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1.6 High Idle

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#1
juliancates

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This new-to-me 1.6 tends to idle at around 2k.  (It also surges when the RPM is held slightly higher, but that's another problem.)

 

The adjustment screw is all the way down.

 

It's slightly lower with the air valve (AV) blocked off.

 

With the AV blocked, connecting TEN-GND in the diagnostic connector doesn't have a noticeable effect.

With the AV unblocked, connecting TEN-GND will decrease it slightly.

 

The thing that really gets me, is that spraying starting fluid in the vicinity indicated in the attached photo causes the idle to go DOWN significantly, to around 1k or so until it evaporates.

 

Also, I'm not sure what A/F is supposed to be at idle, but this seems to idle rich at around 12, sometimes high 11's. Not sure if that matters.

 

Given the above, any thoughts on what to chase down next? (And why the heck are ISCV's so dang expensive?!!)

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#2
juliancates

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*sigh*

 

Haven't changed a thing.

 

With the car cooled down and the AV still blocked off, went to go start it up after that last post just to make sure I was remembering everything right.

 

It started REALLY rough, almost as though it didn't want to run before eventually smoothing out.  Thought I heard a loud pop from the AFM as it was running rough. Then, when it smoothed out, it idled at ... around 1K.  Connecting TEN-GND has no effect. Spraying the starter fluid still has the same effect.

 

Going to let it cool down again and see if it stays the same.


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#3
OrangeCrush86

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I have the same problem with my car. I never solved it so my car just idles at 2k. More air is better right?

 

Honestly though I'm almost certain I need to replace the idle air control assembly, but it isn't worth $500 bucks to me.



#4
Steve Scheifler

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Yes, most likely to be the IAC, though you most recent experience sounds a littke off. But that may be other adjustments you’ve made playing with it. You can clean it, check electrical connections and such, but I rarely hear of anything working short of replacement. They are ridiculously expensive even after-market but a used one is of course a total crapshoot at this age.

Has anyone had success actually repairing them. Dave?
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#5
Steve Scheifler

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The rich idle is common when the AFM has been “tuned”. It isn’t intentional, just a side effect of getting more power where you need it.
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#6
Tom Hampton

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The high idle happened to me once after a wreck.  The hood bent down and impacted the IAC valve and bent the bracket down.  Only figured it out after taking all the intake plumbing off and noticing that the butterfly was not quite closed.  It doesn't take much.  


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#7
Dave D.

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Remove the connector and check the resistance on the IAC, the shop manual has the spec(at home now, so I don't have AllData to look it up). The internal connection from the activating coil to the pins of the connector break. The problem is that they are potted in, so you have to cut open the area,dig out the potting to get at the wires( I reseal with black RTV). Then resolder the broken connections and recheck the resistance. As with any other issue in this world, there is a youtube how to and also there are posted repairs with pics on some Miata sites, just Google Miata IAC valve problems!


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#8
juliancates

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Well, after sitting all night, I hit the starter and it fired right up, no hesitation. Idles at 1K. Only thing different at this point between now and the past few weeks is that the air valve is blocked off. I couldn't get it to do the surging thing, either. 

 

Idles way more rich at 1K... like 9.8.

 

I'm loathe to change anything at this point, though I'm not sure about the legality of blocking off the AV. Might have to do the freeze/boil thing and make sure it's not getting stuck open.

 

Which leads me to another question... do we even really care about idle on these cars? Shouldn't they spend most of their time at WOT anyway? If it comes back, how much do I worry about it?


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#9
OrangeCrush86

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Idle doesn't really matter, but if you let the car warm up and go into closed loop you should not have 9.8, it should bounce around 14.5 so you might have other issues.

 

It would be interesting to dyno a car with the AV blocked off, and then dyno again with AV forced full open. Maybe there is 1 hp there ;) .



#10
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https://www.miata.ne...faultcodes.html


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#11
juliancates

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Thanks for that, filing away for future use.  No codes now.


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#12
Steve Scheifler

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Idle doesn't really matter, but if you let the car warm up and go into closed loop you should not have 9.8, it should bounce around 14.5 so you might have other issues.

It would be interesting to dyno a car with the AV blocked off, and then dyno again with AV forced full open. Maybe there is 1 hp there ;) .


For a stock street car yes, but there are limits to how much the closed loop logic will trim the fuel. Assuming that the AFM has been “adjusted” significantly, and likely rotated physically, it may not be capable of attaining target AFR.
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