Plug gap/spark duration testing

After recent discussion about plug gap and spark duration, I started thinking about doing some controlled testing on a known good vehicle for reference. Only three spark plugs were harmed in the making of this thread.

For reference, I started out with three new(er) plugs on bank two of my '08 Escape. This is where we started out:



I then hammered and bent the gaps around to this:



The plugs then got reinstalled in the engine like so:



Now, before I smashed the plugs, I used my Launch to observe the spark duration. These are scan tool PIDs provided by the OEM data on SOME tools. Launch will read them, Forscan will read them. Snappy (on this car, anyway) will not.



The cool part about the Launch is the integrated min/max and average measurement. Really helps picking out the one that doesn't look like the others. Anyway, bent plugs went in and I took some secondary waveforms:

#4


#5


#6


The difference between the small and normal is obvious. :ohmy: The bigger gap, not so much. I dunno that I'd be able to pick out an issue between normal and big based on this waveform alone.

The difference in 'ringing' at the end of the burn line is also evident. The small gap has a solid ring, suggesting that there's still electrical potential, despite the 2+ ms burn time.

So how well does the scan data pick up on the difference? Surprisingly well.



It's no substitute for a scope, but it's not wrong. I'm willing to bet these values will match the scope very closely. How much value you find in these PIDs will depend on your personal skill level and available tooling.

I still have the plugs if anyone has more questions/suggestions. While we're at it, the PCM decided to flag a few misfire during testing... Anyone care to guess which one flagged miss counts?

[quote="Tyler" post=40530

I still have the plugs if anyone has more questions/suggestions. While we're at it, the PCM decided to flag a few misfire during testing... Anyone care to guess which one flagged miss counts?[/quote]


I 'd say the misfire is on the cylinder with the small gap - #4 -......for the spark not being hot enough

A bit surprising there is virtually no difference in the firing kV of the three cylinders, there should be a difference.

I suppose you tested at idle, If so then the firing kV seems to be way to high! This might be a clue why there wasn't a pronounced change in the cylinder #6 with the holy gap. (Did you use the snap-on secondary probe or another one? Maybe it was just a generic inductive probe that does not reflect true firing kV?)

Tyler wrote: So how well does the scan data pick up on the difference? Surprisingly well.

It definitely shows the tight gap on #4. Interesting thing about the wide gap on #6 is that it's showing a longer burn than #3. Do you know the history on the plugs in bank 1? Also is it possible to 'scope secondaries on bank 1?

juergen.scholl wrote: I 'd say the misfire is on the cylinder with the small gap - #4 -......for the spark not being hot enough

That's the one! To be honest, I'm surprised it didn't run worse than it did. :silly:

A bit surprising there is virtually no difference in the firing kV of the three cylinders, there should be a difference.

I suppose you tested at idle, If so then the firing kV seems to be way to high! This might be a clue why there wasn't a pronounced change in the cylinder #6 with the holy gap. (Did you use the snap-on secondary probe or another one? Maybe it was just a generic inductive probe that does not reflect true firing kV?)

Yep, idle, and I actually used the banana plug end of a test lead. :blush: That'd be why firing KV is way too high. I probably should have used a regular voltage scale instead, so nobody gets mislead by the readings.

Interesting thing about the wide gap on #6 is that it's showing a longer burn than #3. Do you know the history on the plugs in bank 1? Also is it possible to 'scope secondaries on bank 1?

I wondered if someone would notice that. :lol: I'm honestly kinda concerned about #3 after looking at the readings. All six cylinders got Motorcraft plugs last year, and bank one got Denso coils. Maybe I bent an electrode over? :huh:

Tyler wrote: I wondered if someone would notice that. :lol: I'm honestly kinda concerned about #3 after looking at the readings. All six cylinders got Motorcraft plugs last year, and bank one got Denso coils. Maybe I bent an electrode over? :huh:

You post strange looking data on a diag forum and folks are gonna notice :lol:

Even on the before PIDs bank one looks to be a shorter burn. Maybe that's the different coil packs. There also appears to be a 1/4 longest to 3/6 lowest pattern going down each bank in that data too.

Regards #3 if you bent it that would probably have tightened the gap rather than opened it up. Do you check the gap on new plugs before you install them?

Matt T wrote: You post strange looking data on a diag forum and folks are gonna notice :lol:

Yeah but this is my wifes daily! :silly: I can't have her driving around with short spark duration. Now I gotta pull the stupid intake off, again.

THANKS, MATT.

Even on the before PIDs bank one looks to be a shorter burn. Maybe that's the different coil packs. There also appears to be a 1/4 longest to 3/6 lowest pattern going down each bank in that data too.

I noticed that as well. For comparison, I took these numbers off an '07 Escape (same engine) that got Motorcraft plugs and FVP coils. It does not display the same pattern.

Do you check the gap on new plugs before you install them?

Sometimes? :whistle: Usually depends on the plug brand and application. On EcoBoost and forced induction engines, every time. On NA engines, I'll usually just compare the plugs to each other. Motorcraft plugs get checked in either case, since they package their plugs without shields over the electrodes.

So I just had to see if moving ignition parts around on bank one would make any difference. :blush: Here we are:



It... kinda did? I looked at both plugs - not much to see:



Eh, it doesn't misfire normally, so I'm gonna let that go. :lol: Speaking of misfiring, I went ahead and took the plug gaps on bank two to their extremes. #6 can't be measured with a gap gauge. #4 is completely closed.



I'm very surprised at #4. There just can't be much spark taking place... Plus, it's nearly a dead misfire now. I check the secondary waveform:



If anyone can explain what I'm looking at here, I'm listening. #6 is more of the same:

Back in the pre-eletronic era spark plug gap with distributer ignion were much narrower plug gaps especially on european manufactuers cars. Cars with Lucas and Bosch ignition systems generally ran 0.025" (0.625mm) gap. The narrowest gap spec I ever encountered was for the Rolls-Royce B series engines which specified 0.015" (0.375mm). Electronic ignitions have made a huge improvement particularly improving cold starting , reducing ignition servicing and vastly control over ignition timing but I have never been convinced of the benefits of the wide spark plug gaps specified on modern cars.

Slope of the burn lines are interesting --- narrow gap so should less combustable fuel-air mixture between the electrodes ?