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Old 10-30-2015, 05:23 AM   #1
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Default ISC valve issue?

Hey guys.


My car is a '92 1.6 miata running on an MS3 built by rev.

Right now im trying to figure out why my idle is off.

When I test the idle by setting the pwm duty value to 0% while the car is warm, the idle wont go below 1300. Setting it to 100% brings the car up to 2350.

My idle is set to open loop just for easier troubleshooting.

Im wondering if this is just a setting I have configured wrong that is keeping my car from going below 1300 rpm, or is it possible my ISC valve is gummed up/failing? Im not really sure how to go about testing if my ISC is closing properly.

Any ideas?

Last edited by Fatvod; 10-31-2015 at 12:16 AM.
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Old 10-30-2015, 05:46 AM   #2
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The pwm range that actually does anything effective is around 10-20% wide, not 100%. With how I have my idle bleed screw set my range is 28-38. Numbers outside that range are unpredictable in how they'll respond, I attribute it to the solenoid valve maxing out or flailing around randomly.

Try to find the bottom of the range that works well, if you lower the number and the idle goes up or doesn't go down, you've gone too far. Adjust the idle to around 750 with the idle valve fixed at that number you just found, then adjust as necessary.
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Old 10-30-2015, 03:43 PM   #3
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Wow, that did the trick. The low end of my range is 28 also!

Thanks so much, that saved me from pulling apart the valve and cleaning it like I first assumed!
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Old 10-31-2015, 12:17 AM   #4
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So, the lower bound was pretty obvious when I reached it. But the upper bound can go to 100% without any oddities. It just continues to raise the rpm.

How did you determine your upper bound? Did yours actually run into issues after a certain range? And if so, what were these issues?
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Old 10-31-2015, 12:47 AM   #5
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Is there ever any reason your ECU is going to need to demand more than 2,000 or so rpm?

That's how I set my upper limit, it's about what it takes for 1500-2000rpm at full cold. There's really no reason I'd ever need anything more. Anything higher is asking for trouble with closed loop idle.
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Old 10-31-2015, 01:01 AM   #6
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Thats what I figured, I set the upper bound to a value that idles around 1700rpm, should do the trick.

Thanks so much for the help!

Last edited by Fatvod; 10-31-2015 at 03:38 AM.
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