Waveform Diagnosis - I am all Ears.

2004 Jeep Liberty Limited 3.7L automatic w/AC
Misfires at Idle
DTC's: P0300, P0301
Equipment: Wyzeprobe, Picoscope 6 Automotive

Just started playing with my picoscope with a Wyzeprobe. I am gradually learning this, so any input would be greatly appreciated. This is a vehicle a friend has temporarily with a 2004 Jeep Liberty 3.7 L , automatic w/ AC. DTC's set: P0300, P0301. I pulled this waveform off #1 COP at idle, I see that it is running lean and possibly some carbon tracking from coil itself. What does everyone else see?

When I'm diagnosing misfires on one cylinder, I like to scope a few of the other cylinders for comparison. While there is an ideal secondary scope pattern, there are small, subtle differences from car to car. Definitely scope more than one for comparison. This is especially helpful when the misfire is subtle. Dead-hole misfires are more likely to have a secondary pattern that doesn't require comparison.

Also, in my experience, the best way to see burn voltage spike from a lean cylinder is to snap the throttle. At idle a lean cylinder may hide itself in a secondary ignition waveform.

Your waveform shows a decent burnline and burntime. The coil got lots of energy left as shown by the ringing after the spark extinguishes.

I don't see indicators of a lean condition nor carbon tracking. A complete secondary analasys must include snap throttle. On some systems you can disconnect the injector and you will not see the fault at idle. Get used to the snap throttle test.

Agree with Juergen and Eric, no obvious faults so far. Sometimes that spike at the end of the burn line can show a lean? Some systems show that pattern on known good cylinders. :silly: As indicated, snap throttle is the ticket.

Definitely compare the firing line to other good cylinders. If this cylinder is low compared to others, you might be looking at an AWOL rocker arm. A quick audible relative compression test could confirm this.

Thank you! I keep forgetting that I need to test under loaded conditions and try to recreate the problem.

If it misfires at idle I wouldn't consider needing to test under load as that important - because the fault is happening at idle already. Flooring the car on take-off might show up a weak spark if it gets a lot worse under those conditions - higher compression means it's harder for the spark to jump the gap and the spark finds an easier route. If a vehicle misfires a lot under idle I'd leave it idling for a while then pull out the spark plugs - maybe post a picture of them, that can give many clues.
Also, with a "cop" system someone showed me a trick - tap each coil with the handle of a screwdriver - sometimes if a transistor is breaking down this will make it happen when you tap it and show up the coil as faulty.
The other option is to remove the spark plugs and put them each in a different cylinder, taking note of their new positions, and also swap the coils around to different cylinders - then see if the fault moves with the spark plug, or the coil, or stays on the same cylinder which would mean it wasn't the spark plug or coil.