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Bad IAC valve or possible boost leak?

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Old Dec 17, 2013, 01:06 AM   #1
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Bad IAC valve or possible boost leak?

Hey guys so lately at when i'm driving and i come to a stop light and i push in the clutch to roll to a stop my car will act like it wants to die for a second until the idle evens out a little and even sometimes when i'm just idling the idle will move around a little and sometimes it will make a misfire type sound and the AFR's will jump really rich for a second then will lean out some and then start to even out again. I've heard that this might be because i have a bad idle air control valve. I'm just hoping its not a boost leak or anything like that lol. Any ideas or similar experiences? i just wanna know for sure its the IAC Valve before i go off and buy one lol. Thanks guys
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Old Dec 17, 2013, 01:25 PM   #2
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Originally Posted by mrm95 View Post
Hey guys so lately at when i'm driving and i come to a stop light and i push in the clutch to roll to a stop my car will act like it wants to die for a second until the idle evens out a little and even sometimes when i'm just idling the idle will move around a little and sometimes it will make a misfire type sound and the AFR's will jump really rich for a second then will lean out some and then start to even out again. I've heard that this might be because i have a bad idle air control valve. I'm just hoping its not a boost leak or anything like that lol. Any ideas or similar experiences? i just wanna know for sure its the IAC Valve before i go off and buy one lol. Thanks guys

From my experience, when my IAC went bad on my old evo, the car wouldn't high idle on cold start, it would fumble around at ~500rpm until warm, and then go to 800ish rpm. My idle would dip when coming to a stop as well. Typically the IAC would go bad on 03/04's from what I had read, I believe the 06 has a different, updated IAC.

Why would you hope it's not a boost leak? Boost leak testing is something you can do quickly, and by yourself. A new oem IAC is around $400...

What you describe could be caused from many things. Such as running a VTA bov with a MAF...etc
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Old Dec 17, 2013, 01:52 PM   #3
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Start with a boost leak test first, you can also test the IAC valve.
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Old Dec 17, 2013, 02:03 PM   #4
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When my IAC went out, my evo would not idle at all without giving it some revs. Drove fine but would die at any light or stop sign..... Check to make sure the vacuum line under the throttle body is plugged in. Mine popped off regularly until I zip tied it down finally. It also threw a check engine light for me and I had a rough, inconsistent idle.

Is your CEL on?
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Old Dec 17, 2013, 02:56 PM   #5
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A vacuum leak will give problems at start - you need to keep the gas pedal depressed or the engine will die. After warmup the car drives normally.
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Old Dec 17, 2013, 08:48 PM   #6
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What your describing is a common problem with the evos. Me and a guy named shaesta are trying to track down exactly what is causing it, but it could be anything from your dv to ps pump sensor ( a suggestion by db) to something in your stock iacv tables in the tune. Shaesta thinks he figured his out with a used synchronic ( or however you spell it) dv but is unsure if the vacuum line setup he has is safe. I'm still testing but I'm down to a small leak from my biss screw that I'll b fixing as soon as my mailman stops stealing my mail, the dv and my tune. All the other stuff I have replaced with no effect on the problem.
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Old Dec 17, 2013, 10:18 PM   #7
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Thanks for all the help guys.

Am i being specific enough or do you guys need any more details?
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Old Dec 18, 2013, 06:45 AM   #8
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My car used to do similar things. It is an 03. By adjusting my low load low rpm timing higher, getting my maf and fuel trims dialed in, and raising my idle rpm to 1000 with proper iac adjustments it has been working well. My car does have cam's, intake, forge rs bpv, etc.

It seems like you may need a bit more fine tuning. I didn't really see what mine was doing until I stared at the data logs and saw the load jump up and the timing go low.

Good luck.
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