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?

 

Copper spark plugs ?

When the engine is under high resistance driving load the compression on the engine is at it's highest vs. a unloaded engine

The air gap resistance goes way up with a very long equation

I had the similar problem with a GM using platinum plugs and iridium's would be the same ( exotics )

 

It appears the boot is quite old, as well as the red boot. I would venture to say you have a small crack in the tower of the coil itself, carbon tracking out from under the boot, or a crack in the boot itself. All should be easy to replace. I have seen many a crack in the coil itself. Electricity is simply looking for ground, and it will find it one way or another. Hope it helps.

Jack

 

I had this condition for a few years so I have late last year replaced the coil with a brand new one and the problem is still there. Also as I said it has new ignition wires as well.
This evening I also replaced the spark plugs. 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 tank?

 

I sure wouldn't think so? Have you tried using a silicone sealer on the tower to seal the high tension lead boot, when you push it into the coil? Put some around the tower just before you push the lead in, and let it set 24hrs, to cure. Might help might not. You are experiencing something rather strange. As I said electricity is simply looking for ground. Usually it's pretty simple to isolate things, to prevent this from happening rubber boots, etc??? Thinking about it. The resistance in it's normal path through the high tension lead, has increased to where it is as great, if not greater then the resistance to where it is jumping? Maybe you have a bad high tension lead? Just a few more thoughts? Hope they help.

Jack

 

if i'm not mistaken those coils were high power or something along that stuff, i have tried to replace mine with standard bought from online jag spares and it lasted one month, other thing are cables, if they are not oem or known good quality they may arc from new and almost surely will after some time, then parker point about plugs is valid place to check.
I would bet on cables, try to temporary wrap it in isolating tape and see how it goes

 

SOLVED!!!
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.

 

If it's running well with the new stock plugs? Why bother. Keep the Platinum's for the next tune up. To much hassle, especially if it's running good. Just my thoughts.

Jack

 

Hi Steve,

My personal experience with the AJ6 engine is that it runs best on the original cheap Champion copper plugs. This seems to also be true of the AJ16 engine in the X300s.

I have marked your thread RESOLVED!

On a separate note, since you had three different threads going on this topic, I have combined the two threads on distributor orientation/installation so you now have only two related threads.

Cheers,

Don

 

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.

 

Unfortunately it is NOT resolved Don.

 

Hi Steve,

Sorry to hear the disappointing news. I have unmarked your thread RESOLVED.

Regarding the engine ground strap, you can't assume anything. You really should remove it, thoroughly clean the terminals at both ends, inspect for corrosion on the braided cable where the terminals are crimped on, clean the body and block where the cable connects, and reinstall the cable. Remember that the starter draws more current than any other component in the vehicle, so corrosion on the ground strap may have a less noticeable effect than for the ignition. Given the transformer effect of the coil, the ignition output is very high voltage but very low current, so, theoretically, it is possible that corrosion on the engine ground strap could prompt the coil output voltage to find a lower-resistance path to body ground.

Cheers,

Don

 

OK, I will take a closer look at the engine ground strap. Any idea where it is actually located on the car?

 

Hi Steve,

Off the top of my head, one end of the engine ground strap connects to a starter mounting bolt, and the other end connects to the body near the front left inner wheel well.

Cheers,

Don

 

Earlier this evening after getting the lawn here cut for the first time this season, I started up my 1994 XJ40 with the spark jumping problem to make sure the spark jumping problem was still occurring. It was so I jacked it up and crawled under the car to look at the ground strap.
I did not actually remove the ground strap since this was here in my driveway where it is not the best surface for safely jacking up a car. I did however stick my head under there to look at it and reach my hand under there to see if it was particularly loose or corroded. I did not see anything immediately obvious that seemed to be a problem.

Next I got out some silicone dielectric grease to try that suggestion. I removed the coil wire from the top of the col and smeared a generous amount of dielectric grease around the outside of the center tower of the coil. I then used a pair of needle nose pliers to gently bent the contacts on the metal part of the coil wire that normally sticks down inside the center tower of the coil out a little bit so it would make good contact with the metal inside the center tower of the coil. I reinserted the coil wire onto the center tower of the coil and started the car up. It seemed greatly improved so next I took the car for a ride around the block. It does seem like there has been a definite improvement in the way the engine runs. I still don't know why I need to do this but I will keep checking it out and update here with any changes.

 

Grounds can be a terrible pain in the ***. They can look fine but the corrosion is often down between the metals where you can't see it. That is why everyone keeps reiterating to remove the ground straps, and clean the connecting parts, and areas around them. On the weather issue. Excess dampness / humidity will actually cause the plug wires to increase their resistance. We had a 71 Thunderbird that when it was cold damp or rainy outside, it wouldn't start, take a hand held hair dryer out, and heat the distributor cap, and plug wires for 5 minutes or so, and it would start fine. We finally fixed it by taking the cap and wires off, and spraying them with clear lacquer paint. Which sealed the wires from the moisture. Problem solved, I also had to do the same thing on my 550 Honda, as if you got caught in the rain it would lose a cylinder or 2, and you'd have to set under an over pass until it stopped raining. Lacquered the wires, problem solved. I'm not saying that's your problem just a thought from past experience. You're definitely experiencing something strange.

Jack