Violent shudders at low rpm and high load: Spark blowout?

BLOZ UP

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#1

Violent shudders at low rpm and high load: Spark blowout?

10-28-2013, 03:58 PM

Ok, I'm not entirely sure what I'm experiencing.

The problem I'm having is that at low RPMs, 2500-3500, and high load (boost), the car violently shudders. Pretty sure it's misfiring, as my datalogs are showing errant RPM readings. This only happens in 4th and especially in 5th gear, at 100% throttle, below 3500 RPM. Full throttle at higher RPMs results in normal ass hauling.

I'm seeing 2psi at 2500 RPM, 15psi at 3000, and 18psi at 4000.

I think it's spark blowout. I've encountered spark blowout (I believe) before on my old AE, but only at high RPM and gapping my plugs down to .030" or so fixed it. I've tried as low as .025" in this case, and while it seems to improve the symptoms, it doesn't go away.

My next theory was that my worn out, inefficient engine was surging. Mapping out 5th gear pull datalogs over my compressor map, that doesn't seem likely (I'm using VE values from Megasquirt, which might be off, but are eons better than guessing).

So what gives? Attached files

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G-E

Junior Member

Posts: 6320
#2

10-28-2013, 09:07 PM

I'd assume the timing is going screwy, either too much since you don't have egr, or your timing is wobbling due to bad signal… if it's the latter, only a scope will show anything meaningful
BLOZ UP

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#3

10-29-2013, 09:02 AM

Hm, good idea. I briefly checked it at the time, but I can post the map. IIRC it was around 18 degrees @ 3000 RPM @ 15 psi

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BLOZ UP

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#4

11-12-2013, 10:10 AM

Gapping it at 0.030" except on #4, which is at 0.040", solved it. Also pulled a couple degrees timing down low. It looks like cylinder #4, which has low compression was the culprit, causing a chain reaction of misfires. Setting it to 0.035" or lower and it doesn't fire at idle. Setting it lower than 0.040" and it misfires at boost. Got me.

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FlawleZ

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#5

11-13-2013, 10:21 AM

What were the compression numbers ? Doesn't make any sense increasing gap improved combustion consistency on a low compression cylinder.

Originally posted by Andrew84zx
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BLOZ UP

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#6

11-13-2013, 10:56 AM

FlawleZ;n763615 wrote: What were the compression numbers ? Doesn't make any sense increasing gap improved combustion consistency on a low compression cylinder.

Why not? You decrease gap on increasing compression (boost) since the amount of air the spark has to cross is greater. You increase gap when the amount is less, otherwise it may not ignite. At idle that cylinder never fired and eventually fouled with a small gap. Not sure what happened in boost, but this is what's working.

Compression is 110psi (1-3, 5, 6), 75psi or so on #4

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