So over the past week or so I have been struggling with what turns out to be an ignition misfire on my MY94 XJ6 but I did not know what was causing it.
My initial thought was that I had some sort of fuel supply issue. So this evening I swapped my fuel rail for a special one where I have drilled a extra fuel tap port back by the fuel supply inlet on the rail and hooked up my pressure gauge to the system.
But after doing that, I started the car up and ran it for a while it in my shop but the fuel pressure did not seem to be the smoking gun I was hoping for. But just to make sure I left the special rail on the car and drove back home from my shop.
Midway home the car started acting up again so once I reached home, I left the car running in the driveway while I popped the hood open and prepared to hook up the fuel pressure test gauge.
Just before I hooked up the fuel test gauge, I reached over and manually twisted the throttle with my hand. At this point it is very dark and there are no lights in the area when I park this car in my driveway.
As I twisted the throttle while leaning in from the driver's side of the car, I noticed some small blue sparks flashing over on the front of the passenger's side inner fender.
So I walked around to the other side of the engine compartment and again leaned over to gave the throttle a twist with my hand while I watched the inner fender for the jumping sparks.
Ah HA! There are twin sparks jumping across from the inner high voltage tower on the coil to both the positive and negative low voltage posts on the coil!
So now I have my smoking gun as to the cause of the misfire and just have to try replacing either the ignition coil and/or the ignition leads.
My thought is that it is the coil but I will just have to see what happens when I examine it closer tomorrow when there is more light.

Are the coils used on the XJ40 anything special or should a standard coil function fine?

 

Hi Steve,
I would definitely do the plug wires first. The coil either work or does not. Its the wires that age and start allowing spark to leak. The hoarder in me likes to keep an old set of wires around for problems like this, that way I can swap one out and look for change. I dont think the coils are "special" per se, a plus and a minus tower and the Hi tension tower. There is that silly rubber cap though, but its not part of the coil. Good luck.

 

The puzzling thing about this issue is this problem comes and goes. Sometimes the engine runs fine but other times the misfire is so bad that it is difficult to drive the car.
As you say typically coils AND plug wires either work or they don't. Except possibly when the plug wires are damp or dry they can act differently.
For this spark energy to want to jump across these poles of the coil, the normal electrical path has to have some really serious resistance since the boot over the end of the coil wire fits down over and almost completely covers the center tower of the coil.
From what I seen last night in the dark the spark must have either been at the very bottom of the coil wire boot or actually sparking through the boot.
Yesterday when it was acting up the weather was snow and wet on the roads.
I just ordered a couple spare rotors and some ignition wires from Rockauto but that stuff won't arrive for a few days.

 

check the main engine grounding lead , it may be broken , or the bolt might be loose, or maybe the cable has gone green inside causing grounding issues

BB

 

Yesterday I pulled the coil off, cleaned all the accumulated grease and oil off of it and the ignition module right in front of it.
This all built up over a period of time because I had a leak for a while at the upper timing chain tensioner plug.
I also cleaned all of the wire terminals that bolt onto the coil. I also looked at the brass terminals on both ends of the coil wire.
I cleaned the heat sink plate that the ignition module sits on and added new dielectric grease when I reassembled the ignition module onto the aluminum heat sink plate.
I also carefully tried wiping down the coil wire. Then later in the evening I took the car for a drive. On the way out on the drive the car ran fine.
Then on the trip back home it started acting up again so when I got back to the driveway I once again popped the hood open and looked for the jumping sparks.
Same damn thing, not as bad since the weather was a lot dryer that day than what it was the other day but still the same basic problem.
I will look at the engine ground today.

 

Why would you use that there?

I tend to stay away from that stuff as it has caused me lots of pain preventing good connections. Only use it as an external coating if you HAVE to, never use it within a connector or ground point..JMO

Larry

 

The dielectric grease between the bottom of the ignition module and the aluminum heat sink plate is there to allow heat to be efficiently conducted down to the heat sink plate.
This is the same idea as putting a dab of similar heat sink grease on the top of a computer chip before the heat sink is strapped down to the computer motherboard.
There is no electrical signal passing through this interface. Just heat being conducted away from the ignition module.

 

Heat sink grease maybe. Is it possible the module needs to be grounded through the casing? Probably not, but like I said, not much use for that stuff anywhere on my car.

Larry

 

So one other piece of information is that I have the jumping spark occurring between the positive terminal of the coil and the central tower. This is what I witnessed the other night when it was a LOT dryer than the first night where I noted the spark jumping between the central tower and BOTH low tension electrical connections. So again I am confused as to why this spark jump occurs between the central tower and the positive side of the coil.

 

Just for clarification for other readers, dielectric grease and thermally-conductive grease are not the same thing.

Dielectric grease is usually a silicone-based product designed to insulate electrical or electronic components and serve as a lubricant and moisture barrier.

Thermally-conductive grease or heat sink grease may also be silicone-based for good dielectric properties, but also contains metal oxide powders to provide good heat transfer.

Using dielectric grease instead of thermally-conductive grease may not promote proper heat dissipation from the hot component to the heat sink. The paste used between the ignition module and heat sink plate should be thermally-conductive.

Cheers,

Don