Scope Results question for Idle Miss 04 Jeep TJ

Just bought my first cheap scope, and used the youtube channel to begin to learn how / where / when to use it. 

I've been chasing an intermittent rough idle, with intermittent felt misses, and a slight lean condition all only at idle for about a year now on an 04 Jeep TJ with a 4.0L engine. I've had a cylinder 1 misfire code show up 3 times months apart that cleared itself after a few drive cycles. I've replaced pretty much every sensor, the coil pack, the plugs, the injectors, checked fuel pressure, compression (static cranking compression anyways, haha). Unfortunately every time I try to get help with a diagnosis (either on Jeep forums or local Mechanics) to avoid throwing more parts at it, eventually people just start recommending parts to throw at it. I can't believe I didn't realize the power of a scope years ago TBH. Oh, and the fuel trims are usually +5-9% only at idle, go back to near 0 off idle, and there is no intake or exhaust leaks based on me smoking it multiple times, and running a vacuum in reverse through the tailpipe.

Anyways, long story long, I've scoped the primary ignition coil driver circuits (Wasted spark, PCM drives each coil to ground as needed) and my limited understanding says it maybe needs a new PCM. Hoping someone with a bit more experience with this can confirm, or deny that assumption. Since it's a cheap scope (I didn't know better until it was too late) I can't export data or scroll about, so I hope y'all can forgive me for the hard to see video of the scope with finger smudges that is too dim for daylight.

Here's the video: 

If you pause it and scroll around, you'll see some decent looking ignition patterns (I think) interspersed with some cycles that look to me like there just is absolutely no voltage being induced in the secondary coil. Just the PCM pulling the voltage down, but no response at all:


As well as some weird and squirrely firings that have way shorter on times:
 

I am guessing the intermittent completely flat line on the scope is the oscillations after firing being large enough to retrigger the display. I scoped multiple driver circuits, and the video is of the one that shows these anomalies most often but they do seem to be there in all 3.

My assumption based on what I'm seeing is that the PCM has weak ignition driver transistors that are able to pull the voltage to zero, but intermittently can't sink hardly any current(high resistance), so there is no magnetism to energize the secondary coil at all? Or is there another reason I haven't considered that could cause the driver to drop to 0 volts for a coincidentally similar amount of time as an ignition event without the secondary being energized intermittently?

What kind of sample rate does that scope have?
Does the scope have a "trigger" that can be set? If so, set it higher than the sparkline voltage.

some cycles that look to me like there just is absolutely no voltage being induced in the secondary coil. Just the PCM pulling the voltage down, but no response at all

 An amp clamp to view the coil current would answer this question.

The scope sample rate is 2.5MS/s, and the bandwidth is 200khz.

The reason I set the trigger below the sparkline, is that the voltage spike before the sparkline seems intermittent, and so I don't get to visualize the non-ideal events. I mean, if that's even possible to have a tiny spike like shown below? But that's what the scope says. It's possible the scope is just not picking everything up, but here's why I think that is not the case:
1) In this new video, I do see the same total dropouts, although they are less frequent today.
2) The 12v line shows some downward spikes in between the events on this coil, however they always line up in 3rds, with the more regular slight drops when the other 2 coils fire
3) The patterns of the higher spikes seem to follow a more periodic pattern than purely random to my eye, but that's subjective.

So I am hopeful I'm not just looking at noise here.


I will get a low amp clamp ordered, it'll take a while to ship in. In the meantime, if the computer controls the ground, and if the coil is indeed dropping out intermittently but we're still seeing the 12v drop to 0 at the correct time, are there possibilities here I am not considering? I mean, maybe a better question is do these videos suggest anything other than 'Get a better scope'?

Oh, and in case it's not obvious, I pause the video twice, and slow it down to 1/4 speed twice to try to show these events I'm talking about, since we can't just look at the data after the fact on this cheap scope.

After a think, I realized that this maybe looks like an intermittent short of the primary coil (or the feed and control wires shorting together).

Since there 'should' be current flowing when the voltage drops to 0, but there is no inductance spike when it goes back up to 12v, it would suggest there was no current moving through the primary coil and so no magnetic field to collapse, ie there's an easier path the current is taking.

And since the primary coil is only an ohm roughly, it would be pretty easy for this short to just act like a voltage divider, making the coil charge with a fractional current amount, which would explain the meandering spike levels when it is 'working'. This would mean the high spikes seen in the video intermittently would be the only times the primary is actually getting full power.

I'm hoping someone can give me a 'That's wrong because', as I can logic what I know, but have no idea what I don't know, so am likely missing something obvious.

And if my logic does check out, I guess seeing full current when my clamp comes would confirm that the current is still happily making it home to the computer, just via a different path. Which would mean the faultiest new coil pack ever that didn't happen to change the symptoms, or the 12v feed is melted alongside a coil control wire somewhere. Maybe?

Your logic is sound. A primary current waveform will answer some questions.
 

I got an amp probe, all current ramps for the ignition looked good. I also upgraded my scope, as I really couldn't trust what I was seeing, and having to record video of data flying by was a nightmare. Since getting the new scope, I haven't seen any primary voltage complete dropouts like before, but also the fuel trims have been sitting within 3% of zero now on both banks, so I'm not sure whether it's just randomly better for now or the dropouts never existed. Hopefully time will tell as I keep testing.

I am however seeing this when I measure voltage (yellow) and current (green) from the alternator, with blower on high and headlights on:
 

That current spike pattern happens regularly, but as far as I can tell it's not timed to any one cylinder (usually ~120 crankshaft degrees between spikes). The voltage regulator is in the PCM. Is this a bad alternator? I can't imagine what would cause that huge alternating current spike periodically like that with rock steady voltage, and haven't found any example waveforms that look anything like that.

How many amps does that spike convert to with your current clamp?

It could just be the cooling fans turning on or some other cycling load.