Supercharged KA24 misfire under load

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Grantspec
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Supercharged KA24 misfire under load

Post by Grantspec »

I have a supercharged ka24de that is having serious ignition issues. Here is the rundown.

I previously ran a chipped ecu with an altima MAF, 370cc injectors, and a walbro fuel pump. AFR's were good but at about 75% load it would immediately start misfiring to the point where it would no longer continue to increase RPM. I tried new plugs, brand new magnecore plug wires, gapped the plugs down to .028 (NGK bkr7e's). Still no luck.

Fed up with the issue and 20 year old nissan wiring. I replaced the engine management with megasquirt and a brand new engine harness. It is now a MAP based speed density setup running a FORD EDIS ignition system which is a waste spark coil pack setup. Still using 370 cc injectors and walbro fuel pump. I began tuning yesterday and am having the exact same ignition misfire issue. AFR's were 11.5:1 and at about 8 psi boost it begins violently misfiring. I have tried richer and leaner to no avail. I feel like at this power level there should not be this issue (should be around 200-215 hp at 10psi). I checked compression and it is good at 180 psi cylinders 1-4.

To summarize. I am getting a misfire under high load situations before and after standalone fuel management and complete ignition system change. Compression and AFR's are good.

Please someone give me some clues of things to check I am out of ideas..
Grantspec
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Post by Grantspec »

My main suspicion at this point would be the timing chain tensioner going bad causing the cams to be at unpredictable angles at a certain power level.

Anyone else see this as feasible?
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Jmcc
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Post by Jmcc »

since you got a ms. whats the voltage at that rpm it starts missing?
Grantspec
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Post by Grantspec »

From looking at my data logs it looks like 13.6.
sh1tsweak
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Post by sh1tsweak »

check fuel pressure while its acting up? you changed everything but the injectors and pump by the sounds of it. check for boost leaks too. i guess if you have MS you arent running a maf anymore so boost leaks shouldnt do that really... you still have the butterfly power valves in the intake manifold?? my friends turbo svt focus did the same thing until we unhooked his.
Grantspec
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Post by Grantspec »

Fuel pressure seems to be good. As i said it doesn't seem to be leaning out. The wideband gauge is pegging lean whenever it begins misfiring. But this is due to the un-combusted oxygen going into the exhaust.

The butterfly flappers have been removed.

I appreciate the input folks, please keep it coming. I am at my whits end with this thing. Hasn't ran to its potential since I supercharged it a year and a half ago.
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joshpntbl07
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Post by joshpntbl07 »

wouldnt a missfire cause a rich condition because of unburnt fuel?
Grantspec
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Post by Grantspec »

wouldnt a missfire cause a rich condition because of unburnt fuel?
No, O2 sensors do not measure fuel they measure the unreacted oxygen in the exhaust stream to determine the AFR. If you have a misfire, all of the oxygen that goes into the combustion chamber is left unreacted and will register on the gauge as a lean condition.
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Re: Supercharged KA24 misfire under load

Post by klattr1 »

Aren't things like this fun to figure out? Just a couple ideas:

Is it misfiring at 8 psi in high RPM's only or does it occur at lower RPM's too at full boost? Have you tried gapping your plugs down even further?

Can you narrow the problem down to a particular cylinder or does it occur on all 4? Could it possibly be an injector issue?

What about a potential discrepancy in the crank trigger sensor signal (this is probably not an issue though since you had the problem on factory style ECU)?
Grantspec
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Post by Grantspec »

Ok, so I gapped the plugs down even further and that seems to have done the trick (down to .020). I had ruled this out initially because i had tried this with the chipped ecu setup with no success. But all seems to be well now. Thanks for all the input fellas.

I'll post some pics and info about the megasquirt setup in my build thread in the supercharged KA section shortly.
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Post by klattr1 »

it's always something simple :)

Lowering the plug gap might be a bandaid on the problem though. I was around .030" gap on stock S14 ignition at 10psi on my supercharged setup. Your gap of .020" seems a little low. What is the voltage coming into each coil? Also make sure the belt isn't slipping that spins the alternator.
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all shal perrish
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Post by all shal perrish »

incorrect dwell settings could contribute to weak spark as well....
just sayin
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nismoautoxr
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Post by nismoautoxr »

in your ignition settings on your megasquirt ...what is your dwell settings?

If it is lower than 3.0-4.0 MS then that will absolutely cause this .

I am running a very similar setup as you . SC ka with a megasquirt but I am using LS2 coils in wasted spark and if I go below 3.0ms it will misfire above 4k badly.

Also are you running a narrow or wideband 02s? If you are relying on AFRs in Tuner Studio through a narrow band o2s then you may not being getting the whole story on your afr's when the problem happens .

Are you using a boost sensitive FPR? I have had injectors that if pushed beyond 60PSI of FP ...they would not squirt but this is with bosch top feeds though (maybe others too IDK)so you really need to be able to see accurate AFRs through a wideband and FP under boost .

Good Luck,

Ricky-
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95 SE ,LS1 N/A and T56, 230/232 @.050 cam, MS3 with MS3X,GC coilovers,17x8.5 Enkei RPF1 on 255 40 17 RS3 Hankooks, 377HP/351TQ , megasquirt3 fully sequential
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