Cracked Spark Plug....
 

Bit of a story for you guys, firstly its not a Subaru before anyone looks at the pics and says it the wrong plug ;)

Well, I've tracking down a rogue albeit minor misfire on the one cylinder bank on my Jag V12, and seeing the car has no intelligent misfire detection as per modern cars, I thought before pulling things apart and stabbing in the dark trying to find which cylinder(s) was having problems. I hooked up the oscilloscope into the laptop (picoscope) and had a look at what was happening.

Low and behold, the HT output on cylinder no 4. looked well dodgy. So that was the first spark plug I removed. To confirm it was ignition and not compression/fueling related I swapped it to a different (good) cylinder (no.6 in my case). The misfire moved with the cylinder. So that ruled out everything but the plug itself.

On initial inspection it looked fine, but on a second closer inspection something caught my eye, I intially thought it was flaking carbon but wasn't so sure, so out came the magnifying glass......

A crack! Not on the centre electrode or insulator as one would expect though, but on the outer around the negative electrode! Now I've seen various broken/failed/fouled spark plugs over my lifetime, but this one is a first....and just goes to show how something so easily missed can cause running problems.

If it hadn't been for the scope showing me which cylinder was misfiring, I would have to pull all the plugs out and probably wouldn't have given them such a close inspection or mixed them up. To the naked eye it was very easy to miss. :o

Take a look: These pics were taken via the magnifying glass with a LED Lenser shining on the plug, without it the crack looks like a flake of carbon.

Now in a normal 4pot engine I'd have changed the plugs straight away without doing any tests just to see if the problem was cured, but seeing this is a 12pot, I didn't want to waste time and money on something that may not be the problem. Luckily in this case it was.

Shouldn't this be in other marques ? :thumb:

Nice find though, I've seen lots of naff plugs but never one like that.

dunx

I was thinking it was abit too techy for OM.

And it could apply to scoobs, as they can be a bit temperamental when the boost is wound up. :)