1995 Chevrolet Astro IAC Valve problem

Tiny
KRISNUDA
  • MEMBER
  • 1995 CHEVROLET ASTRO
  • 4.3L
  • 6 CYL
  • 4WD
  • AUTOMATIC
When I put parking in my van after driving the Service Engine Soon light comes one. After putting the van in drive it disappears. It was scanned and it shows IAC Valve problems. I replaced it, the code was erased and all that is supposed to be done. The thing is that I got the light on again in parking after driving it a while. I scanned it again, and it shows me IAC valve problems again. There is also a strange smell like paint or nail polish. Any idea what is causing this problem?
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Saturday, April 4th, 2015 AT 6:45 PM

5 Replies

Tiny
CARADIODOC
  • EXPERT
The common mistake is the diagnostic fault code did not say to replace the idle air control valve or that is was defective. Fault codes never say to replace parts. They only indicate the circuit or system that needs further diagnosis or the unacceptable operating condition. This valve has a very low failure rate, at least as far as electrical failures are concerned. You didn't list the mileage so I can't make any judgements related to that, but based on age, I'd start by checking the wiring for insulation that's rubbed through and a wire is grounding out, or possibly a corroded or stretched terminal in a connector. Your mechanic can also use a scanner to run the valve through its range of travel to evaluate its operation.
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Saturday, April 4th, 2015 AT 7:09 PM
Tiny
KRISNUDA
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The mileage is 165,000. I was told to check the evaporative system too (I think that's the way it is called), which was done today to see if there is any leak, but the mechanic could not find anything. I would like to know if there is a kind of diagnosis that can be done to find where the problem is. Thank you for your response.
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Saturday, April 4th, 2015 AT 7:19 PM
Tiny
CARADIODOC
  • EXPERT
First you have to know the exact fault code number. There are about a dozen related to the idle air control valve, and they mean very different things. That system doesn't have anything to do with the evaporative emissions system. Also, that system, which is also referred to as the "vapor recovery" system, isn't monitored on '95 and older models, so while it could have a leak, that won't be detected by the Engine Computer and it won't set a fault code.

To find a leak in the vapor recovery system, you really need a smoke machine. That allows you to inject a white, non-toxic smoke at 2 pounds of pressure, then you can watch for where it sneaks out. Other than that you're limited to a visual inspection, and that is only good for real obvious problems. Most leaks in this system are way too small to see by eye.

For the idle air control valve circuit, I already mentioned checking for bare wires and corroded connector terminals. Beyond that the testing gets a little more involved. You can use an ohm meter to test the resistance of the wires and the motor that runs the idle air control valve, and I mentioned the scanner for running the motor through its paces to see if it responds properly. There really isn't anything you can do yourself when a scanner is required.
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Saturday, April 4th, 2015 AT 8:17 PM
Tiny
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The code that is shown in the scanner is 35.
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Saturday, April 4th, 2015 AT 8:26 PM
Tiny
CARADIODOC
  • EXPERT
35 - Idle air control sensor circuit fault

As you can see, fault codes don't get real specific. To add to the confusion, the idle air control valve, (automatic idle speed motor), is not a sensor as the code implies. AND, ... It's not a motor as we normally think of one that spins. This unit has four wires and four internal electromagnetic coils. It doesn't have brushes like most motors do. The Engine Computer pulses the coils with varying voltages and polarity to set the armature to the desired position, and as it rotates slowly, it extends a valve that is on the end of a threaded shaft. As that armature turns, it extends or retracts the shaft and valve to vary how much of a controlled air leak is exposed. As it opens the passage to allow more air in, the computer lengthens the amount of time it holds the injectors open to allow more fuel in. That is how the computer adjusts idle speed and holds it steady.

Remember too that this valve only affects idle speed, and nothing else. If idle speed is too low, you just have to hold the accelerator pedal down 1/4". The engine may be hard to start if you don't do that, and it may tend to stall at stop signs.
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Saturday, April 4th, 2015 AT 8:45 PM

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