Looking for help pinning down a misfire in speed density

[djamesp]

Started having a random miss that I cant pin down to a single cylinder by disabling injectors and running speed density so no misfire data either. I can see the narrowbands/AFR spiking but not sure if there's a way to figure out which cylinder is the culprit maybe with filtering or something? I have an IAT in the intake and trimmed out all the MAF wiring so I can't just throw one in and test that way. Tried logging currently firing cylinder but it doesn't keep up even at low rpm. Any advice appreciated, thanks!

RCSB V3 - 4L80 - 2BAR RTT.hpt
23-09-16 16-45-04 missing.hpl

[gametech]

Sometimes you have to go back to physical diagnostic work. Just check the temperature at each exhaust port.

[blindsquirrel]

O2 will go 'lean' when there are misfires (they don't detect unburned fuel).

[gametech]

O2 will go 'lean' when there are misfires (they don't detect unburned fuel).

Unless he has 8 o2 sensors, that would only narrow down which bank had the misfire.

[blindsquirrel]

True, but his use of the word 'spiking' had me wondering if he was seeing a rich (high mV) reading and attributing that to the misfire. And narrowing it down to one bank or the other is better than nothing.

[Matt Vardaman]

Started having a random miss that I cant pin down to a single cylinder by disabling injectors and running speed density so no misfire data either. I can see the narrowbands/AFR spiking but not sure if there's a way to figure out which cylinder is the culprit maybe with filtering or something? I have an IAT in the intake and trimmed out all the MAF wiring so I can't just throw one in and test that way. Tried logging currently firing cylinder but it doesn't keep up even at low rpm. Any advice appreciated, thanks!

RCSB V3 - 4L80 - 2BAR RTT.hpt
23-09-16 16-45-04 missing.hpl

What I usually do is unplug one coil pack at a time to figure out which cylinder is misfiring. I had a bad br7ef spark plug new out of the box. It was half ass firing. I am running the 2bar os, and the injector disable labels do not match. So much contradicting information about the labels, so it's easier to unplug a coil or injector.

[kingtal0n]

wait what injector labels dont match? then the injector are wired incorrectly lol

SEQ-EFI is sequential fuel, each injector fires a specific number of degrees before the intake valve opens, the ECU identifies it correctly to do that

ECU firing the wrong injector at the wrong time leads to misfires... could be ops problem seen it a bunch of times alrdy

[kingtal0n]

Started having a random miss that I cant pin down to a single cylinder by disabling injectors and running speed density so no misfire data either. I can see the narrowbands/AFR spiking but not sure if there's a way to figure out which cylinder is the culprit maybe with filtering or something? I have an IAT in the intake and trimmed out all the MAF wiring so I can't just throw one in and test that way. Tried logging currently firing cylinder but it doesn't keep up even at low rpm. Any advice appreciated, thanks!

RCSB V3 - 4L80 - 2BAR RTT.hpt
23-09-16 16-45-04 missing.hpl

The timing jumps kind of crazy in your log, up and down, like a staircase. Timing is very low under light load. Is this thing very very heavy truck? I don't like the timing I see.
high timing at idle and low load can cause misfire feelings, like near idle in gear. But not usually while driving.
We need more info, when does it feel like misfire. What gear or what speed at what time. Did you check the injector wiring matches the disable injector sequence.


I also notice the a/f kind of leaning out. I hope you are not in closed loop with it doing like that. I didnt look at the tune file.

[djamesp]

Damn. I tried a few misfire diagnoses methods but didn't think of testing individual temps. Thanks for the tip I'll give it a whirl.

Bank 2 is the one leaning out.

I can't get anything definitive by disabling injectors or unplugging coil packs. Each cylinder RPM drop is about the same. Also its intermittent, cold starts and random periods during driving everything is spot-on, feels normal, narrowbands look right, etc. and most often has to be under load to really see something is happening in the logs.

Its a 2wd RCSB :') should be the opposite of heavy... I had not checked the timing tables in a while I'm not sure how I ended up with the values I did but they were very low and not smoothed well. I bumped everything outside of boost up closer to stock, a real short drive didn't show any evidence of changes. Is timing supposed to be a nice smooth line w/o any jumps? I will double check injector wiring, skipped over it bec this is a new thing, been running well for 250miles or so after a rebuild. The injector sequence (when disabling cylinders in HPT) goes by firing order, right? No closed loop. One thing I noticed was weird blips in the fuel pressure sensor readout, its untrustworthy as far as actual psi goes but I've not seen a periodic dip in it like that before and of course I cant see the manual gauge under the hood while driving. Spot on 58psi at idle w/vac gauge disconnected though.

I know my way around a wrench well enough but this is my first deal tuning and of course I'm getting it all from books, forum posts and dubious youtube channels without any feedback. So its entirely possible I am making a catastrophically dumb mistake in the tune, and I'm betting on that after a nearly perfect compression test, swapping coils/wires/plugs, regapping plugs, noid lights, beating my head on the wall, etc. Used this as an excuse to buy a multichannel scope so as soon as that shows up hopefully I will have more info.

I attached another log, 6min in is a good example. Converter locked, pedal relatively stable, AFR/narrowband voltage and timing all over the place.
23-09-19 19-42-49.hpl

[djamesp]

Sorry I meant the narrowbands usually read rich but drop <100mv for misfires. I usually watch my AFR readout from the wideband that displays the opposite way, thats how i came up with "spike".

[Alvin]

I'm not seeing any strong indications of a miss.

Can you describe the miss? When is it worst? What plugs and gap?

[djamesp]

Sure, to me it feels just like a standard-issue P0300 truck, kinda dogged out, exhaust volume drops off a cliff and sounds muffled, etc. Difficult to describe I may have to drive again to be sure. Driving I can pretty clearly hear where it's not hitting if I'm not mistaking the sound for something else. It is 4" straight pipe w/2.5 dual exits it's hard to miss the, uh, miss. If that's what is. What else would have the o2s intermittently oscillating like that? Tr6 plugs 0.030 just tightened up from 0.035, didn't change.

Having looked at this some more sometimes the LH o2 goes nuts sometimes RH, hard to tell cause the LH makes em both go wonky. Worst is low speed high load (pulling hard out of a corner) and light throttle cruising at Hwy speed.

[Guy With A Chevy]

I had hell tracing a cracked spark plug down. Barely found the answer today after posting for about it for awhile. Must have barely split the filament inside but the temperature difference was surely there on the single exhaust tube. I used a water bottle with a pinholed cap and squirted it on it. I just had no clue that my mail tune was set to ignore misses.

Your saying yours is possibly at speed? Tried vehicle in drive with ebrake on? I know my truck ebrake will hold 1200rpm in drive for diagnostic purposes might help you just please be careful.

Why would his truck not be logging this miss? The most clear way to check for a suspected miss like this is put it in torque lockup and accelerate and if it hesitates through the band it probably has a mechanical problem. Mine had been doing it the last few weeks trying to tune hesitating hard especially in torque lock up or at low rpm. Once it fought its way past the miss it was ok. It was said by someone a bad tune should not cause the symptoms of a miss. Yours also looks correctly setup to sense misfires as far as the P0300 settting goes

[Alvin]

I assume you've already swapped plugs? Even if they are fresh.. one might have been damaged. I've had a handful of brand new spark plugs be bad for no apparent reason. Pull them out after hundred miles and they literally look like I just put them in.. no carbon on them or anything.

A squirt gun is a great tool for finding miss. As silly as that sounds. If its missing you'll see the difference in how water evaporates off the header. Its trickier with manifolds.

And to the guy above me.. Its quite normal to disable misfire diagnostics if the car has a cam. The misfire diag is looking for the engine not rotating smoothly. Well that includes cam lope.

[Guy With A Chevy]

I assume you've already swapped plugs? Even if they are fresh.. one might have been damaged. I've had a handful of brand new spark plugs be bad for no apparent reason. Pull them out after hundred miles and they literally look like I just put them in.. no carbon on them or anything.

A squirt gun is a great tool for finding miss. As silly as that sounds. If its missing you'll see the difference in how water evaporates off the header. Its trickier with manifolds.

And to the guy above me.. Its quite normal to disable misfire diagnostics if the car has a cam. The misfire diag is looking for the engine not rotating smoothly. Well that includes cam lope.

Ok good to know Alvin as I plan to keep it re-enabled for quite some time being it was built by me. Thank you again for your help getting that solved

And to James I was just sharing my actual miss diagnostics and outcome as it was actual. The clear indicator there was a problem accelerating, you can see here the RPM sags.

I don't want you chasing what could be nothing but check everything. If the vehicle is kicking 40-60mph light acceleration you probably have an issue.

[djamesp]

Yeah per my understanding once the maf is disabled all misfire logging goes out the window. Every bistory/current/total count is zero. I've been trying to find someone I trust enough to power brake it when I'm under it/in front of it but hasn't happened yet. I can't get a clear indication on timing, compared to my stock file ive pulled 5? at the most from the spark table which is just a saftery margin while I work on VE/PE/etc. Other tunes for similar setups go way higher, but I'm getting knock retard at low load with a ~12.5 AFR. nothing is rattling or anything obvious for false knock which is why I kept it so low.

As far as individual cylinder temps, after idling about half an hour I had a couple that were 10?F off, is that enough to make a difference? I couldn't measure passenger bank well everything is heat soaked from the log/turbo/downpipe but I'll try more this weekend. Had not thought of squirt gun.

I forgot to grab my screenshot of the oscilloscope readout off the crank sensor but the pulses looked irregular as hell big upswings downswings way too many pulses (10-12) between plug #1 firings. Granted it is running 80# injectors at 600rpm I'm not. Expecting factory smooth running esp at low rpm/idle but it's definitely different than it was a week ago without having changed anything.

The plugs have under 1k on em and ran fine for months, premature failure happen as often as being bad out of the box? Will try swapped my out plugs tonight.

If I come up with enough evidence that it's not a mechanical issue I might get a mail tune just to get this bitch rolling in a way thst doesn't scare me and play with it from there.

[djamesp]

OK so I attached a pic of the cylinder numbering when I go through the scanner functions window to disable individual injectors and if I understand correctly how theyre numbered this is way off. My problem a) i was completely neurotic over making absolutely sure the injector plugs all ended up back in the same place but also b) it wouldn't be possible to make some of these incorrect swaps, like 1 & 8 (hptuners numbers, not GM) being next to one another on the LH bank. Any input on this? Would be great if it was just swapped injectors. Also attached a pic of the crank sensor wave with intermittent noisy sections, I'm new to oscilloscopes just wondering if that noise supports my actually having a miss or not.

cylinders.png
crank-and-plug-1-scope.png

[kingtal0n]

OK so I attached a pic of the cylinder numbering when I go through the scanner functions window to disable individual injectors and if I understand correctly how theyre numbered this is way off. My problem a) i was completely neurotic over making absolutely sure the injector plugs all ended up back in the same place but also b) it wouldn't be possible to make some of these incorrect swaps, like 1 & 8 (hptuners numbers, not GM) being next to one another on the LH bank. Any input on this? Would be great if it was just swapped injectors. Also attached a pic of the crank sensor wave with intermittent noisy sections, I'm new to oscilloscopes just wondering if that noise supports my actually having a miss or not.

cylinders.png
crank-and-plug-1-scope.png

That cylinder order looks like misfire city. Unless there is some kind of bug or mislabeled in the software type of thing. I suggest trace the injector wires back to the ECU and make them match the actual cylinders is my vote, even if the software is not correctly labelled it should still match at the ECU wire to wire. I am curious about this result. You do understand #1 is the front of the engine, drivers side (right hand side in the picture at front) and number 2 is front and left, other side, etc...

[djamesp]

100% yup per GM driver bank front to back is 1357 passenger 2468. i just tied an tucked the harness a month ago Don't suppose I could cheat and swap firing order around in the tune to check..? Oh no then everything would be out of sync haha. New plugs and wired showed up last night so I'll take a crack at that first just in case.

Someone had mentioned that my log wasn't evidencing any serious miss, Im genuinely curious what else would do the up and down chart for the narrow band? It's much worse once it's hot and to double check I stuck my head in the wheel wells and theres a real obvious back/after-fring sound like a hollow thump at random intervals that sort of coincide with the lean spikes and noise at the crank sensor on the scope. I got a current probe coming tomorrow so I can really power balance it.

I'm sure this isn't relevant but I've been chasing a charge issue, new alternator, newer 6/22 battery load tests fine & checked all terminals and grounds but I never see any higher than 13.5v on the gauge or PID. Pretty sure it's supposed to be 14.5 and I'm not sure if that'd make a difference to sensor readings or anything.

[kingtal0n]

You check battery voltage at idle and with a bit of RPM, cold and hot. Don't worry about the computer display voltage there is a drop because of diodes which protect the computer it will never read as high as the battery.

How about disable the coils 1 at a time and see if they are correctly labelled. That would narrow down the wiring or ECU related aspect. Because the injector can fire at the wrong time and the engine will run but like shit. If the coil fires at the wrong time it wont run at all.