[hotrod182]

So during the course of a qtr mile run, I am getting these strange misfire/20:1AFR spikes. You can see the AFR in the mustard color line in the logs below.
Sometimes it comes right during the shift, sometimes in the middle of the pull, etc.

The first log shows it occuring around 6500 rpm in the gear. The 2nd log around 6300 rpm. So somewhere around 120mph.

The 3rd log shows it not occuring till the shift into 5th gear. (130mph)
This run measured 124mph in the qtr mile (Vbox)

These spikes used to happen only when using the dryshot. However all these run were just plain 91/meth runs.

Not sure if it is a lean out situation causing a misfire. Or a misfire causing the lean 02 readings.

Each and everytime this happened, I immediately checked for faults. None were stored (except the typical catalytic conversion faults). I had the autoclear off in hopes of capturing fault. On nitrous, if the misfire was bad enough, I could get random cylinder # misfires. 2,4,6, etc. Generally, I had to stay on the throttle to get it to happen.

As you an see in these logs, I just kept the throttle planted. The episodes come and go away by themselves. Again, no limps, no codes.

At first I was suspecting a bent valve, broken spring, etc. But the more I think about it, I don't think a mechanical cylinder problem could manifest it self at 120mph, and then correct itself and pull cleanly to 145mph.

Plugs only had about 3000-4000 miles on them. I replaced them a few days ago. No improvement,

I replaced the HPFP a few days ago, no improvement.

All Injectors were replaced about 6 months ago. (recall)

Wonder if there is a logical way to measure the LPFP output under these conditions, but what numbers are normal for these conditions?

I did have a crank sensor gap fault a couple months ago, but it was a one time occurance, and during a trans mis shift/over rev. I don't think a crank sensor would cause a sudden spike/misfire a correct itself during a run, without triggering some kind of fault code.

I would almost suspect another bad injector, but again, why no fault codes, and random cylinders have recorded misfires at times.

[hotrod182]

Here is a vdieo showing what the misfire sounds like. This was on the dryshot and with open exhaust so you can hear it better. You can hear it around 90mph and again at 110mph. It never backfires or pops, it just kind of makes a fluttering type noise, and power goes kind of flat.

Also, here is a log of a quick 2nd-4th highway pull (40-120) with no spikes showing during that run. So perhaps from a stand still, or speeds above 130 it may have started to spike?

[GeorgiaTech335coupe]

Sounds like a misfire to me. I've had PLENTY of them. Dealing with some right now as a matter of fact. When I get my car to misfire it sounds like that at the beginning of the misfiring. When I stay on throttle it gets worse though then throughs an SES light. Yours seems to recover. Sometimes when my car misfires, it will throw an SES light, but won't through any codes. This can get really annoying when trying to troubleshoot. Also, I have replaced injectors with less than 6 months on the clock. See if your car misfires when lugging the engine. Start at a slow speed in 4th or 5th gear and then go WOT. From what I've found this is usually the easiest way to get a misfire to reveal itself.

[JamesM3M5]

Misfires shoot unburned fuel AND oxygen into the exhaust, which is why you see the severe 50:1 spikes. O2 sensors read excess oxygen content in the exhaust versus atmospheric reference (which is why you shouldn't mess with the O2 sensor wires near the sensor end, the reference comes from outside the sensor at the wiring outlet), and a misfire has lots of extra oxygen left over.

Chase down your misfire problem.

[themyst]

Bad o2 sensor maybe?

[hotrod182]

Originally Posted by JamesM3M5

Misfires shoot unburned fuel AND oxygen into the exhaust, which is why you see the severe 50:1 spikes. O2 sensors read excess oxygen content in the exhaust versus atmospheric reference (which is why you shouldn't mess with the O2 sensor wires near the sensor end, the reference comes from outside the sensor at the wiring outlet), and a misfire has lots of extra oxygen left over.

Chase down your misfire problem.

Yup, exactly what I was thinking. Misfiring spike in 02 in the exhaust stream. Of course by looking at the fuel trims, it looks like the DME is pulling fuel during these episodes too. So again, lean misfire, or misfire causing the DME to go on the reactive and lowering fuel to that cylinder? Its so hard at this point to differentiate between cause/reactions in the feedback system.

Originally Posted by GeorgiaTech335coupe

Sounds like a misfire to me. I've had PLENTY of them. Dealing with some right now as a matter of fact. When I get my car to misfire it sounds like that at the beginning of the misfiring. When I stay on throttle it gets worse though then throughs an SES light. Yours seems to recover. Sometimes when my car misfires, it will throw an SES light, but won't through any codes. This can get really annoying when trying to troubleshoot. Also, I have replaced injectors with less than 6 months on the clock. See if your car misfires when lugging the engine. Start at a slow speed in 4th or 5th gear and then go WOT. From what I've found this is usually the easiest way to get a misfire to reveal itself.

Sounds like a good idea. I will try lugging it in taller gears to see if it manifests itself. Perhaps I won't have to do a 130mph run everytime to evaluate this problem!

Originally Posted by themyst

Bad o2 sensor maybe?

I thought about it, but never have any precat o2 sensor faults. Looking at the waveforms on both sensors with the BT looks good. They both intersect and diverge pretty evenly on the realtime display. And again, no faults. I mean if someone had this exact same misfire problem and resolved it with a 02 sensor change, I would definitely like to hear about it.

Someone thought carbon buildup could cause this????
And does it seem like a possible cylinder mechanical problem? I don't know though, sure seems like a mixture problem more than mechanical, as mechanical problems seem to be persist even at higher speeds, etc.

[JamesM3M5]

Carbon? When was the last time you had the intake cleaned? Mine was a 47k mile street car when I bought it Sept, and the carbon build-up was HORRENDOUS. Cleaned it out, put in new plugs, and it runs like a scalded cat.

[Lulz_M3]

Might be a far cry, especially since your on stock twins but maybe your blowing out spark? Weak coil maybe?

[rader1]

Maybe you should install a set of aftermarket widebands just to verify? I find it odd with all your testing you haven't done this already.

[themyst]

Could also be tune hardware related. I've heard of piggybacks failing before.

I seriously doubt it's plugs injectors or coils because they are pretty much brand new. My plugs have 29k miles on them and I've never misfired due to a hardware issue. Only tune related.

In hotrods cause it's likely something less common.

[vasillalov]

Do you have away of logging the two O2 banks simultaneously? If so, it can pin point which bank of cylinders is misfiring.

[Jeff@TopGearSolutions]

How about coils?

[hotrod182]

Originally Posted by Jeff@topgearsolutions

How about coils?

Believe me, if it were a cylinder misfiring fault coming up, I would be immediately swapping that coil with another cylinder etc. But again, no specific cylinder misfire fault coming up. I could monitor and record the lambda activity during the misfire. But of course the voltage will be spiking way up when this is happening. But perhaps I could at least narrow it down to a cylinder bank? Not sure how a tune could work good through most of the run and induce just these sporadic misfires for such a brief period.

[Jeff@TopGearSolutions]

Originally Posted by hotrod182

Believe me, if it were a cylinder misfiring fault coming up, I would be immediately swapping that coil with another cylinder etc. But again, no specific cylinder misfire fault coming up. I could monitor and record the lambda activity during the misfire. But of course the voltage will be spiking way up when this is happening. But perhaps I could at least narrow it down to a cylinder bank? Not sure how a tune could work good through most of the run and induce just these sporadic misfires for such a brief period.

Perhaps its all the coils, seriously. Wouldnt spark blow out from weak coils produce misfires?

Do you have a BT tool? I'm assuming you have auto clear off?

[hotrod182]

Originally Posted by Jeff@topgearsolutions

Perhaps its all the coils, seriously. Wouldnt spark blow out from weak coils produce misfires?

Do you have a BT tool? I'm assuming you have auto clear off?

Yup, auto clear is off. I would suspect a coil if a specific cyl misfire came up. But for it to only happen with nitrous at first, and then now even off nitrous, something is becoming more persistent. I'm not sure how likely it would be for a coil to act up for a brief second, and then work fine from 120-145mph.

[Jeff@TopGearSolutions]

Originally Posted by hotrod182

Yup, auto clear is off. I would suspect a coil if a specific cyl misfire came up. But for it to only happen with nitrous at first, and then now even off nitrous, something is becoming more persistent. I'm not sure how likely it would be for a coil to act up for a brief second, and then work fine from 120-145mph.

I'm not 100% on coils either but at a minimum you can start changing things out from lowest to highest cost to see if any improvements arise. It sounds like your problem is hardware related which only leaves a few pieces : plugs/coils/vanos/injectors/fuel pumps.

After that you are diving into a mechanical problems....

Pull your plugs and atleast check the current gap.

[hotrod182]

Originally Posted by Jeff@topgearsolutions

I'm not 100% on coils either but at a minimum you can start changing things out from lowest to highest cost to see if any improvements arise. It sounds like your problem is hardware related which only leaves a few pieces : plugs/coils/vanos/injectors/fuel pumps.

After that you are diving into a mechanical problems....

Pull your plugs and atleast check the current gap.

Plugs were just replaced last week with brand new ones. Didn't make a difference. Hope its not vanos. I mean if it is, it would be nice to get some kind of fault code! With no codes, I feel like I am just shooting in the dark. I don't want to keep buying parts with no resolution. To bad I didn't still have my coupe. I could have been swapping parts left and right, and could have narrowed it down quickly.

[Joshboody]

Originally Posted by JamesM3M5

Misfires shoot unburned fuel AND oxygen into the exhaust, which is why you see the severe 50:1 spikes. O2 sensors read excess oxygen content in the exhaust versus atmospheric reference (which is why you shouldn't mess with the O2 sensor wires near the sensor end, the reference comes from outside the sensor at the wiring outlet), and a misfire has lots of extra oxygen left over.

Chase down your misfire problem.

Hey James
Can you confirm that widebands will read different afr based on spark or not? My understanding that it will reference oxygen between exhaust and air like you said. BUT oxygen total mass will stay constant whether burned or not. Molecular mass remains the same for each element, they just take different forms during/after combustion.

Anyway, I believe 50:1 would allude to no fuel in 1 or more cylinders. And splitting the bank afr would help narrow the issue.

[starmo]

Could this be a sticking valve/bad valve spring or cam ledge?

[hotrod182]

Originally Posted by starmo

Could this be a sticking valve/bad valve spring or cam ledge?

Car idles perfectly fine and smooth. Really wouldn't notice this at all unless you are going over triple digits. I wonder if there is some kind of fuel pickup in the gas tank which can be causing some kind of starvation. It almost seems like it may possibly be worse with lower fuel levels.

[Jeff@TopGearSolutions]

Originally Posted by hotrod182

Plugs were just replaced last week with brand new ones. Didn't make a difference. Hope its not vanos. I mean if it is, it would be nice to get some kind of fault code! With no codes, I feel like I am just shooting in the dark. I don't want to keep buying parts with no resolution. To bad I didn't still have my coupe. I could have been swapping parts left and right, and could have narrowed it down quickly.

Ah thats right you changed them again.

If you have BT tool maybe you can start logging other things.

[OpenFlash]

Originally Posted by JamesM3M5

Misfires shoot unburned fuel AND oxygen into the exhaust, which is why you see the severe 50:1 spikes. O2 sensors read excess oxygen content in the exhaust versus atmospheric reference (which is why you shouldn't mess with the O2 sensor wires near the sensor end, the reference comes from outside the sensor at the wiring outlet), and a misfire has lots of extra oxygen left over.

Chase down your misfire problem.

Spot on