Coil on spark plug igntions.

[OP800]

I need some insight on specifically what happens at the plug when the gaps cannot be fired for any reason such as a very lean misture, excessive gap or a short.
The plugs have center electrode resistance anywhere from 500 to 5000 ohms.
Does the spark attempt to find ground to the shell in these cases, causing random missfires?
How does the voltage drop accross the resistance affect the outcome?
In the specific situation the coil is right on top of the plug by spring connection inside a rubber boot.
The coil resistance is 7000 ohms, the plugs are in the 3000 ohm range.
I am trying to track down the reason for random light load missfires that go away with a plug changes but return in a short time and go away again with another new set of plugs.
This last set was selected for lower center electrode resistance and still in testing stages.
If the missing returns, I am next going to replace the boots.
It may be the boots have grown in size and donot fit the plugs tight. As soon as the dielectric grease dries hard the leakage begins.
The coils may be pretty hot for voltage.

 

[Fabrico]

4 inch gap! We're talkin ignition coils, not Tesla coils! :-0

 

[LordMalaK]

No, really. Not as much in COP systems , more in the older GM DIS systems. I've actually heard the 'crackle' of spark jumping thru bad insulation from burned wires as far away as 10 feet from the car.

This is why they put warning stickers on cars, warning of lethal voltages.

 

[patdaly]

With modern injected engines, wouldnt the leaner idle mixture raise the firing voltage requirements?

Another thought is you may actually have a misfire condition under high load conditions, just not noticeable due to more firings per second.

 

[patprimmer]

If I have an ignition system based misfire only under certain conditions, I jump to the conclusion that those are the conditions under which there is the most resistance to a spark jumping the plug gap.

If a spark won't jump a gap, it will very often jump a smaller gap. If the gap is to small to generate a hot enough spark to fire the charge, it will still misfire. You then know whether it is an insulation breakdown, or insufficient energy to light the charge you have.

If the latter you then need to decide if you need to improve spark energy or improve charge quality.

The ignitability of the charge can be affected by any number of things, like A/F ratio, charge temperature, cylinder pressure at firing point, fuel distribution, fuel particle size, turbulence etc etc.

Cylinder pressure might degrade due to poor ring seal, lack of oil on the bores, poor valve seal, changes to valve timing due to cam drive wear or miss alignment or cam lobe wear.

Cam lobe wear is a common cause of a low speed miss.

Regards

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[OP800]

Pat has pretty well sumed up the issue.
This motor is a Ford 4.6L mod SOHC V8 type that is fully PCM controlled.
There is no ignition timing adjustment possible, or mixture changes unless made in the program.
The 'roll on' from a closed throttle and apparent missing, is at a time when the injector pulsewidth is small and the mixture is very lean.
This is one of the hardest times for spark to fire a lean mix besides when the EGR routine is in play with a hi amount of exhaust gas present.
The plugs reside deep in a bore setting with boots over the coil links.
I find that replacing plugs clears the 'apparent' missing fault so assume the reduced requirement of spark voltage on new plugs really only last a short time before the fault can reappear. The culprit may well be the boots need replacment as well. That is the next action to be taken.
It looks like this system lives close to a marginal condition and does not take much to go over the system limits before the spark at the electrode become unreliable with a vary lean cylinder condition.
The real situation, as I now find out, is the inherent low power tranfer ability of these ignition systems using center series resistance. I have already reduced the resistance by selection and see a imediate difference in plug performance but this is not all that needs to be addressed to totally solve the problem. Were getting close.

 

[Metalguy]

Have you tried new Iridium plugs? Have you opened the gaps to the max. limit to see what happens?

"When the eagles are silent, the parrots begin to jabber."
Winston Churchill