I just pulled #6 spark plug and it looks fine. The coil and plug both have no signs of cracks/tears or burn marks. I don't feel anything (no line of sight) obviously wrong behind the cylinder head. The coil harness insulation looks intact. No signs of chafing. I'm stumped.

Out of curiosity, what is that yellow thing at the back of the cylinder head? Is that a knock sensor?

 

The yellow thing is the cam shaft position sensor.

I think GA_DB9 is correct in that the ground from the ignition coils goes to a point in the area you seeing a spark (which of course you should never see).

Try and check the wire and the bolt fitting shown here, circled in red.

 

I felt around for the bolt and got a lot of dry carbon in my hands. Should have taken a picture first. It doesn't feel loose.

Can I remove the bolt and clean up the surfaces? It it just holding in the wires?





By the way, this black plastic box, I notice that this box has broken clips and has been zip tied. Could someone have been in here before? For what purpose?

 

That box contains the connectors for the oxygen sensors (upstream) probably broke all the tabs that holds the access cover on.

 

Yes, I would remove the bolt and clean everything, also check the crimp on the electrical fitting.

 

Took the bolt out and it looked clean of corrosion. So did the ring terminal and the face of the head. The terminal was also mechanically secure.






I lightly sanded and wire brushed all the pieces till they were shiny just in case. And coated everything in dielectric grease.

Before I started working, when I was running the engine out of fuel, I could hear the sparks during cranking. So I will put in a new spark plug (just in case) and put the coil back and try dry fire it again to see if I can hear the sparks before I reassemble the intake manifold.

 

While you have it all apart it may be good to do a resistance check between the ground pin of each coil plug to that bolt on the side of the cylinder head. It should only be a couple ohms and will probably test fine. It's just piece of mind just since you can't really check it once it's all back together.

 

Good idea. I checked the ground pin to the grounding bolt and it read below 1ohm (my meter is not very accurate at that range). I also checked the ground pin to the alternator body and it read the same.

None of the coil harness has very positive engagement with the coils anymore. Its all super brittle. When I did the coils last year, I found a few tabs broken off the harness and just lying there. But the connector still fitting tightly.

These cars have reached an age where I think replacing the engine harness is a good idea.

 

Just started the car and its still happening but its much less often now. It can go for minutes without happening and then suddenly start sparking (and correspondingly misfire).

It sparks in different places each time I think. I did see two very clear and big sparks happen in these two places.

 

Could you clarify on your picture exactly where the sparks are jumping to and from.

Just to confirm that the engine has a good ground, run the engine again but this time have a single jump lead clamped between the alternator bracket and a good chassis ground (or even better back to the battery negative terminal).

 

The spark sound is mostly contained in an area near #6 cylinder. This is quite clear when listening to the engine.

But most of the time when it sparks, I don't see the sparks.

The OBD2 misfire count only seems to show up for #6 as well. Is this a red herring? I only pulled and inspected #6 coil. I regret not looking at all the coils on that side. Maybe I should have replaced #6 coil as well. I only replaced the spark plug on #6.

Occasionally, I will see a spark. Maybe 1 out of a 100 sparks. It doesn't happen in the same place each time. The most recent places where I saw the sparks were in the 2 green circles in the picture above. This is also the first time I've noticed the spark happen away from #6. I have not had the car running long enough (>10min) to see if it will spark in that place again.

Previously I had seen more regular sparking near the PCV metal tubes near the grounding bolt. But I'm not seeing that happen anymore. Or at least not in a regular enough fashion to be able to visually see it.

I don't really see a direction with the sparking. It sort of spontaneously occurs.

I will connect the alternator body directly to the negative on the battery and see how that goes.

Some general questions;
1) Is there any other possible cause to this other than an ignition coil leaking spark into the cylinder head?
2) Why is the ground to the coils significant? Earlier DB9s didn't even have a dedicated individual coil ground and I assume relied on the spark plug body?
3) Are the ignition coils active high or active low? I'm pondering over the idea of bench testing them all.

 

I think V12AM is trying to see if your main ground is perhaps faulty. If so then a charge can possibly build up and take the path of least resistance to ground. That might include through the air intermittently. If you want to check the main ground, it attaches to the front right of the torque tube to ground the drivetrain to the chassis. Check both ends. Others have indicated the connections points can become corroded or loose.

 

I tried grounding the alternator directly to the negative terminal on the battery and I was still getting sparks.

Since I'm only seeing the misfire count for #6, I'm going to replace the coil there even though I physically can't see any cracks or burns on that coil. And if that doesn't do it, I'll send it out to someone. As I don't know how to troubleshoot any further.

The chassis grounds on the right-hand side frame rail all look clean and secure.

 

As for as grounds- Make sure you check BOTH sides of your battery ground ( at the battery terminal and at the point the battery connects to the grounding point of the car). Other than that the voltage is leaking somewhere! Could be #6 plug, plug wire or coil loose connection or crack/defect? Wow you've done allot!

 

Just got done replacing #6 coil and after 20mins of running, no sign of sparking or misfire. Will button it up, reset the ecu and go for a drive later.