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.

enter image description here

You can see the two electrodes are touching. Inspection of the piston didn't reveal anything.

  • Strength of the spark pulse was as it should be with the tester setup (commensurate with the other leads)? When you have the tester hooked up, is this inline with the spark plug wire attached to the tester and the other end of the tester attached to the plug? Or is it just the wire to the tester to ground? Apr 8, 2017 at 15:33
  • @Pᴀᴜʟsᴛᴇʀ2 Tester to engine ground, I had the tester adjusted to around 20kV and it reflected on the probe.
    – Ben
    Apr 8, 2017 at 21:41
  • I'm wondering then if it's a bad coil pack? If the #2 output is weak, it may not be good enough to jump the gap of the spark plug (and/or under compression), but may be good enough to register through the test equipment. Apr 8, 2017 at 21:43
  • @Pᴀᴜʟsᴛᴇʀ2 I didn't see any evidence to suggest the coil pack was bad. The fix was actually pretty dumb.
    – Ben
    Apr 8, 2017 at 21:49
  • Do we know anything about the condition of the plug? I'm assuming the answer isn't as simple as "the plug was shot" or "not gapped correctly"
    – Zshoulders
    Apr 10, 2017 at 15:14

1 Answer 1


Barring some kind of basic hardware problem (such as: plug wasn't gapped, connector was dirty, etc), and knowing that the compression test was good, I would guess that the ground isn't grounding on that coil. It wouldn't work without a return right? It worked when the tester was in and grounding.

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .