AFRs bouncing at idle, could it be my LC1 sensor? - Miata Turbo Forum -Boost cars, acquire cats.

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Old 08-15-2016, 02:39 AM   #1
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Default AFRs bouncing at idle, could it be my LC1 sensor?

The other day I had my car tuned by Savington (91 with MS2e and LC1) and he told me my idle problem was most likely an exhaust leak near the wide band sensor. Ok, so the wideband is in the end of the down tube right before the cat. Pulled the v bands apart and pulled out the cat. Looked like a very small amount of exhaust might have been leaking out of the space were the two ends of the ring come together that sit in the groove between the flange on the pipe and flange on the cat. I used some Super Copper gasket maker in the grooves between flanges and on the ring and bolted it back together again. It was up on my lift at home so I climed in and stared it up. The AFRs bounced around as usual and I looked and felt around every joint of e exhaust system including the WB bung and couldn't see or feel any leaks.

Now I'm wondering if it could be the WB sensor is going bad? I did a free air recalibration when the cat was out but I'm not sure what else I can do? I got the sensor second hand and it seems to be doing the right things when I drive the car but it idles like ****. (See video below) Is it time for a new sensor?

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Old 08-15-2016, 02:48 AM   #2
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Doubtful. Your idle isn't perfect, you can hear it hunt.
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Old 08-15-2016, 05:33 PM   #3
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Are you running sequential injection? I recently rewired the harness on my 90 (w/MS2e) for sequential and the first thing I noticed was the increased AFR stability at idle.
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Old 08-15-2016, 06:11 PM   #4
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No, just standard batch with Force Flow injectors. The MS2e was built to my specs from Reverent and his base maps have been know to be solid.

I have a feeling I screwed up the idle somehow when I first got it running and started using the autotuning feature on TS. I didn't know the trick about setting it not to auto tune below 1,500rpm so I spent a fair amount of time with autotune on while I was idling. However, I feel like it was still hunting around even from the first time I got it started.

I've been looking up tuning advice for a surging idle but they mostly point toward idle valve PWM values which I not to keen to change since I'm assuming Reverent set them right and I'm more likely to screw something up.

Typically how much do see RPMs and AFRs change while at idle? It seems like videos I've seen of people trying to fix this are better but not super steady like you would see on a car with a stock ECU.
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Old 08-15-2016, 10:06 PM   #5
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I also have 91 with a rev MS2e, but an AEM wideband. I see +- .3 difference on my AFRs at idle with +- 25 RPM. I am still batch and I'm sure I still have plenty of room for improvement.
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Old 08-16-2016, 12:03 PM   #6
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Improperly tuned idle cells.
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Old 08-16-2016, 02:58 PM   #7
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What's your idle pulsewidth?

I don't think it's possible to idle smooth 14.7 with any 400cc+ injectors in batch fire, especially on a 1.6.

Running sequential I am firing around 1.8ms to get to 14.4 AFR. Assuming you are similar, you'd have to fire two 0.9ms pulses per cycle. That's way too small, the actual fuel output will not be consistent especially with changing temperatures. Since the squirts are happening twice, reversion between ports is a real problem. All these things together make it impossible to not have a surging idle.

You need to upgrade to sequential fuel or really fatten up the idle AFR to get into consistent injector pulsewidths.

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Old 08-16-2016, 03:08 PM   #8
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Quote:
Originally Posted by deezums View Post
You need to upgrade to sequential fuel or really fatten up the idle AFR to get into consistent injector pulsewidths.
Like I posted above, switching to sequential injection stabilized my AFR and idle more than the literal months of tuning I had attempted leading up to it.
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Old 08-25-2016, 02:51 AM   #9
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Quote:
Originally Posted by akbloom View Post
Like I posted above, switching to sequential injection stabilized my AFR and idle more than the literal months of tuning I had attempted leading up to it.
I'm not opposed to trying sequential injection if it is reversible every 2 years for Smog (live in CA).

I have an extra injector harness that I could rewire. From what I understand I would run two extra wires to pins on the main harness that the stock ECU doen't use but corresponds to separate injector pulses on the MS.

Is it true that in this senerio if I replace the modified injector harness with a stock one and replaced the MS with the OE ECU it would work like stock (also assuming replacement of stock injectors)?
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Old 08-25-2016, 10:09 AM   #10
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All of that is correct. Yes. I just recently wired my 1990 for sequential. Big difference.

I couldn't grt idle stable at anything more than 12.8:1 or so with the GT500 injectors running batch. Pulse widths were down near 1.3-1.4ms to idle, even with a bumped up rpm.

Now I am idling around 900-1k rpm at 14.7 afr and pulse widths around 1.75ms.

Swaping the harness out, stock injectors back in and stock ecu will put everything back, yes. In the harness I made I used stock injector #3 connector to run #2 (since stock, 1 and 2, and 3 and 4 are pairs) and then snipped the wires for stock 2 and 4 and made them 3 and 4 on the rewired harness. There are pictures in my build thread.
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