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Old 08-27-2010, 07:48 PM   #1
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Default random sustained 5000rpm idle

I can drive the car, get out of the car, and it will randomly rise to 5000rpm for about 5-seconds then stop. I am confused. I loaded my .msq again and the problem still persists. I'm also getting a random backfire on upshift that I've never had before. I haven't changed the tune or the hardware and suddenly the car is missing at high rpm again, random backfire, and this idle ****. I have about 20-miles on the plugs.

I wonder if something electronic is going south.
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Old 08-27-2010, 07:50 PM   #2
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I can drive the car, get out of the car, and it will randomly rise to 5000rpm for about 5-seconds then stop. I am confused. I loaded my .msq again and the problem still persists. I'm also getting a random backfire on upshift that I've never had before. I haven't changed the tune or the hardware and suddenly the car is missing at high rpm again, random backfire, and this idle ****. I have about 20-miles on the plugs.

I wonder if something electronic is going south.
When my Idle Air Motor circuit died... I had the same **** happen. You may wanna look at that first since you described my car for about a week prior to it going south completely. Granted this was on an Adaptronic; but the circuit is pretty much the same anyhow...

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Old 08-28-2010, 12:28 PM   #3
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did the vacuum line to the MS become unplugged and/or split?
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Old 08-28-2010, 12:44 PM   #4
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did the vacuum line to the MS become unplugged and/or split?
doubtful considering it drives wonderfully as usual with random weird symptoms. I'm going through every wire again today and check everything.
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Old 08-28-2010, 12:53 PM   #5
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Wirelessly posted

MS vac line shouldn't be big enough to allow a 5000 rpm idle. In fact when mine got unplugged it still idled at the same rpm, just at a 10:1 AFR since MS thought it was at 100 kpa all the time. Any idle up from the vac leak was trumped by stumbling from being pig rich. I also have a parallel install with the OEM ECU controlling idle so maybe that makes a difference. But I agree the the IACV would be a suspect.
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Old 08-28-2010, 01:00 PM   #6
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the backfires thing made me think that if the map line was compromised it would fuel pig rich.

only way to increase rpms at idle is to bypass the TB butterfly...so there's a leak somewhere, and the IACV could be the actual cause. When mine went out it did something similar...i had to unplug it and bring the idle speed screw all the way in and i was still idling at 1500RPM.
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Old 08-28-2010, 01:01 PM   #7
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the backfires thing made me think that if the map line was compromised it would fuel pig rich.
Its backfiring on high rpm upshifts only, and pretty random. However, I should note that it was pretty cool and made the cars behind me swerve, stop, and someone caught up to me to let me know that a huge fireball came out of my car.
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Old 08-28-2010, 01:46 PM   #8
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A 5000 rpm idle shouldn't be anything electrical. You don't have drive by wire throttle, I doubt the idle valve can do a big enough difference to rev the engine to 5000.

In other news:
https://www.miataturbo.net/race-prep-75/itt-weight-loss-throttle-body-style-49344/

With no AC or PS, and after tuning it with a hot engine, I have a rock solid idle, beyond it being slightly rough when cold.
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Old 08-28-2010, 01:53 PM   #9
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A 5000 rpm idle shouldn't be anything electrical. You don't have drive by wire throttle, I doubt the idle valve can do a big enough difference to rev the engine to 5000.

In other news:
https://www.miataturbo.net/showthread.php?t=49344

With no AC or PS, and after tuning it with a hot engine, I have a rock solid idle, beyond it being slightly rough when cold.
I was outside of the car looking at the t-body and it rose to 5k rpm without touching it; definitely not mechanical. The throttle is not moving, it has to be the idle valve.

Thanks for the link, I may do this today in my mission to simplify everything...if its possible on a 99.
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Old 08-28-2010, 02:14 PM   #10
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Its backfiring on high rpm upshifts only, and pretty random. However, I should note that it was pretty cool and made the cars behind me swerve, stop, and someone caught up to me to let me know that a huge fireball came out of my car.
so whats the problem rockstar? lol
does your rpm drop right away if you put the car in gear? or put it in neutral and back in a gear? my car does that every now and then, only stays at 2500rpm but if i cutch in and put it back in gear it drops back to idle right away.
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Old 08-28-2010, 02:25 PM   #11
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so whats the problem rockstar? lol
does your rpm drop right away if you put the car in gear? or put it in neutral and back in a gear? my car does that every now and then, only stays at 2500rpm but if i cutch in and put it back in gear it drops back to idle right away.
Nope, unchanged with gear unless I mechanically engage the clutch.
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Old 08-30-2010, 12:21 PM   #12
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Sounds like either a pretty decent sized vac leak or bad idle valve.
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Old 08-30-2010, 12:42 PM   #13
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Sounds like either a pretty decent sized vac leak or bad idle valve.
I let it idle for about 30-minutes yesterday and can't seem to find anything in terms of wiggling wires or visually inspecting them so I guess its the idle valve. Its time to remove it all-together.
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Old 08-30-2010, 12:49 PM   #14
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Try unplugging the valve. If there's a short in the wire between the ECU and valve, it would cause the valve to open fully.
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Old 08-30-2010, 12:53 PM   #15
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Try unplugging the valve. If there's a short in the wire between the ECU and valve, it would cause the valve to open fully.
Pulling the wire makes it "chug" at idle.
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Old 08-30-2010, 01:14 PM   #16
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Can you hook up the laptop and see what duty % the MS is commanding when you get the 5000rpm idle?
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