Customer brought this one in, he had replaced the plugs and wires as general maintenance no codes or complaints before the work. After the R&R the truck had a noticeable misfire the tail and dash lights stopped working and the check engine light was on a with a p0302. After the light came on he swapped the old #2 wire and plug back in without any change.
A little bit of testing
Did a quick relative compression test and it looked pretty good.
I also took a look at the primary coil ramp since the fusebox was right next to the battery. In it you could see that the coil was firing without any missing peaks or any evidence of a short.
Hooked up an inductive probe to the #2 spark plug wire and... There's no reading? Played around with the time base a bit and still nothing. I also swapped the probe around to different wires to make sure it wasn't the setup. Fortunately the #2 plug wire is somewhat easy to get at and was able to get my hand down there to pull it off. Hooked up an adjustable spark tester to confirm and what do you know... I'm getting spark and my secondary pattern is showing up on the scope.
So the old wire is in decent enough condition and isn't arcing (which wouldn't really explain the lack of a secondary pattern anyways). So what's going on?
There probably isn't any real correlation between the misfire and the tail/dash lights not working so I'll limit it to the spark issue.
Why would I have spark when the wire is hooked to a tester, but not when it's attached to the spark plug?
The plug in question.
You can see the two electrodes are touching. Inspection of the piston didn't reveal anything.