MY94 spark jumps on coil and causes misfire (see video on YT)

For a few years I have been struggling to identify a problem with the ignition system on my 1994 XJ6. When it is at part throttle or somewhat steady state going down the road or even just running here in my driveway I have a bad misfire that is caused by spark jumping across the terminal on the coil. It is somewhat intermittent so it doesn’t happen 100% of the time but it’s often enough that it makes it somewhat nerve wracking to drive. The strange thing is if I floor the throttle when it is doing this the problem goes away. See the following video I uploaded to Youtube to see how the spark is actually jumping across the coil terminals: The jumping spark can be seen at about 46 seconds into the video.

The things I have done over the years to try to identify the problem are:

  • Replaced the coil
  • Replaced the ignition wires
  • Replace the cap & rotor
  • Replaced the crank sensor
  • Replaced the ignition module
  • Replaced the spark plugs
  • Checked the fuel system pressure

I think the fact that when the throttle is floored the problem goes away is probably a clue but I don’t know what that clue is trying to tell me.

Anyone have any ideas?

Have you observed a correlaton between humidity in the air and the spark jump / mis-firing? Conventional wisdom is electrical ‘leakage’ increases with humidity. Can you induce the spark jump by spritzing the coil with a very fine water mist? If so that will be helpful with troubleshooting.

You did not list the rubber boots covering the coil terminals as items that have been replaced. Have you tried coating the insides of those boots with dielectric grease and sealing the gaps between the boots and the top of the coil boot with dielectric grease to see if that stops the sparking?

The rubber boots are part of the new spark plug wires.
I have not tried using dielectric grease on the boots on the coil wire.
I have never had to do that on any other car I have worked on in my life.
But I am willing to try anything at this point.
Today was a very dry day here in NE Ohio so very little humidity and no rain or dampness at all.

Hello Steven - it has been my experience, that when a spark is jumping over, there is a lesser resistance to a ground source for this to happen - this path may be caused by an accumulation of dirt/grime for example - in looking at the video you posted, I am thinking that this may be the case of why the high voltage (center terminal on the coil) is jumping across to that other terminal - if the resistance of your coil wire, to the distributor, the resistance of the distributor rotor, the resistance of the spark plug wire, and the resistance of the spark plug, all are so high of a resistance, as compared to the air gap resistance to the side terminal on the coil, then the spark will try to jump over to that path of lesser resistance - you could try cleaning the top of the coil with alcohol, between the terminals; also make sure that the center coil terminal boot has a very tight fit on the “nose” of the coil - changing your spark plugs to a smaller gap, and changing your spark plug wires to a lesser resistance (as measured with an ohm meter) may well help your problem - Tex.


What kind of plugs do you have installed?

definitely a weird issue…

Looking at my XJ40’s ignition coil, i cant see any rubber caps, yet it runs flawlessly…

have you tried running it without those rubber caps. Sure, normally they would isolate but maybe yours are bad for some weird reason… Maybe its worth trying it out.

Good luck and keep us updated

I had NGK BKR6EGP platinum G-Power plugs in there but this evening I removed those and replaced them with brand new set of Jaguar EBC11480 plugs which are the OEM Champion plug RC9YCC. This made no change to the condition.

Could this be caused by water in the fuel?

Setting the distributor to the correct position at TDC seems to have fixed the issue.
It wasn’t off by much but I guess it was off enough to affect the resistance of the spark path.
Thanks to everyone who responded with hints and suggestions.
Now I am trying to figure out if I should put the NGK BKR6EGP platinum G-Power plugs back in the engine.

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Engine is designed for copper plugs. Use those. I changed plugs once in 12 years/100,000km and only because a guy in France had OEM ones and thought it would be cool to run them. The plugs I replaced 50,000km ago were in the car when I bought it and even after an unknown amount of miles showed no degradation in performance or appearance.

cool to see you have fixed it :slight_smile:

OK, I was wrong. Resetting the distributor to a slightly different position only had a temporary effect on the problem. I think the solution was actually due to a change in weather rather than the actual change in distributor position. Since then with the advent of colder weather this problem has reoccurred. Today the car failed to start several times and when it was starting the spark jumping on the coil was so bad that the car was doing a lot of hesitation and bucking while driving around. This problem is driving me crazy since I have now been trying to solve it for at least two years now.

But it still makes no sense to me why a change in weather would cause this extra resistance in the ignition circuit to the point where this large gap with lots of insulators in the path would still have a spark jump problem. I am very very stumped by this problem. I was thinking it might be a ground strap on the engine but then it occurred to me that there is absolutely no issue with the engine spinning over briskly on the starter. Particularly when the engine won’t start. The starter spins the engine over just fine for extended periods when I am trying to start it.

what happens if you take the old rubber caps off?

Would covering the area with some non conductive material help at all?

Maybe you can measure your resistance in your wires? I just installed a new rotor, distributor cap and wires. I could tell you my resistance and if yours is way higher something needs to be replaced

I was thinking about using some globs of silicon sealant or maybe some dielectric grease between the coil wire boot and the center tower on the coil.

It has also been suggested that I check the ground strap on the engine.