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cgrey8
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How much should an IAC flow when closed?

Post by cgrey8 » Tue Dec 18, 2012 11:34 am

On the new 331 I'm still tinkering with, I've got an excessively high idle even when the Idle Air Controller (IAC) actuator valve is unplugged. The MAF is registering the higher airflow so there's only 3 components that pull air through the MAF. The PCV, TB, and the IAC valve.

The TB is confirmed closed as far as it can go. I completely removed it and took a close look. The set-screw was actually not even being touched. The blade was closing to soft-bind making it kind of sticky. While the TB was in my hand, I adjusted the idle screw so the TB lands on the screw, but still closes as far as possible without bind.

I confirmed that the PCV is flowing while at idle. If I squeeze the line OR cover the inlet, the MAF drops by 4-5kg/hr and idle drops accordingly, but not low enough for my liking.

So that leaves the IAC valve.

I removed the valve and blew through it just to see if it was closing completely and it isn't. The valve is flowing a fair amount of air while in the closed position. I can fairly easily blow through it. Using a screwdriver, the valve easily moves from it's resting closed position all the way to full open, but doesn't appear to completely return to the closed position which explains why I was getting idle air control above the resting point. It's almost as though the return spring is just a little weak allowing it to stay open more than I'd think it should. I took the actuator off and sprayed both the actuator and the valve with Teflon spray to see if that would help. The moving parts move freer now, but the valve still doesn't close down completely. Although it is possible the flow of air an engine would produce would be enough to push the valve closed. Air does flow through the valve in the direction that would tend to close the valve, not open it. Perhaps with the parts freed up, the valve will close when on the engine? We'll see.

But I didn't stop tinkering there. Notice these older valves have the actuator portion screwed to the mechanical valve portion:
Image
I removed the actuator off the valve and noticed that with the actuator removed, the valve actually closes off. It pisses a tiny amount of air when I blow through it, but I feel definite resistance to flow. Playing around with the actuator and the valve, it seems there's a very weak set-spring pushing on the actuator plunger that is just barely stronger than the spring in the valve thus with the actuator in place, the valve is held slightly cracked open. I found I could separate the actuator from the valve by about an O-ring's thickness and the valve seems to close all the way. So if just spraying the valve doesn't free it up to close down better on the engine, I'm going to try sandwiching an additional O-ring or two between the actuator and valve and see how that works.

Now my question to those out there with experience/knowledge of these older style (pre-95) 2 piece IAC valves, is it normal for them to be cracked open like this? Or is this an indication that the valve is just worn out and needs to be replaced? Looking on AutoZone, replacements are ~$55 and are special order. While I don't mind paying that if that'll fix the problem, I don't want to buy it if the new one is going to behave exactly the same way.

Thoughts?

The good news is I tentatively seem to have identified where the high idle is coming from. I'll hopefully confirm this afternoon.
...Always Somethin'

89 Ranger Supercab, 331 w/GT40p heads, ported Explorer lower, Crane Powermax 2020 cam, FMS Explorer (GT40p) headers, aftermarket T5 'Z-Spec', GUFB, Moates QuarterHorse tuned using BE&EA

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Re: How much should an IAC flow when closed?

Post by IMINHELL » Tue Dec 18, 2012 2:30 pm

My best guess is that at warm idle the valve is designed to be open slightly based on stock airflow requirements. We can assume that a zero point shouldn't be closed since that will stall the engine out, no air. So it'd make sense that full closed isn't possible.

One thing I'm curious on though is the many different part #'s for the valves. All the Ford ones are interchangeable for the most part. The flange is the same. Begs the question of why there are different parts for different vehicles,,, almost like it's the stop point that changes. Say a 4cyl IAC has a lower stop point than a V8 one?
Least that's my take on it.
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Re: How much should an IAC flow when closed?

Post by Stengun » Tue Dec 18, 2012 3:29 pm

Howdy,

To to your local U-Pull It/Pick-N-Pull and get look at a couple of used ones and see if they close all the way.

Pick out one the appears to be good and see if it makes a difference.

I'm not a big fan of swapping parts as a way to troubleshoot a problem but sometimes that's what you have to do.

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Re: How much should an IAC flow when closed?

Post by 85GT » Tue Dec 18, 2012 4:15 pm

Think I have an original fox valve on the shelf. I'll check tonight and let you know.
85GT, 302 w/Dart Windsor Jr heads, Crane 2030 equiv. Performer 5.0, 75mm TB, 88mm slot MAF, 34lbs injectors, BBK shorties, 5spd converted to 4R70W with Baumann controller, 9" rear w/3.25s
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Re: How much should an IAC flow when closed?

Post by 85GT » Wed Dec 19, 2012 12:13 am

No luck. Amazingly, I must have throw it out. Got another at other house but that won't be until holidays. I got the autozone one when I screwed up the one I use to have here. I wanna say I think i recall doing the 'blow' test on it and nothing seemed strange as in air leaking.
85GT, 302 w/Dart Windsor Jr heads, Crane 2030 equiv. Performer 5.0, 75mm TB, 88mm slot MAF, 34lbs injectors, BBK shorties, 5spd converted to 4R70W with Baumann controller, 9" rear w/3.25s
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Re: How much should an IAC flow when closed?

Post by cgrey8 » Wed Dec 19, 2012 8:12 am

I always assumed the IAC shouldn't flow any air when closed. And any "minimum" air needed for the engine to run should be adjusted via the TB. So you basically unplug the IAC, and then set the idle screw to get the engine to just barely idle while warm. Then plug the IAC in and let it handle idle speed from there. That way, the IAC isn't supplying ALL the air, and thus it has a much wider range of RPM control.

Anyway, I found 2 Orings last night of moderate thickness and sandwiched them in between the actuator and valve. That seems to work great from a blow-test standpoint. I spent last night taking the intake and VCs back off to apply silicone gasket to the VCs again before hopefully buttoning everything back together for the last time. I should have it driveable again this afternoon to actually test the IAC hack.

But if anybody can get me some feedback on whether the IAC should or shouldn't flow a minimum amount of air when in the closed position, that'll kinda tell me whether I really should buy another one or just ride with what I've rigged up (again assuming this works in practice).
...Always Somethin'

89 Ranger Supercab, 331 w/GT40p heads, ported Explorer lower, Crane Powermax 2020 cam, FMS Explorer (GT40p) headers, aftermarket T5 'Z-Spec', GUFB, Moates QuarterHorse tuned using BE&EA

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Re: How much should an IAC flow when closed?

Post by StangD » Wed Dec 19, 2012 8:27 am

cgrey8 wrote:I always assumed the IAC shouldn't flow any air when closed. And any "minimum" air needed for the engine to run should be adjusted via the TB. So you basically unplug the IAC, and then set the idle screw to get the engine to just barely idle while warm. Then plug the IAC in and let it handle idle speed from there. That way, the IAC isn't supplying ALL the air, and thus it has a much wider range of RPM control......
That's exactly my understanding how thy work as well. I run a FOX IAC and TB on my SN95, I will see what it is doing.
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Re: How much should an IAC flow when closed?

Post by decipha » Wed Dec 19, 2012 12:04 pm

it shouldnt flow anything when closed, it should be seated and sealed and should take adequate force to unseat it manually

when they fail they typically hang open just as you observed

henry made different isc's to conform to different mounting locations and clearances

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Re: How much should an IAC flow when closed?

Post by TwinSCs » Wed Dec 19, 2012 12:34 pm

I agree that the IAC should not flow anything when closed. However I suspect engine vacuum is required on the correct side of the IAC to keep it seated.

The reason I say this is because I was experimenting with a few different IAC configurations. When I had the IACs installed "backwards" The idle speed would shoot through the roof like it was stuck open. When I would flip the same IAC back around idle speed would be fine. So unless I had several bad IACs, it leads me to the conclusion that vacuum pulls it closed and the electronic signal to it opens it according to duty cycle.
81 Mustang, 91 5.0 shortblock, GT-40 heads & intake, TFS1 cam, 24# bluetops, Explorer TB and FEAD, 73mm C&L Maf, BBK shorties, T-5 with 3.73s. Running GUFB on A9P with QH.

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Re: How much should an IAC flow when closed?

Post by cgrey8 » Wed Dec 19, 2012 12:43 pm

Based on the design of this IAC I'm working with, I agree that the orientation that the IAC is mounted make a difference. For the 2-piece IAC I'm using, the hole closest to the actuator/connector should always be the side that leads into the engine. The far hole should be the one pulling air from the MAF.

The newer style IACs that don't come apart may not behave the same way. I don't know. The 97 Ranger I have has one of the newer style. If I think about it, I'll take a look and see if the same rule applies to that one or not.
...Always Somethin'

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Re: How much should an IAC flow when closed?

Post by 85GT » Wed Dec 19, 2012 3:06 pm

cgrey8 wrote: For the 2-piece IAC I'm using, the hole closest to the actuator/connector should always be the side that leads into the engine. The far hole should be the one pulling air from the MAF.
You sure about that? The mustang has the actuator end pointing at the MAF side. The farthest hole looks like it would be on the vacuum side of the TB blade.
85GT, 302 w/Dart Windsor Jr heads, Crane 2030 equiv. Performer 5.0, 75mm TB, 88mm slot MAF, 34lbs injectors, BBK shorties, 5spd converted to 4R70W with Baumann controller, 9" rear w/3.25s
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Re: How much should an IAC flow when closed?

Post by cgrey8 » Wed Dec 19, 2012 8:02 pm

Hmm...then disregard. I might have the thing installed backwards then. :oops:

BTW, I got the engine running and with the O-ringed IAC, the engine now has a much more controlled idle. Although it's still a little high. But one thing I figured out is the "minimum" ISCDTY value 11.88 that my tune seems to command is enough to open the valve a tad even with the Oring in place. If I unplug the IAC, the engine just barely idles below 600RPMs and if I pinch the PCV, it dies. If I plug the IAC in, the minimum seems to hold idle in the mid 600s which is fine by me. And my MAF readings are down from 30kg/hr to around 20-22kg/hr. I saw a few times where it got down to 19. That's MUCH more in the neighborhood I expect low RPM idle airflow to be given the 302 using the same intake tract would idle at 15-17kg/hr.

What I need to identify now is if 11.88 is the absolute minimum ISCDTY possible or if there's some clip I've got set in the tune that I could lower to allow the tune to go all the way down to 0% if needed to get idle down to my desired RPM of 625-650. The 302 had a solid idle right at 600, but this engine doesn't seem to like that. It will idle at 600, but it is rough and isn't stable at that RPM at all. It's not a stock cam, so that's hardly a surprise.
...Always Somethin'

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Re: How much should an IAC flow when closed?

Post by cgrey8 » Wed Dec 19, 2012 10:48 pm

I just looked back at some old pics of the 302 when it was installed. Yep. I got the IAC installed backwards.
Image

All-n-all, I'm glad I said what I did now so someone would catch that, correct me, and help me figure out my problem. I'll see if I can get that turned around before driving the truck to work tomorrow.
...Always Somethin'

89 Ranger Supercab, 331 w/GT40p heads, ported Explorer lower, Crane Powermax 2020 cam, FMS Explorer (GT40p) headers, aftermarket T5 'Z-Spec', GUFB, Moates QuarterHorse tuned using BE&EA

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Re: How much should an IAC flow when closed?

Post by liljoe07 » Wed Dec 19, 2012 11:52 pm

LoL! It does look better the wrong way!
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Re: How much should an IAC flow when closed?

Post by TwinSCs » Thu Dec 20, 2012 1:17 am

liljoe07 wrote:LoL! It does look better the wrong way!
LOL, that's sort of how I discovered it. I was trying to run the Explorer IAC backwards on a Fox because it looked cleaner.

I figured the newer technology couldn't hurt either.
81 Mustang, 91 5.0 shortblock, GT-40 heads & intake, TFS1 cam, 24# bluetops, Explorer TB and FEAD, 73mm C&L Maf, BBK shorties, T-5 with 3.73s. Running GUFB on A9P with QH.

decipha

Re: How much should an IAC flow when closed?

Post by decipha » Thu Dec 20, 2012 2:30 am

iac is clipped by the transfer!!!

also the min airflow debycp

i recall there being 1 more but dont have my laptop to look

however, henry may have done that due to the hardware outputs, im not sure if this applies to the intel ic's but on the atmel (what i use for the d.e.c.) there are a couple pw outputs that cant operate stable below approx 10% dc , its only logical to use those for isc since youll never be that low anyway

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Re: How much should an IAC flow when closed?

Post by cgrey8 » Thu Dec 20, 2012 5:06 pm

Well, no surprise with the IAC mounted correctly, I get perfect control of the engine RPM without any sandwiched O-rings. And I was able to revert back to my 302's tune with only a few things tweaked for the 331. And the engine runs even better now. Thanks to 85GT for pointing me to the fact that I had the thing mounted backwards.
...Always Somethin'

89 Ranger Supercab, 331 w/GT40p heads, ported Explorer lower, Crane Powermax 2020 cam, FMS Explorer (GT40p) headers, aftermarket T5 'Z-Spec', GUFB, Moates QuarterHorse tuned using BE&EA

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Re: How much should an IAC flow when closed?

Post by 85GT » Thu Dec 20, 2012 9:59 pm

Excellent. Gotta love those simple fixes.
85GT, 302 w/Dart Windsor Jr heads, Crane 2030 equiv. Performer 5.0, 75mm TB, 88mm slot MAF, 34lbs injectors, BBK shorties, 5spd converted to 4R70W with Baumann controller, 9" rear w/3.25s
A9L running A9P bin via Quarterhorse w/LC-1 WB

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