Shorted Spark Plugs Cause Misfire Code

I have a 1996 Ford Explorer with a 4.0-liter OHV engine. It developed a misfire on cylinder 4.

It was overdue for the replacement of spark plugs and wires, and after I replaced them the problem cleared for about 10 days, which some mechanics call a "perfect repair." Then the misfire on cylinder 4 happened again. I checked the new wires and checked for spark from the ignition coil pack. No issues were found there. Since the plugs were new I felt no need to check or replace them. After long and involved diagnostics, including a check for vacuum leaks, bad injectors, and EGR problems, I went back to the basics.

I used my timing light to see if the misfire on cylinder 4 was intermittent or random. I got no spark indication from the timing light. All the other cylinders gave me a steady, rhythmic flash of the timing light. I was getting spark from the ignition pack -- continuity on the plug wire was verified but still no spark indication. I put a spark tester at the end of the wire and got a spark and a flash from the timing light. I put the plug wire back on the spark plug and got no flash of the timing light.

I swapped plugs 4 and 5 and the problem moved to cylinder 5. I got a new plug and things were fixed. For another month things went well and then I got misfires on cylinder 4 again. I did the same diagnostic routine with the timing light and it looked like the plug had failed again. I swapped it out with a new one and the problem went away.

Not to let this go without further investigation, I checked the last spark plug that was replaced to fix the misfire. I expected it to be open as the timing light did not flash -- indicating no current in the spark plug wire. The plug showed continuity with some resistance from the noise suppressor resistor. I did not understand the result, but then checked the plug for leakage from the core to the shell. It was a dead short. All that ceramic, and new at that, and a dead short. I did an autopsy on the plug and cut the metal shell off with a dremel tool. What I found was a small metal shard embedded in the ceramic material that was under the metal shell. It was a dead short from there to the plug core. I dug through my toolbox for the other plug that failed and found the same thing. Both were new Autolite spark plugs.

What I concluded is that the shorted spark plugs were loading down the coil pack and not allowing enough surge current to trigger the timing light through the induction pickup. I sent the failed plugs off to Autolite and am waiting for their response on what I found.

Joe Olejar is a senior engineer with CenturyLink.

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