Treatise on Spark Plug Wires


Contributed by drbob

For about a month I have been casually chasing an intermittent miss in my '92 Explorer. Over the course of that month I replaced the plugs, changed the oil (not related...), and ran a couple bottles of FI cleaner through it. I also checked the wires with a DMM for continuity and total resistance. Doing each of these steps individually was intended to help me identify the real problem.

The old plugs looked fine, Bosch Platinum's with 15k on them. They were replaced with Autolite Platinum.

Wires tested fine with the DMM, with resistance reading about 4K on the shortest wire to about 8K on the longest. Well within reason I thought. I did a physical inspection when I changed them, with no damage noted. These were Motorcraft wires, replaced at 55k. car now has about 115k on it, so they were due at least according to history.

Anyway, new wires eliminated the miss completely. Even though the old wires tested well and inspected well, new ones let it run a lot better. Besides the erratic miss under load, the idle is a lot smoother too.

FWIW, the new wires are Borg-Warner replacements, at about $20 at Pep Boys. Compare to another set of Motorcraft at about $40 at the local regular parts store.

So, if you are having any of the symptoms above: slightly rough or erratic idle, or a hesitation while accelerating that might be otherwise diagnosed as a fuel injector problem, think seriously about the wires as a possible solution. Looks like the magic number on wire life is about 50k, at least for me and Motorcraft wires.

How to Get that Last Plug

Apparently, the easiest route to changing the rear plug on the passenger's side of the vehicle is to get at it through the wheel well.


Contributed by Ken H.

I just purchased a '92 Explorer Sport with 136K on the odometer. During the test drive, I didn't notice any problems; however, the previous owner had recommended changing the fuel filter because of a "bump" at 55 MPH. The previous owner gave me the fuel filter he had just purchased. Sure enough, I noticed a definite "bump" on inclines at 55 MPH, in overdrive, and under slight acceleration. This "bump" was repetitive and appeared to be more serious, like transmission serious. The whole vehicle "bumped" and jarred.

Skimming through the excellent service records that the previous owner kept, I found that the fuel filter was replaced two years ago. Reading the Haynes manual I had purchased for the vehicle, I discovered that the fuel filter was not recommended to be replaced unless clogged. I changed the filter anyway; and of course, it was not clogged.

Skimming through the service records once again, I found that the plug wires were never replaced. Still thinking I had a transmission problem, I decided to change the plug wires just in case. Changing the plug wires worked, no more "bump".

What's peculiar is that the idle was never rough, and there was never a stumble in immediate acceleration. This accompanied with a first time Ford owner, the general used vehicle fears, and the "I told you so!" ribbing I was going to get from my Chevy loving family, caused me to suspect the worst. I can only assume that there was not enough spark energy to combust the fuel completely under a low RPM load in one or more of the cylinders.

 


Contributed by Matt C.

Something that should probably be mentioned about the Explorer (at least a '91 4.0 liter engine) is that the DIS? (distributor) is not in the same order as the plugs. Pay close attention when changing those wires.


 

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Updated November 21, 2000

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