just in case the "fly by wire" throttle in my significant other's Kia misbehaves and needs me to fix it. Thanks!

No. After seeing the diagrams supplied by @J_C I agree that the air supplied through the IAC comes from the intake/throttle body, but how can you argue that turning the adjustment screw does not effect idle speed when it most certainly does (and did). I'm un-watching this thread at this point as I have no further interest in reading about how different systems work or electrical theory. Feel free to continue offer your expert opinions.

 

[donalds]

and I don't believe everything I see on YouTube

Agreed. I prefer books but they are harder and harder to find on this subject anyways

 

[410Fortune]

I like Hitachi, they make nice stuff....even elevators!

 

[imp]

So can we summarize this discussion by stating that the $10 "adjustable" IAC does not allow setting the idle speed by rotating a screw, because that's the job of the PCM, and that screw is probably some factory adjustment for spring tension or rod length on an inferior product with poor tolerances?
Also, if one looks at J_C's diagrams, it appears that the purpose of that little cap on the top is to equalize the pressure inside the damper (that probably exists to smooth the rapid "clicks" of the solenoid) - obviously nothing to do with combustion air, which is always supplied filtered and metered.

And yes, do explain how the DBW self-checks -- just in case the "fly by wire" throttle in my significant other's Kia misbehaves and needs me to fix it. Thanks!

@1998Exp
I've been sort of shut out now by others, for some reason it's always helped me to understand the theory behind some A.H.'s design, and seems you see it too!

Early 2000s, one or two Japanese makers implemented DBW without doing enough homework. No fail-safe built in, their cars occasionally went wide open throttle suddenly without any notice. OK maybe at freeway speed, NOT GOOD standing at a red-light. Think several people were killed. They had massive recalls.

Ford did good homework. Don't know about KIA, though. To do DBW all you need is a variable resistor which moves it's wiper contact by a moving gas pedal. Plenty of possibilities for failure there. The TB throttle plate, TP, is spring-loaded b y a spiral spring which forces it closed. There is a positive stop, factory-set, which prevents the plate from jamming shut mechanically, sticking, as if you're old enough to remember, happened on carburetors if you backed the idle speed screw way out. Against that stop, almost no air can pass, though. A small plunger working against the main spring pushes the plate open until the plunger hits it's own stop. That position, with the TP open slightly, you can feel how much if the air inlet tube is removed, by pressing the TP closed fully using your fingers, releasing it, it returns to the slightly "cracked" position. That is the "off" position, engine shut down.

The Throttle Position Sensor, TPS, measures the amount the TP moves. A foul-up there can also cause unwanted throttle opening. The closing of the TP to it's stop, as I just described, and letting it go back against the plunger stop, produces an electrical output very predictable, by the TPS. PCM "remembers" that small output from the first "KEY ON" sequence, as each time the key is turned to "ON", the servomotor closes the plate tight, then releases it, in about 1 second. During that time, the TPS output is checked; it's always the same, unless something goes haywire. Then yer in BIG TROUBLE!

So you turn key on, and quickly go to "START". By then, the "prove-out" has already been done. Every single time the key is turned to "ON". There is one other fail-safe, at the gas pedal. The "Gas Pedal Position" Sensor, the variable resistor, is two separate resistors, moving in opposite polarity directions as the pedal is depressed. This results in a new zero voltage across the two wipers, checked by PCM, constantly, ready to force engine shutdown, possibly, seeing a Pedal sensor failure.

Did we need this DBW ****? In a word, no. But I consider the fact that while it's complicated, it eliminates certain numbers of possible trouble spots: throttle cable, IAC problems, while greatly enhancing idle quality and throttle response. I have it on my '04 Explorer. One day, my vehicle went into "Forced Engine Idle". At that time, I didn't know diddly-squat about DBW, or that I had it. But I by God won't entrust fixing to a dealer. That's why I went ahead to learn how it works. I replaced the complete TB, and she runs beautifully. Thanks for reading this long mess! imp