Recurrent plug 4 misfire fixed!

[greasemanicure]

I've posted several threads on my attempts to correct misfires on plug 4 on our 96 ex 4.0 OHV. I made the rounds through changing the plugs and wires some months ago. Changed the coil pack. Latest change was the injector for cylinder 4. Improved things but did not clear the P0304 code.

Went at it again this afternoon. Using a spark tester, checked for spark at the coil pack and at the end of plug wire #4. Good solid spark somewhere above 35kv. Checked the wire resistance at 9.5 ohms. Checked the coil pack at 11.6 ohms. Put a timing light on wire #4. No flash. Plug open??? Pulled the plug and checked from the top to the center electrode. Not open. For no reason in particular I checked resistance electrode to shell. Should be open, no? Read 6.67 ohms! This is a motorcraft AGSF42FM that I put in maybe in June and the ex has not been driven that much since then with all the misfires.

Change the plug to a new one (measured open between shell and electrode) and the misfire went away.

Anyone ever see a new plug be shorted out of the box??? First time for me.

Joe

 

[Iron Weasel]

Anyone ever see a new plug be shorted out of the box??? First time for me.

Joe

Anything that's mass produced is bound to have a faulty unit show up at some point. If you use some arbitrary math and say that 0.05% of the product has a manufacturing defect and there are 10 million units produced in a year. That's 5000 faulty spark plugs that are in the distribution system. At some point, someone will end up with one of those plugs.

It's uncommon to have that happen, but it's not impossible.

 

[swetrid]

I have seen it before but usually is plug getting cracked during installation.

 

[zukman]

I had the same issue in the Expedition. Took the plug back and got a new one. Truck now runs great.

 

[CDW6212R]

Very good Joe, thanks for the reminder. I've had a miss before that was from a bad new plug also. It is very rare and not obvious when it happens. This thread should remind everyone to consider this each time they change plugs.

 

[Runnin'OnEmpty]

I ran into a bad out-of-the-box plug a couple years ago;
it was either a Motorcraft or Autolite. I do remember that
it was manufactured out of country; maybe the quality control
isn't as good as when they were domestically produced...?

I also once installed a new Champion plug that was bad.....

 

[greasemanicure]

Curiosity got the best of me and I did an autopsy on the defective plug to see how it failed. The results are attached. The picture is of the innards of the plug once the metal shell is removed. The heavy dark lines running parallel to the plug body are marks from my wheel when it cut through the shell metal.

The arrows point to a line in the ceramic material. The arrow aimed at the close up inset points to an "edge" to the line that protrudes from the ceramic body. The fine line on the ceramic has all the appearance of a carbon track but the protrusion can be snagged by a fingernail or a pick. Whatever it is is electrically tied to the electrode and would have shorted out to the shell that was removed. Resistance to the electrode tip is about 0.9 ohms and resistance to the cap is 5.5 ohms, likely the value of the resistance built in for noise suppression, if they still do that. FYI, Runnin'OnEmpty, this one was labeled "Made in the USA".

I've started a flow chart of the process I will follow in the future to diagnose single plug misfires. When I'm done I will scan and attach it.

Thanks for all your help and advice, and for the kind words.
Joe