2003 Pajero / Shogun / Montero 3.5L misfire (6G74 engine)

Hi, first post and straight in with a question about my Pajero or more commonly known as a Montero. I have fully rebuilt the engine and it has run perfectly for a couple of hundred KMs then developed a horrible misfire at idle. It will rev fine, although can feel it missing. Pulling the connectors off the coils one at a time it appears to be mainly cylinder 1, possibly also 3 but hard to tell. It is a wasted spark system so 3 coils, and swapping coils doesn't make the misfire move cylinders. There is a strong smell of fuel and it sounds like a popping noise in the RH exhaust manifold, again hard to tell exact;y but leads me to think ignition related and cylinder 1 and or 3.

Please can someone sanity check my scope traces?

Firstly, cam and crank sensors:


Power Transistor signals from ECU:
Pin 13


Pin 12


Pin 11


And the corresponding outputs to the coils:
Pin 3


Pin 2


Pin 1


So all of the above to my (very untrained eye looks ok)

I'll post next the spark waveforms...

So now to the waveforms off the HT leads

RH bank in order 1, 3, 5
 
 
 

I'm assuming the inverted waveforms are normal? Are they?

LH bank 2, 4, 6
 
 
 

The waveforms were a bit hard to get, I have cherry picked the best images and there were some odd shapes in the full sample but that appeared to be the case on all HT leads. I wouldn't say that number 1 was particularly any worse than the others, but difficult to tell.

Really looking for some guidance here, as I'm a bit stumped.
Cheers

Please can someone sanity check my scope traces?

Firstly, cam and crank sensors:

Looks correct to me. Mitsubishi has a known good shown in their SI, if you want to compare:

 

Power Transistor signals from ECU:

Not to nitpick, but I think the signals on pins 11-13 are actually the coil control signals from the power transistor, not the ECU.

Were you using an attenuator to capture these signals? Because otherwise the amplitude seems low for a coil negative waveform.

And the corresponding outputs to the coils:

Again, being nitpicky, but these are transistor control signals from the ECU to the power transistor. Waveforms look lovely.

So now to the waveforms off the HT leads

...

I'm assuming the inverted waveforms are normal? Are they?

The inverted waveforms are normal, yeah, just part of waste spark systems.

On each coil, you'll have one cylinder that's firing with normal polarity (positive spark plug electrode to negative electrode), and the companion cylinder will be opposite. You can use the Pico to invert the waveform and make it right side up if you want.

Bank one seems normal enough? But bank two has very clear 'tails' at the end of their burn lines. Bank one doesn't have any. Makes me wonder if there's something wrong with all cylinders on bank one, not just #1 and #3.

That, coupled with the noise you noted from the right bank makes me wonder if bank one is mechanically out of time. That'd explain the misfire, the noise, and maybe the secondary ignition waveforms. The cam/crank won't show it because the single cam sensor watches bank two, not bank one.

I'd be tempted to do a relative compression waveform next, if possible.

Hi, thank you so much for your detailed response.

To confirm, yes I was using an attenuator

For the pin numbers I relied on this image in the manual:


Which seems to align with this


I must have mixed them up when numbering the images

Regardless, good to know that waveforms for the primary look ok.

 I'd be tempted to do a relative compression waveform next, if possible.

EDIT - so a bit of YouTube later and I managed to do a relative compression test. I did also scope the cam sensor to find where #1 is, but the waveform is distorted as the starter doesn't turn the engine at a constant speed.

Can you deduce anything from this?


Although not identical in height they don't seem relatively that far off. The rise in current also seems to correspond evenly to either a leading or trailing edge of the bank 2 cam sensor. I don't know if that's significant or not.

Keen to get your opinion
Cheers

Thanks again!
Cheers