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Old 02-04-2007, 01:53 PM   #1
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Angry boost oriented miss

I've been driving around for over a month on mechanical boost (~8psi) trying sort my EBC issue. Prior to that I'd run on a MBC at ~12-14psi w/o issue. Now I'm back on the MBC w/o any changes to it or anything connected and I'm getting miss (feels like a misfire) right at peak boost. As soon as peak boost is reached I get the miss, and it continues throughout the acceleration at that boost level. If the throttle is backed off and boost drops to around 9psi the miss goes away. I thought maybe it was because I'd widened the plug gap to .040, but that wasn't the case. Installed new plugs at .035 and nothing changed. Next is plug wires. Any other suggestions for checking?

Datalog of the last run linked below of the last run for those interested. Fuel/Inj look steady, I suppose AF is bouncing around from unburnt fuel. - rob

http://ebersol.com/FE3/DL020407a.TXT
or
http://ebersol.com/FE3/DL020407a.TXT.zip
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Old 02-04-2007, 01:58 PM   #2
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maybe even .030 might help...

what heat range are the plugs?
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Old 02-04-2007, 02:01 PM   #3
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I'd try dropping the gap down to .025 or something similar and seeing how it behaves. If the miss goes away my guess would be either wires or one of the coils.
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Old 02-04-2007, 02:07 PM   #4
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Wow man, if it's not one thing, it's another.

I've not been able to get away with a gap that large at 9 psi, so it may be worthwhile to close em down some and see if the spark isn't getting blown out.
I have a good spare set of wires you can try, but it's not really worth the drive is it. If your wires aren't all that old, I'd think it's maybe the coil, or a bad connection at the coil.
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Old 02-04-2007, 09:11 PM   #5
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I'm running the autolite 3922s- I think they're a 7. My tune is rich now, I know, but I can't see how the cold weather (the only thing that's changed) could cause this. I've got a collection of plug wires I can swap out, coil too. Checked in to the gap, and as mentioned, most run a gap around .025 with boost. I thought I read the AFloyd was running .040- but with ngk iridiums I think. So I can crank down on my gap. And tweak my tune now that I've got a good log.

Ben- This is the second time I've driven my car since I met up with the miata crew!

I hope it's the gap- haven't really had much trouble except for the EBC and that looks like it's actually a component on the ecu, which is going to require some solder work if I can actually find a replacement. Meantime, I'll just stick with the MBC. - rob
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Old 02-05-2007, 03:18 AM   #6
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Quote:
Originally Posted by m2cupcar View Post
I'm running the autolite 3922s- I think they're a 7. My tune is rich now, I know, but I can't see how the cold weather (the only thing that's changed) could cause this. I've got a collection of plug wires I can swap out, coil too. Checked in to the gap, and as mentioned, most run a gap around .025 with boost. I thought I read the AFloyd was running .040- but with ngk iridiums I think. So I can crank down on my gap. And tweak my tune now that I've got a good log.

Ben- This is the second time I've driven my car since I met up with the miata crew!

I hope it's the gap- haven't really had much trouble except for the EBC and that looks like it's actually a component on the ecu, which is going to require some solder work if I can actually find a replacement. Meantime, I'll just stick with the MBC. - rob
I do run stock heatrange Denso Iridium IK20's. They are gapped stock because trying to gap them risks breaking the tip. I too had a miss and it turned out to be my igniton coil. So it was swapped out for a one with 50k miles. Since I have run up to 16psi w/o issue. If you are using copper plugs a gap of .030 should suffice, if it doesnt then check the coil and possibly the iginitor if using the 1.6L ignition ( not sure if you are, just saying ).
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Old 02-05-2007, 03:21 AM   #7
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hey andy what measurement did you use to confirm a bad coil i checked all my ohms and mine specs out but do you know of another measurement?
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Old 02-05-2007, 03:25 AM   #8
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hey andy what measurement did you use to confirm a bad coil i checked all my ohms and mine specs out but do you know of another measurement?

Yeah, i checked the resistance and mine was out of spec. Even if your coils do pass the resistance test, it still doesnt necessarily mean that they are good.
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Old 02-05-2007, 03:28 AM   #9
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Good to know thx.
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Old 02-08-2007, 03:19 PM   #10
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Leaned out the fuel near 12psi and swapped out with some other used injector wires and it's 90% better. Hmmm... I need to find out what kind of OE wire setup I can use. I mounted the coil pack at the back of the head, but the engine is just long enough so that one of the wires isn't long enough (the number 4). So I have use two number 3s- which means two sets of Miata plug wires. I'm thinking wires.
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Old 02-09-2007, 01:26 PM   #11
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Not sure if it applies here, but my DSM will missfire at full boost (22psi) if the mixture gets much richer than 10:1.

It's easy to spot when it's that rich since it leaves a smoke trail behind the car.
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Old 02-09-2007, 01:50 PM   #12
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Not that rich. But not that far off either. I had some 10.3:1. I haven't looked at the logs yet to see what changed. I need new wire regardless.
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Old 04-24-2007, 12:24 PM   #13
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new wires fixed my miss
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