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Old 01-01-2011, 09:16 AM   #1
bhawk
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Default how does idle air control valve work?

We are plagued by the high idle condition that many ranger owners seems to have. I've read many threads on this forum on the issue but none really address my question. How does the device work?
It seems from my looking at it off the truck, when it is disconnected, the default position is to block the passage so no air can by pass the throttle plate and get into the intake. I assume that when it is connected to voltage, the solenoid pushes against that rod and opens the passages somewhat. Correct?
So, when the truck is cold and you're starting it for the first time, does it need air and therefore the solenoid needs full voltage to push that plunger open all the way?? or is it the opposite, on a cold start there should be no voltage at the solenoid so that it blocks air to the engine?
I'm really trying to figure out why disconnecting the IAC after the truck is at operating temp causes our idle to move down from a high of 1000 rpm to a real nice 650 rpm. But with the IAC disconnected, it is very hard to start and if it does, you have to hold the gas pedal down to hold it at 1000 rpm.
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Old 01-01-2011, 10:22 AM   #2
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Quote:
Originally Posted by bhawk View Post
We are plagued by the high idle condition that many ranger owners seems to have. I've read many threads on this forum on the issue but none really address my question. How does the device work?
It seems from my looking at it off the truck, when it is disconnected, the default position is to block the passage so no air can by pass the throttle plate and get into the intake. I assume that when it is connected to voltage, the solenoid pushes against that rod and opens the passages somewhat. Correct?
So, when the truck is cold and you're starting it for the first time, does it need air and therefore the solenoid needs full voltage to push that plunger open all the way?? or is it the opposite, on a cold start there should be no voltage at the solenoid so that it blocks air to the engine?
I'm really trying to figure out why disconnecting the IAC after the truck is at operating temp causes our idle to move down from a high of 1000 rpm to a real nice 650 rpm. But with the IAC disconnected, it is very hard to start and if it does, you have to hold the gas pedal down to hold it at 1000 rpm.
I cant help you with how the iacv works but I was wondering if you checked/changed the ECT sensor and is the iacv new?
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Old 01-01-2011, 10:29 AM   #3
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The IAC is controlled by the computer. The computer takes many other inputs from other sensors, and the IAC plunger is adjusted accordingly to adjust engine idle.

The IAC has a stepper motor. It has several different settings like steps that it positions to with different voltage applied to it by the PCM.

So it should move, and not to just one extreme or the other. And if the idle falls when unplugging it, it doesn't mean it's bad. It could just be doing what it is supposed to, according to the other sensors input into the PCM (MAF, ECT, HO2S's(oxygen sensors), etc).

IAC's usually get dirty, sticky, and clogged with carbon. You can try to clean it and the passages leading to it. Also, they usually make a whining humming sound when going bad.

High idle is also a symptom of a false reading ETC (trying to warm the engine up....constantly with a high idle), or a vacuum leak (or just false incoming air reading from the MAF).

Try unplugging the MAF and ETC, see what happens.
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Old 01-01-2011, 03:45 PM   #4
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thanks for the advice.
The engine coolant temp sensor is new, replaced it couple months ago. No change in idle then. Put a new air intake temp sensor in the intake chamber and no change in idle. Changed the MAF sensor couple days ago with one that works on another ranger. No change. Changed the TPS to one off a truck that worked, no change. When we unplug the IAC it idles down to 650 or so, real nice, but once it is plugged back in, it goes to 1000 rpm at idle. When shifting (manual trans) the rpm's hold at what they were when we hit the clutch, no quick drop in rpm's on shifting which is the real aggravation here.
I guess we'll have to bite bullet and buy a new IAC. Tried a used one yesterday and a little change for the better, but not much.
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Old 01-05-2011, 09:34 AM   #5
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If the RPM sticks, I'd suspect either of 2 things.....

Vacuum leak after the MAF or IAC.
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Old 01-05-2011, 09:53 AM   #6
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Thanks guys for the opinions. I've searched high and low for a vacuum leak and can't find one. I recently changed the IAC to another used one off an old parts truck my dad has. It brings the idle down to 900 rpm at operating temp. So that isn't too bad. I suspect if I bought a new IAC it would come down even more.
On a disgusting note, I live in Canada and I went to price out a new IAC at Napa here. They sell Echlin and quoted me 209 bux plus 12 percent tax. I go on the Napa USA online site and the same IAC by Echlin sells for 83 bux. Go figure. We constantly get gouged by parts suppliers here in Canada and this is just another example.
He also quoted me 109 bux for a new IAC by Standard, but he seemed to imply it wasn't the best quality, he pushed me to the Echlin one. No way am I spending 209 when I can pick one up in ND on my next trip there for 83. Thanks again boys.
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Old 01-07-2011, 12:17 PM   #7
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Here is what my IAC was doing when it was going bad

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Old 01-08-2011, 10:51 AM   #8
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Default Iac ?

Hey Bhawk I didnt see anything about yr of your truck, I have a good little collection (2 or 3) IAC's here that came off of a 91, 92, 93, or 94 explorer with the 4.0. If you need one I can save you alot of money. And I know for a fact they are good and work.

Oh Hey there it is,, It says a 94,,,

Last edited by AaronC47; 01-08-2011 at 10:53 AM. Reason: I was momentarily unable to see the writing on the screen, I think I had a brain fart.
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Old 04-17-2011, 07:00 AM   #9
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To update this thread, the high idle that bothered us for a long time has been solved. It was not the IAC. It turned out to be a leaky lower intake manifold gasket. After replacing the gasket, the idle goes down to 700 rpm or so hot, and never exceeds 1150 or so on a cold start. Runs and drives so much nicer, and fuel economy has improved significantly.
You cannot discover a vacuum leak at the joint between the lower intake and the cylinder head with propane or starter fluids, or the like because that joint is UNDER the valve cover gaskets. I discovered that on tear down. Spraying those products will show you if you have a leak at a vacuum hose or fitting, or a leak at the join between upper intake and fuel rail manifold, but they are of no help with the key joint under the valve covers.
Thought I'd close this topic with the solution that worked for us.
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