IAC stuck shut causes 'lean' DTC?

I subscribed to the AUTOTAP newsletter because I thought there might be a gem here and there... well the last one certainly was! Or rather a headscratcher:
QUOTE "Recently the check engine light came on my neighbor's Windstar and the car did not idle properly. The RPM reading was about 500RPM at idle. The engine idled so rough that the accelerator had to be pressed slightly to prevent the engine from dying. While driving, we also heard some mysterious whistling sounds but could not locate its origin. The whistling sound is more pronounced when the engine is running under load. At 60mph the car ran well, but stop or slow for a turn and the engine would die unless we punch the accelerator.
When we first performed the diagnostic test with AutoTap we found error codes from two oxygen sensors, namely P0171 System too Lean Bank 1 and P0174 System too Lean Bank 2 implying lean fuel mixture. We believe the source of our problem could be caused by a vacuum leak, however we are aware of other issues that could cause a lean mixture, such a faulty gas filter, and plugged injectors.
We tried using several methods to locate vacuum leaks - .... The mechanic quickly found the problem to be the Idle Air Control Valve. Apparently the valve was stuck shut all the time because of carbon deposits on the rod which connected to the valve.
.....
Looking back on our troubleshooting procedures, the cause of the problem is obvious. The Idle Air Control Valve should have been the first sensor/actuator to focus our attention on. The actuator is very accessible and easy to experiment with. The reason we did not focus on that valve was because we didn't know it existed or what it did!" UNQUOTE http://www.autotap.com /
So... let me get this straight.. the guy buys an autotap diagnostic tool, but doesnt know that there's such a thing as an IAC.. and, further, what it does?
What's more... the fact that the IAC is to blame for the 'lean condition'?!!!
1. The guy should spend the time to study up on modern FI
2. I think he needs to clean his MAF as well.
--
Yeh, I'm a Krusty old Geezer, I believe you're never too old to learn...
what stifles that is forgetting what it was you just read.
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On Mon, 03 Apr 2006 16:09:49 GMT, Backyard Mechanic

BM I can't help but wonder what would happen with the IAC stuck open too far. Would the ECM then reduce fuel delivery in an effort to reduce the idle speed to the correct value resulting in a lean fault? Never tried or saw this. I suspect that it would result in a high idle speed possibly with a lean fault just like a vaccum leak. I don't believe the ECM would know the difference.
On the other hand, if the IAC is not allowing enough air in, the ECM will reduce fuel delivery resulting in bad idle quality and low RPM because the EGO sensors would be signaling rich. I have seen a number of low/poor idle conditions because of stuck/defective IAC's but they didn't give a lean code and usually not even a fault code. I have seen a couple of rich codes that were corrected with a new IAC. As far as the fuel filter goes, I haven't seen an EFI system filter that would supply enough fuel to drive the car and result in poor/lean idle conditions. Most of the restricted filters I've seen cause problems started by limiting the speed the vehicle could attain under load or the acceleration rate of the vehicle because that is where the juice is needed. Me thinks a little study time is in order for the technician on this one.
Lugnut
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I agree, a stuck-open IAC IS exactly like a broken vacuum fitting. Had that fitting break happen on my taurus and the result was a very high, erratic idle. But the EGO sensors arent in play (open loop) most of the time you'll see the symptoms of either stuck closed or open.
Nope.. the more I think about it, the more I'm convinced the guy written about had TWO problems or rather one BIG ONE.. filthy AIRSTREAM passages. And he'll end up cleaning the MAF.. or, worse, replacing it if he's that clueless.
And if the Autotap guy.. may not even be a tech.. had done a search on 'whistling' he'd have seen correlating posts regarding IAC.
And the 'fuel filter' symptoms are also dead obvious... a clogged filter cant cause poor idle while running good otherwise.
--
Yeh, I'm a Krusty old Geezer, sometimes I just marvel...

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Seems that the art of basic troubleshooting based on a knowledge of how the system works is about dead. If the computer can't tell many of them nowadays, it will remain a mystery until the shotgun parts replacement procedure exhibits results.
Lugnut
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A stuck IAC should have kead to idling problems which should have been the first problem seen. The story seems bogus to me.
Ed

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