IAC stuck shut causes 'lean' DTC?

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Backyard Mechanic - 03 Apr 2006 17:09 GMT

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.

Signature

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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lugnut - 03 Apr 2006 18:21 GMT

>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
[quoted text clipped - 42 lines]
>
>2. I think he needs to clean his MAF as well.

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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Backyard Mechanic - 03 Apr 2006 18:48 GMT

>>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,
[quoted text clipped - 33 lines]
>
> Lugnut

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.

Signature

Yeh, I'm a Krusty old Geezer, sometimes I just marvel...

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lugnut - 03 Apr 2006 21:45 GMT

>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
[quoted text clipped - 12 lines]
>And the 'fuel filter' symptoms are also dead obvious... a clogged filter
>cant cause poor idle while running good otherwise.

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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C. E. White - 04 Apr 2006 14:34 GMT

A stuck IAC should have kead to idling problems which should have been the
first problem seen. The story seems bogus to me.

Ed

> 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
[quoted text clipped - 42 lines]
>
> 2. I think he needs to clean his MAF as well.

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