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Old 08-11-2015, 10:20 AM   #1
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Default IAC Valve stuck open? Plugged in goes straight to redline

Car: 95' 1.8 Turbo on EMS

Here's the results I got w/a multi meter on the plug, what do you guys think? Is this normal?

11.6 ohms

7.2 V

9.25 V @ idle

When unplugged it starts a little rough, but with my foot on the pedal and can get the idle to normalize fairly quickly. When i plug in the IAC you can hear it make a sucking sound immediately and it wants to rev all the way to red-line. Not nice when the engine is cold.

Can anyone help me with this. I'm close to buying a new IAC for $60, but I don't know if this is more of a wiring issue or equipment issue. Ran fine for years and I didn't change anything. We did auto-x that day, but it ran fine until a mile from home when the revs just went into the red suddenly after using the cruise control switch. Car is running a EMS...no cel on the dash.

I can't seem to find anyone that's had this issue on not just a miata, but any car before. Really bizarre. The IAC was removed and cleaned and looks fine, but who knows that's going on within the black cylinder of mystery attached to the casting. Hopefully someone can help here.
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Old 08-11-2015, 10:25 AM   #2
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When you say plug, do you mean the electrical harness or do you mean the tube?
if electrical, then it's likely a wiring issue
if tube, then it's likely stuck
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Old 08-11-2015, 10:32 AM   #3
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When I say plug I mean the green plug that goes into the harness connector mounted to the motor.
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Old 08-11-2015, 10:49 AM   #4
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what ECU is controlling it? and have you verified the computer isnt trying to ramp it open?
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Old 08-11-2015, 10:56 AM   #5
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Its a AEM EMS II . I've had it plugged in, but not really familiar with the software or how to look at that. There are some idle parameters, but nothing that stuck out as adjustable as it pertains to the IAC.
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Old 08-11-2015, 10:58 AM   #6
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Old 08-11-2015, 12:24 PM   #7
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Google "AEM EMS jonathan fasking"
Start at Part 1, watch the entirety of the videos without skipping a beat and pay attention.

That'll give you a quick overview of the system and how to use it.
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Old 08-11-2015, 12:56 PM   #8
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Yes I realize there are videos out there to learn the software, but all things considered how could the computer possibly change it's own internal parameters suddenly? Can it just decide that the IAV goes wide open all the time. I'm wondering more so if the voltage and ohms are normal and if it's more likely a wiring issue or the valve itself. Again unplugged it's fine, plugged in it surges...just touching the connector to the pins causes this surge, so it's instantaneous.
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Old 08-11-2015, 01:06 PM   #9
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Well if you don't have the slightest idea what the **** you're doing, it's very easy to connect the wrong wire, set the wrong duty cycle, etc.

If you wanted to be able to say "car not vroom anymore, spoonfeed pls" you should have gotten a car with a warranty and factory wiring. We have no idea how you miswired or misconfigured your ECU, sensors and solenoids.

Hit the tutorials, grasshopper. Also, go back over your install instructions and see if you missed anything when installing and setting it up.
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Old 08-11-2015, 01:17 PM   #10
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Listen dick....The tune was done 3 years ago as was the EMS, installed by a shop not myself. It's run fine up to this point, no wires are crossed or have been altered since I've owned it. If it was wired incorrectly it would have had this issue from the start. I'm not asking to be spoon fed I'm just thinking this is an odd thing, and I can't find anyone that's had this happen to them. Once a tune is set the computer can not go inside and change it's parameters, and there are no codes, so there's nothing to go on which also leads me to believe it's the sensor itself.

go troll somewhere else, or at least read the post entirely so you don't look like a fool.
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Old 08-11-2015, 01:24 PM   #11
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i loled.
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Old 08-11-2015, 01:27 PM   #12
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Now only if we had props...
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Old 08-11-2015, 01:29 PM   #13
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I dunno what kind of responses you're expecting on this forum, where the average user rolls their own engine management or at least installs and tunes it themselves.

No one here is going to know your setup as well as you do. Either you get enough expertise to lay hands on it yourself or you pay others an arm and a leg to do it for you. We can't really do much for you over the internet.

If my idle control valve was fucked up and went wide open every time I plugged the harness in, I'd probably fiddle around in the ECU to see if it was going full open on purpose or if it was a hardware malfunction. If it's a malfunction, just buy a new one.
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Old 08-11-2015, 01:39 PM   #14
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I answered his question in post 2. He refuses to brain
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Old 08-11-2015, 02:16 PM   #15
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If someone would be willing to take a multi-meter do their IAC and tell me if they're getting the same or similar readings that would be helpful. Beyond that yes I'll have to dig into the tune.

I'm not refusing to "brain", I was just looking for someone that my have had a similar issue in the past. Or someone that knows the EMS that could guide me through where to go within the software. A buddy of mine who knows MS took a look and couldn't figure out where these parameters were set. That was after watching 3 of the videos on youtube. There's so much detail in AEMs software that it makes a guy like myself that has never tuned a car intimated, but I'm willing to learn.
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Old 08-12-2015, 07:49 AM   #16
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Not sure how much help this will be but here goes. The IAC is pulse width modulated so the voltage shouldn't change. The resistance you are quoting might only be useful if someone else had an EMS as that's a value coming from the EMS. Most of us have MS so even if we went out and read these values for you it might be useless or lead you down the wrong path.

Now if you were to read ohms across the IAC valve you can at least compare that to a known good valve.

Your best bet here is opening up the software and looking at the data stream from the EMS. Look for idle % or something like that. If its commanding high with it plugged in, that could be the start of your diagnosis. And since the idle circuit "should" be a closed-loop circuit, you might have to do a good bit of diagnosis.

Or just spend the $60 and eliminate the IAC valve from suspicion. Good luck.
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Old 08-12-2015, 11:31 PM   #17
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Sounds like something is making the pulse widths fail to modulate.
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Old 08-13-2015, 11:08 AM   #18
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sounds like that might be it. I should have some time this weekend, so I'm going to look into it a little more. Thanks for the input.
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