Idling at an AFR of 20-21, what's the cause? - Miata Turbo Forum - Boost cars, acquire cats.

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Old 09-16-2008, 08:44 PM   #1
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Default Idling at an AFR of 20-21, what's the cause?

Recently I've been getting AFRs of 20-21 at idle. It slowly works its way up from about 15 or so and stops and alternates between 20 and 21. Before I would idle at around 14-15. I'm running a boost setup with the voodoo box but as far as I know that only comes in under boost. Also when I'm driving under cruising conditions I'm getting a pretty steady 14.5-15 AFR and a good 12 under boost. So it just seems my idle AFRs are messed up. What could cause this?
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Old 09-16-2008, 08:48 PM   #2
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vacuum leak?
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Old 09-16-2008, 10:27 PM   #3
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misfire...
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Old 09-17-2008, 02:11 AM   #4
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wouldn't a misfire manifest itself under cruising too? How could I determine if it's actually a misfire?
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Old 09-17-2008, 02:16 AM   #5
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does it idle rough when it says 20-21???
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Old 09-17-2008, 02:19 AM   #6
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Idles fine when it reads 20-21.
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Old 09-17-2008, 02:22 AM   #7
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that's funny cause when mine used to idle in the 20's when it was messed up, it was rough as hell and missing.... so maybe its lying to you
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Old 09-17-2008, 01:37 PM   #8
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What type of WBO2 do you have?
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Old 09-17-2008, 01:40 PM   #9
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someone just posted a thread about this same problem, and they ended up having barometric correction on by accident worth checking
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Old 09-17-2008, 02:45 PM   #10
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Dead injector? I have to disagree about the misfire. It usually does not cause a lean condition. It will make the car have a nasty vibration that you can feel at idle and cruise.
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Old 09-17-2008, 04:44 PM   #11
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I think you'd feel a dead injector in much the same way you would feel a misfire. Bottom line, one cylinder isn't firing.

A WB02 doesn't detect fuel it only knows about O2. A misfire due to too much fuel could still be seen as lean.
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Old 09-17-2008, 05:09 PM   #12
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Wouldn't a dead injector also affect my cruise and boost AFRs?
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Old 09-17-2008, 05:29 PM   #13
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What WB02 do you have? IF you can, I'd do a free air calibration.

You are running both stock and WB02?
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Old 09-17-2008, 06:00 PM   #14
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I'm running my normal O2 sensor and a JAW WBO2. Already did a free air calibration when I put it in. It was working great ever since then about 3 weeks ago. Now it just suddenly botched at idle and fine under cruise and boost? That's what makes me think it's not the WB unit.
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Old 09-17-2008, 06:47 PM   #15
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If you get raw fuel on the wideband, it will read dead lean like that. I would try and make sure that you arent in fact running very rich.
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Old 09-18-2008, 12:58 AM   #16
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if i were running very rich wouldn't that be reflected in my cruise and boost AFRs?
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Old 09-18-2008, 02:15 AM   #17
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Not if the condition only exists at idle. I'd say that since it is only happening at idle your problem isn't mechanical.
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Old 09-23-2008, 07:40 PM   #18
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Got some new developments if anyone can make heads or tales of it. Well, after driving for about 20 minutes the AFR will read ~15 at idle pretty consistently. On start up that's different, though, it will hold 15 for a good minute before it starts to climb up to 20.

When driving I've noticed some new sounds coming from the exhaust/engine. It's sort of a grumbling or gargling sound and muffled popping when I let off the gas. It sounds similar to the sounds heavily modified turbo cars make on deceleration with all the backfiring and whatnot. Granted it sounds kind of cool, it's really worrying me. Otherwise the car feels completely normal. The idle MAY sound a little different but that could be me being paranoid.

Also now my boost numbers are around 14 opposed to the desired ~12. Sometimes I can see 12 for a second and it will hold at 10.18 for about 5 seconds then go back to normal reading.

When I get the chance I'm going to check my timing, plugs, wires, and injectors. Not sure where else to look. Could a faulty AFM cause this problem?
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Old 09-24-2008, 07:16 AM   #19
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take a piece of paper and put it next to your exhaust (when your car is warmed up.) see if paper bounces like crazy. if it does - its a misfire.
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Old 09-24-2008, 07:56 AM   #20
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Popping is usually unburnt fuel burning up further down stream (cat, pipe)
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