92 Jeep Cherokee 4.0L - Ignition Waveform Analysis

Hi All,

I have checked several cylinder ignition wave forms and they all pretty much look like this. Is this an acceptable waveform?

Waveform Video

Also, this is the Coil negative terminal that is back probed to check the ignition signature. This doesn't look like the signature example provided by the MODIS. What are your thoughts about this?



Steven

The captures on your video aren't stable enough to be useful you can fix this by moving the trigger voltage setting upwards.




Also if you clip your ht probe to main coil to distributor HT lead and slow you time base (trace shows a longer time displayed on screen) you can capture a spark parade of 6 cylinder this is called a "spark Parade"--- which is very useful for tracing misfires. With a spark parade you look for a cylinder that the spark voltage pattern looks different from the rest ---- what ignition waveform guru Jim Morton calls looking at the forrest before you look at the trees. If you see a spark that dosen't fit in then go in and look at the individual spatk voltage.



Unfortuneatly I can't view your photobucket photo so can't give you any opinnion on that you can however post jpegs directly on the forum using the attachments button

Thanks Andy. I will modify the trigger to get the firing event captured better for single cylinder waveforms.

On the parade capture, I did try that and it only gave me 4 firing events. I thought I was doing something wrong there. Perhaps I was "zoomed in" too much with the shorter time base?

But I do have a concept question when doing that... When placing the probe on the coil wire (lead between coil and dist cap), aren't I only looking at the signal between those 2 points? OR, do I get to see the entire path from the coil to the spark plug, for every firing event for every cylinder? I'm trying to visualize what's going on when checking there.

As always, I appreciate you helping me learn these techniques and concepts.

Note: I fixed the image on my original post (not going to use photobucket any longer)

Steven

waxerk5 wrote: But I do have a concept question when doing that... When placing the probe on the coil wire (lead between coil and dist cap), aren't I only looking at the signal between those 2 points? OR, do I get to see the entire path from the coil to the spark plug, for every firing event for every cylinder?

When you clamp the probe around the coil wire, you will see the firing event for ALL cylinders. Usually, an inductive pickup is place around the #1 plug wire, also. This is used as a trigger and helps identify firing order.

First off, thank you guys for helping me to figure out how to better use my MODIS.

I have taken a video of the 6 cylinder parade pattern and I see 1 issue that I am concerned with.



Notice that the spark KV jumps randomly across all 6 cylinders at idle. At increased RPM, it goes away. During a snap, it also goes away. But at idle, the spiking spark KV event is taking place.

Is that a normal event or is there something wrong here with the ignition system somewhere?

Thanks guys.

The ignition system looks fine. I suspect you are running a little lean. An unstable (bouncing up and down) spark line is an indication of a lean density condition. The leaner the mixture in the cylinder, the more KV it takes to jump the air gap. Ideally, you want to see about 1.5 KV on the spark line with a duration of at least 1.3ms.

pole71 wrote: The ignition system looks fine. I suspect you are running a little lean. An unstable (bouncing up and down) spark line is an indication of a lean density condition. The leaner the mixture in the cylinder, the more KV it takes to jump the air gap. Ideally, you want to see about 1.5 KV on the spark line with a duration of at least 1.3ms.

Thanks. Glad to hear it looks fine.

Speaking on the lean condition, I've already smoke tested the engine and fixed the vacuum leaks on the older rotted vacuum lines. I did not see any other location leaking any smoke, other than the throttle body bushing where the throttle pivots. It has a very small leak there, which I read was normal. This is a MAP engine and the RPM's are normal (not elevated to indicate a vacuum leak), so I believe I'm good there.

I did run an injector balance test last week and all were about the same. I had 4 injectors exactly the same and 2 others were about half of a psi off from the others. I think I'll look at the injector pulse patterns next to see how they are looking, trying to find one that may be sticking or off to create a lean condition.

Thanks again for your help.

Looks fine, you have take into acount no two firing events are the same especially at idle where the engine computer will be making continuous rich-lean-rich-lean adjustments to the fueling.