Cracked Spark Plug....
01 July 2011, 01:43 AM
#1
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.
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.
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.
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.
Last edited by ALi-B; 01 July 2011 at 01:50 AM.
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