Disconnecting IAC valve to diagnose ?

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cptnsolo77
Posts: 1645
Joined: Sun Jul 29, 2007 4:05 am

Disconnecting IAC valve to diagnose ?

Post by cptnsolo77 »

Im am trying to trace what my vibration at idle may be. I have read that you disconnect the IAC valve ( while running ) it will cause the idle drop. almost to the point of not running, idle erratically or even turn off. Is that true ? From what I understand the purpose of the IAC is....it DOES makes sense. I disconnected it yesterday while it was running and the car idle did not drop or idle erratically. I did set off a check engine light for the IAC so the computer detected that it was disconnected. Im begining to suspect the IAC valve might be the issue. Thoughts ?
* 2003 Vibe Auto Satellite Silver w / Moons & Tunes
* Kenwood Excelon KDC-X597
* Polk DB651-Speakers
* Soundproofcow Roadblock R sound deadening in all doors
* Drop In K & N Filter
* NGK Iridium IX Plugs
* 27 MPG City/Highway with AC on
jolt
Posts: 945
Joined: Sun Mar 02, 2014 2:07 am
Location: Twin Cities, Minnesota

Re: Disconnecting IAC valve to diagnose ?

Post by jolt »

From my understanding of how most IAC valves work, yours did exactly what it should do. The IAC is a air by-pass valve around the throttle butterfly valve. This allows for more or less air to flow into the engine when the butterfly is close thus increasing or decreasing the idle speed of the engine. The IAC uses a small stepper drive motor that the ECM can control the position of. The stepper drive motor is connected to a threaded valve, much like a needle and seat that was used to control the fuel mixer at idle in the carburetor but this valve is bigger. The stepper motor screws the valve in and out which allows less or more air into the engine and changes the idle speed of the engine.

If you disconnect the wiring from the IAC, it just stays at that last powered on position and can not move. There is nothing to drive the bypass needle in or out so the air flow does not change for the idle speed. The ECM no longer is adjusting the idle speed as the motor runs. If all is good with the system, the engine will just stay idling at that same speed. There are other types of IAC out there but this is the most common type found in early throttle body's and the description of the valve in the Toyota manual said the IAC is "rotary".

Vibrations are mostly caused by mechanical issues. A cylinder that is not firing properly, which can be caused by low compression, bad camshaft lobe, burnt valve, balance issues with rotating parts of the engine. Four cylinder engines do not run smoothly. This is why the larger four cylinder engines use counter balance shafts to help smooth out vibrations of the motor. The second gen Vibe uses counter balance shafts in the 2.4L engine. The 1.8L does not have counter balance shafts.
cptnsolo77
Posts: 1645
Joined: Sun Jul 29, 2007 4:05 am

Re: Disconnecting IAC valve to diagnose ?

Post by cptnsolo77 »

That was a GREAT explanantion and better than what I found on other sites. Makes sense to me and thanks for replying 8-) Onwards to the next suspect ! :lol:
* 2003 Vibe Auto Satellite Silver w / Moons & Tunes
* Kenwood Excelon KDC-X597
* Polk DB651-Speakers
* Soundproofcow Roadblock R sound deadening in all doors
* Drop In K & N Filter
* NGK Iridium IX Plugs
* 27 MPG City/Highway with AC on
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