Technical head-scratcher: spark plug of Damocles?

Frank called me yesterday about his truck "running VERY rich" suddenly, so I drove to his house this PM & started looking at the ignition system. The dist-mounted TFI is only a few months old (replaced it on the way home from OCBR09), and the cap & rotor weren't THAT bad. The plug wires were original 1988 F150 parts, so I suspected them, but couldn't find any specific problems. The coil didn't seem to be the problem - the engine was missing badly even while the coil was still cold, and he said it would intermittently run fine even when warmed up. So since we couldn't remember how old the plugs were (possibly also 1988 vintage), AND because I hadn't read the TSB about wire routing when we built the engine, we started pulling plugs & unclipping the wires. All of them looked brand new - normal white insulator (not overheated) with no deposits on either electrode, and just a layer of dirt/oil on the exposed ceramic (outside). About halfway through cleaning & gapping the plugs (the R bank), we decided to just replace them & the wires.

So after returning from O'Reilly's, I pulled the L bank, and found this in #8:



WTH???
The ground electrode just MELTED onto the center electrode! Note how clean the insulator is - they all looked at least that good. I scraped the grime off the outside ceramic just to make sure it was only grime & not heat-damage. None of the others was even slightly degraded. And after putting the new plugs & wires on, the engine ran PERFECTLY again, so I don't think there's anything in it that would have caused this. It has a CV intake manifold, so it's not the PCV-reroute problem that trashes '84-93 truck #8 cylinders.

Anybody have an idea why this happened? I'd like to make sure it's not gonna happen again, but another pro mechanic says he's seen one do this, and then never do it again after a new plug. He agrees with me that it has to be simply a defective plug, but that's not a very satisfying or reassuring explanation.

That plug looks awfully dry for having not fired for a while, I would look very closely at the injector on that cylinder, poor fuel delivery could make that cylinder run lean enough to melt the electrode.

The engine was warmed up during the initial test drive, so any fuel on it would have evaporated by the time I pulled it an hour or so later, and then there were a few more hours before I took the pics. And it was apparently sparking intermittently because there wasn't a DEAD miss. But all the plugs looked just like that, other than the melted electrode. And with the new plug (no injector cleaner or anything else), the engine went back to purring & hitting all 8 every time. I'm baffled...

Looks almost like coolant deposits doesnt really appear to be a electode melting.

Just some carbon hung up in the gap, and then it continued to build up....

ADNICK said:

Just some carbon hung up in the gap, and then it continued to build up....

Click to expand...

This is what I am thinking too. However I think it was one piece of carbon that came loose and lodged in the gap of the plug. Not a build up situation. carbon will conduct and act like a dead short. this is what is causing the melting. the plug is dry because it was working just not sparking. It was hot enough to burn. carbon fowling like this does not happen that much with todays fuel but looks like your one of the lucky ones. I bet it never happens to you again.

The green is only apparent under the camera flash - IRL, it has no perceptible color. Plain black. The material is too strong to be simply Carbon powder - it's metal, and that plug's ground electrode isn't as long as all the others. As you can see in the pics, it doesn't reach over the center electrode any more, so there's definitely some steel missing from it. And if it was Carbon, there would be SOME on the insulator. There's none, even down deep.