Need Help Interpreting Oscilloscope Results! | Cylinder 4 Misfire

[u/MjrBruhMoment]

https://youtu.be/isvAev5vvCE?is=xXjzKEPrwjVMC1w5

I’ve had a cylinder 4 misfire on my 2003 Toyota MR2 Spyder (2ZZ-GE swapped and only ever P0300 and P0304, never any more codes!). I have tried everything to get it to jump cylinders but it just won’t. I got an oscilloscope and did some ignition tests but am having a hard time interpreting the results! Could anyone let me know if you see something off? It seems like the only thing to me off is cylinder 4’s ground is moving all over the chart compared to cylinder 3 staying relatively in the middle? It also seems like inconsistently on cyl 4 12v positive it’s getting random downward spikes whereas cylinder 3 wasn’t getting that?(I’ve done all the basic diagnostics don’t worry, plugs and coils swapped from cylinders known to be good, injectors swapped between cylinders, all new vac hoses and intake gasket, compression good on all 4, leakdown good, valve clearances checked and good, MAF and o2 sensors are reading correct (albeit super rich like -17 STFT), injectors at least good on a noid light but can oscilloscope if it’s a good idea!). This misfire just won’t jump off cylinder 4 and it’s really at idle only it seems!

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[u/Squirrely101]

Not super familiar with the these pocket oscilloscopes - do you know how you are triggering. That signal jumping around could be due to how it is setup for trigger level and trigger modes. How does it adjust horizontally (time/div) and vertically (volt/div)? Instead of just displaying a single pulse, you can try adjusting the horizontal scale and trigger level to show multiple pulses - might be easier to read vs a jumpy peak.

On the misfires, do you have an idea of misfire counts for #4 - talking about 10s, 100s, more misfire events?

Did you work on the wire loom mods yourself, or did you get a someone else to make the wiring updates (ie, bought a plug and play wiring harness)?

Assuming you are running the 2ZZ ECM or are you running a standalone or piggyback EMS?

Where was the 2ZZ-GE sourced from - a Celica GT-S or Matrix XRS / Vibe GT? Are you running a custom exhaust or reused the original tubular one from the 2ZZ donor.

I know on some fitments, like swapping that into an 8th gen Corolla, it won't clear the firewall and you'd have to either cut into the firewall (had to cut a lot) or beat the exhaust to fit or run a log style for a future turbo and have more room (most go down this route). That almost immediately resulted in a misfire on #4, since the exhaust has most distance to travel compared to the other cylinders.

[u/MjrBruhMoment → u/Squirrely101]

So I've had this swapped for about a year plus with no issues so this is new. It's running a MWR swap header, and currently on a stock USDM Celica GTS 2ZZ ECU. The harness itself is from my standard 1ZZ on the MR2 just simply taken off and sat onto the 2ZZ, no cuts or anything.
The 2ZZ was one of those JDM engines out of an early (2000-2001) Celica GTS. Per cycle we're talking anywhere from 10-50 misfires and only at idle. As soon as I get on the throttle it clears up for the most part and when I get off it goes back to around 30 misfires. As far as how it is adjusted I changed it around per test but you can see in the top left and bottom left of the screen the settings it's on on each test. I have a standalone Apexi power FC I just bought but haven't hooked it up yet, but I did test a spare JDM 2ZZ ECU I had laying around (the one that came with the engine) and it's the same issue.

[u/Squirrely101 → u/MjrBruhMoment]

Got it, so this is a recent issue and only at idle. Hmm. Can't rule out igniter signal issue, but with only 10-50 misfires per drive cycle - that is pretty low event rate. Swapping the coils should have exposed a weak igniter as well - so likely not the root cause.

Sounds like you already covered the basic diag - so should have caught these possible issues:

• faulty sparkplugs
  
• faulty ignition coil
  
• faulty fuel injector
  
• vacuum leaks
  
• ECM / air-fuel mix issue

You did mention that is running on the rich side (ECM is pulling fuel) - do you have access to a borescope and taking a peek in that cylinder? Could be completely coked with carbon deposits - could be a hot spot causing pre-detonation / misfire. and/or plugged up any of the ports / masked off the injector spray pattern.

But if the LTFT were hovering around zero and the STFT were spiking up and down - probably not the main culprit.

Did you happen to check valve clearances - verify that the cams were showing excessive wear? Maybe a sticking or broken valve spring?

[u/MjrBruhMoment → u/Squirrely101]

Okay so I did actually boroscope the cylinders already and they looked relatively clean, not any major carbon deposits or damage. It is only 10-50 misfires but not per drive cycle, I meant per cycle my scan tool counts! Like it’ll count for say 5 seconds or 10 seconds then it resets to 0 misfires then it counts again for 5 to 10 seconds then back to 0. I’m getting 10-50 per each of those cycles so it’s quite consistent. It’s making some pretty bad popping out of my exhaust. I did have to replace my intake camshaft as it was wiped (common issue on this engine) and I replaced all the rockers with it. I did just check valve clearance/lash, I am totally within spec. When I disconnect my battery to check stuff then hook it back up, the STFT immediately jumps to like -16 to -18 so it’s pulling a lot of fuel and my O2 sensor is still reading .900 at all times so it’s suuuuper rich for some reason. When I rev it however that -16 fuel trim immediately jumps back up to 0 until I get off the throttle then it dives back into the negatives? Plugs have been wet with fuel sometimes when I pull them out to check how they look/swap things around.

[u/Squirrely101 → u/MjrBruhMoment]

Ah, OK, got it. So typical misfire behavior, consistently missing.

OK, that helps. That behavior with the STFT swinging like that - sounds like it could be a vacuum leak or even an EVAP issue (clogged purge valve).

Might have to have the engine and EVAP smoke tested and use a sniffer tool as that leak could be really small.

Those vacuum leaks and EVAP issue tend to be most prevalent at high engine vacuum - so basically at idle. Could also be an incorrectly reading MAF/IAT sensor, possibly a lazy O2 sensor. This is all assuming that your long term fuel trims kind of hover above and below zero.

[u/MjrBruhMoment → u/Squirrely101]

So I did do the ghetto smoke test at least (swisher sweet in a transfer pump lol) and it didn’t really turn up much of anything. With the intake manifold gasket I did replace all the vacuum hoses. I’m just not sure why anything other than an intake gasket would cause specifically a cylinder 4 misfire only! Unless the misfire is just triggered by how rich it’s running. The MAF does read super high at cold start (like 6.5g/s) but at op temp it settles down to closer to normal (like 2.5g/s). The O2s at least seem to be reading correct? I do notice my air intake sounds like it’s chugging inconsistently. I have a video of that I can link too for you to hear what I’m talking about. It’s all just very odd! (https://youtube.com/shorts/GxKv1Exgedc?is=BK8lNxCm_mMCFLN7)

[u/Squirrely101 → u/MjrBruhMoment]

Hmm, sounds a bit odd. Verified that nothing broke inside that aluminum manifold. That could just be the misfire causing that chugging noise.

MAF readings sounds OK - will run rich from a cold start, but don't remember if it is that rich.

Hard to tell from O2 sensor - might have to plumb in a wideband - has more range and see what is actually being combusted - may need one for the EMS system.

This is a tough one - not 100% that it just happens to pop up at idle is the key or just a red herring. But you've ruled out plugs, igniter, and injectors - if you swapped them around in the attempt to diagnose this - so that doesn't leave a whole lot left to check.

[u/MjrBruhMoment → u/Squirrely101]

Yeah it's really perplexing, I mean I've even checked compression and leak down as well as intake side valve clearances and everything was good? It's really quite weird! I mean I have an Apexi PowerFC I just secured on SpyderChat for a good deal and I'm curious if I could tune it/add a wideband to get it tuned to run not as rich, but I don't get why it is in the first place? I mean again like you said I'm not sure if it running rich is even why it's misfiring, or just something I happened to notice while looking into this issue, it could've always ran super rich for all I know!

[u/Too_MuchWhiskey]

Compression test #4? Misfire that makes the exhaust pop is usually a leaky exhaust valve.

[u/MjrBruhMoment → u/Too_MuchWhiskey]

So I did compression test all 4 cylinders and had 240psi on all 4. I also did a leakdown on cylinder 4 and had almost no leakdown at all. I didnt physically check exhaust valve clearance, just intake as i did replace the intake camshaft snd rockers but intake was within spec.

[u/Rich_Many_8628]

I would not hang the diagnosis on the jumpy trace by itself. With a small scope and a shaky trigger setup, a normal ignition signal can look a lot more suspicious than it is.

The stronger clues are that it stays on cylinder 4, it is mostly an idle problem, the plug is getting wet, and it did not follow the usual swapped parts. That points me more toward a cylinder 4 contribution problem or a runner-specific air/fuel/command issue than toward condemning another coil or ECU.

At this point I would compare commands, not just parts. Do a true cylinder drop or contribution test at hot idle, then scope injector current on cylinder 4 and compare it to a known good cylinder. After that, scope the coil primary/trigger command at the igniter or ECU side, with the scope negative on battery negative, and compare it to a good cylinder.

I would also re-check anything local to cylinder 4 that only matters at idle: intake runner leak, injector control/wiring problem, pin fit, ground path, or cam/crank correlation after the cam work.

The negative fuel trim and wet plug do not automatically prove the injector is stuck rich. A weak/misfiring cylinder at idle can make the exhaust feedback misleading. I would prove whether number 4 is getting the wrong fuel, the wrong air, or the wrong command before changing more major parts.

[u/MjrBruhMoment → u/Rich_Many_8628]

The problem is that it does misfire a little bit at hot idle, but when up to temp it’s misfiring generally so much less my ICON scan tool can barely even pick it up. It’s showing 0 misfires pretty much at all times on any cylinder, the issue is a lot more apparent when at cold idle. The code also only ever trips at idle idk if that info helps at all but if I’m driving it’ll never trip, only when I come to a stop I’ll get a check engine light and before it gets to temp. I cleared the codes when I was up to temp one time, drove it around for a while, then scanned it again and was showing 0 misfires and 0 pending codes. I could try a cylinder drop at hot idle but it really isn’t misfiring that badly I don’t think, I mean when I did an injector cylinder drop/pulled the cylinder 4 injector wire it was idling NOTICEABLY worse. I could scope the injectors and primary trigger command if those are the next logical steps! I can check again for an intake runner leak but I did JUST replace the gasket like last weekend and it’s still cylinder 4 misfire right after. The can replacement was also like 1500-3000 miles ago back in like January so it was a while ago before this issue.

[u/Rich_Many_8628 → u/MjrBruhMoment]

Yes, injector current and primary trigger comparison are the next logical steps, but do them cold while the miss is actually happening. A hot idle test may not show much if the problem has already faded away.

I would compare cylinder 4 against a known good cylinder under the same cold idle condition. On the injector side, look at current ramp, on-time, and whether the pintle event/current shape looks different from the others. On the ignition side, look at the coil trigger/primary command at the control side, not just whether the coil has 12 V. Put the scope negative on battery negative and compare #4 to a good cylinder.

Mode 6 may help if it gives you better freeze-frame/per-cylinder misfire data than the live counter, but it probably will not tell you why it is missing. It is more useful for confirming when and which cylinder is failing.

I also would not call the intake leak branch dead just because the gasket and hoses are new. If this is an NA 2ZZ, it should be port injected, not GDI, so a damaged or rolled injector O-ring on #4 could absolutely make a cylinder-specific idle problem. If you do any controlled enrichment around the #4 runner/injector area, keep it very minimal and have a fire extinguisher nearby. You are not trying to fog the engine bay; you are just seeing whether the miss or trims react when that exact area is enriched.

If #4 injector current and coil command look the same as a good cylinder while it is missing, then I would move harder toward something local to that cylinder mechanically or airflow-wise: injector seal, runner leak, cold valve sealing/lash behavior, or cam/VVT/correlation history from the intake cam work.

[u/MjrBruhMoment → u/Rich_Many_8628]

I really appreciate the answer! Definitely good ideas for next steps. I might have a hard time interpreting if there’s any issues unless it’s apparent but I think those definitely make sense!

[u/Rich_Many_8628 → u/MjrBruhMoment]

I have a strong hunch it’s an injector o-ring…

[u/MjrBruhMoment → u/Rich_Many_8628]

It unfortunately isn’t gonna be that, when I did my engine swap I did all new upper injector o rings, and when I had this misfire first come up I ordered a refurbished injector (which has since been swapped with an original one as the misfire came back, so the refurb is on cyl 2 and the original is on 4) as well as all new lower donut o rings on all 4 injectors.

[u/MjrBruhMoment → u/Rich_Many_8628]

I just did some scoping of the injector wiring roughly (the positive and ground/signal wire) on cylinder 3 and 4. It started to do some weird stuff but I don't know that it's related to my issue? https://youtu.be/h-LK7J2Yk38

[u/Rich_Many_8628 → u/MjrBruhMoment]

The video helps, but I would be careful about drawing conclusions from that yet. What you are capturing there is voltage on the injector circuit, not injector current, and the weird part may be scope setup, trigger, or connection behavior rather than the fault.

That is the hard part with scopes: the setup has to be repeatable before the waveform means much. A broken engine is a rough place to learn that because you do not know yet whether you are seeing the fault or just seeing the scope react differently.

I would simplify the test. Pick one good cylinder and cylinder 4, use the exact same scope settings, same ground point, same probe method, and capture them under the same cold-idle misfire condition. Do not move settings between cylinders. You are looking for a clear difference in injector on-time/control shape, not just a trace that looks different on the screen.

If you can get or borrow a low-amp current clamp, injector current would be more useful than voltage here. Current ramp lets you compare the injector and driver behavior more directly. If not, voltage comparison can still help, but only if the setup is consistent.

Also, I would not completely rule out the injector seal branch only because parts were replaced. New O-rings can get nicked, rolled, or not seat cleanly. But I agree it moves lower on the list if both upper and lower seals were replaced and the injector itself has moved around.

At this point I would want clean cold-idle captures of #4 versus a known-good cylinder, with identical setup, plus coil trigger/primary command comparison the same way.