Secondary ignition waveforms analysis - Mitsubishi DIS

Hi, I have been reading and watching videos about the waveform analysis and kinda understand the areas of the waveform, internal/external spark issues, but when it comes to a real world scenario things get weird, for example I have this Mitsubishi V6, slight misfire at idle, no codes, scoping this engine I can see that the spike line is not like the examples, a thin line then we start with the burn line.

Maybe this is a characteristic of the probe/scope, some interpolation or something like that, because I see this on all the cylinders.

This particular system uses coilpacks (1-4, 2-5, 3-6) , front bank is 5/3/1, rear bank 6/4/2, first plug towers 1/2/3 have the positive spark, 4/5/6 are negative so I'm inverting the signal for those.

For example cylinder 1, I can see this small square like signal before the burn line, other than some turbulence and a lean burn at the end looks usual:

But when I get the misfire or bad waveform (cylinder 2):



(cylinder 4):





This indicate an external issue, this is before the burn line so the spark it's taking the least resistance path that is not into the cylinder.

This is so random and cannot find where it's going, no visible damage on cables or coils, plugs are new, cables have good resistance (within spec).

if this were a 2-5 issue that would indicate a failed coil but here I have 2/4, two coils that are close to each other leaking voltage, but also this happens on the rear bank, but if this is an external issue that shouldn't matter right? maybe the wires are crossing the spark here?

I'm putting the probe next to the tower, just a few inches from the boot ( from here I have a good battery ground for the probe).

I also got a similar waveform from number 3 but I think I was pushing the boots and routing the cables:



What do you think about those? If it's leaking but have no carbon tracing (maybe inside the boots) could be internal to the coils, how do you usually get those waveforms? also the weird spike square that is present on all cylinders some scope definition issue?

Thanks

"albertom wrote: Maybe this is a characteristic of the probe/scope, some interpolation or something like that, because I see this on all the cylinders.

Waveforms appear to be clipped at 5V. Most noticeable on the last #3 capture but you can see it on pretty much all of them. So the scope is part of your problem.

Other thing this being waste spark you're likely seeing a mix of combustion and waste spark events. First thing to try would be attenuating the signal so you're getting the full height firing kV. Then you might be able to reliably trigger on the combustion sparks. If that doesn't work you'll need to use your second channel as a trigger. Opposing injector pulse is often close enough to the ignition event to use it. Personally I use an RPM pickup to trigger on waste spark, and distributor, ignitions.