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Spark Plug Wire Replacement

Maintaining your car's connections

Time was when your mechanic tuned up your vehicle you could often get the plug wires installed just for the additional price of the parts, since it only took a couple of extra minutes. And the parts were so inexpensive that many people opted to have the wires changed at every tune-up (15,000- to 30,000-mile intervals). Nowadays ignition tune-ups are less frequently required (60,000 to 100,000 miles), so changing the plug wires might occur less frequently. That's probably a good thing, though, because spark plug wires are now a bit trickier to install and a good quality set can be somewhat costly. But you'd expect them to cost more, wouldn't you, since the wires are built to last longer?

Keep in mind that today's spark plug wires are called upon to do more work than ever before. They must pass higher current voltages more often (due to distributor-less technologies) and in the higher temperatures under the hood. All the while containing the current better (through increased insulation materials) so as not to upset delicate electronic components nearby. Consequently, breakdown is almost inevitable.

Causes of Failure

This electrical failure can be greatly hastened by loose connections at the ignition coil(s), distributor, or spark plugs (causing telltale corrosion at connectors). Over time this loosening can develop from quality variations in the wires themselves, or through normal engine vibration. Other causes include retainer failure, fluid leakage onto the wires (causing the wires to swell and soften), or even poor original routing (too close to exhaust components or components with sharp edges).

When these conditions exist—and they may or may not cause a noticeable misfire—the wires must be replaced immediately, lest they wreak havoc on other components such as ignition coils and nearby electronics, or exhaust system components such as oxygen sensors and catalytic converters. Now that stuff can get expensive!

How Often?

Aside from those kinds of conditions existing, just when should you change the spark plug wires? Generally, at the manufacturer's recommended tune-up intervals where the plugs are replaced, or at five years (60,000 mile tune-up) or seven years (100,000 mile tune-up), even when the plugs don't necessarily need to be changed due to age alone.

If you are not sure that the wires have been changed already (supposing you purchased the car used), there are a couple of ways to tell just by looking at the wire. On domestic vehicles, the factory replacement wires usually don't have the corresponding cylinder numbers printed on them, whereas the original set of wires for the vehicle will. However, on many Asian vehicles, both original and factory replacement wires will have cylinder numbers printed on them and they also will have their date of manufacture printed on them.

Replacement Tips

Here are a few modern strategies to go along with the modern vehicle, once you're ready to tackle the wire replacement:

> Use the highest quality set of wires you can find. A cheap product is ill-fit and of inferior materials, which won't last until the next replacement interval.

> Read and follow all instructions for installation. Even a slight variation can impair performance.

> Observe the original installation carefully, to the point of taking notes, drawing diagrams, or even taking photos. Note the specific routing order and unsecured retainer locations. Record the engine firing order (if distributor-equipped), or ignition coil pack numbering (no distributor), which will either be on the coil or on the underhood emissions decal. If you can't find that information at either location, it would be wise to get a service manual that has that info. Ditto for the distributor-equipped models.

> When removing the old wire from the spark plug, always twist the "boot" first, to break it loose from the plug insulator. Then, when possible, use a special tool, like the one pictured, to pull the wire from the plug. That will save a lot of skinned knuckles and reduces the "carpal tunnel syndrome" effect that this procedure might otherwise cause.

> Do plan to use all of the original retainers, both secured and unsecured, opening them with an appropriately sized (small) standard tipped screwdriver.

> Although some advocate the "one wire at a time" method, it can sometimes be difficult to determine the length of each original wire relative to the replacement wires (even if using a factory set). You may wind up doing most of the job before discovering that the wires you have left are either too short or egregiously long! Not a pleasant revelation if the installation is particularly difficult, such as on a transverse V-type engine. Instead:

> Remove all of the wires and group them together according to which bank of cylinders they came from (V-type engines). The goal is to have a new group of wires that is relatively proportional to the old one. Note any patterns, such as the fact that the wires for one bank may be longer, overall, than the other, and which side has the longest and shortest wires. Then, start to group the new wires together to match the old, keeping in mind that all of the wires in the new set are going to be a small percentage longer than the corresponding old wires they replace.

> For installation, start each bank with the plug wire that mounts the lowest in the secured retainers, connecting the wire first to the spark plug and then working back to the ignition coil pack or distributor.

> Make sure to bleed trapped air from underneath the wire "boot" at the coil or distributor connections (pinching the boot between you fingers usually works).

> Double check your installation, especially in areas adjacent to other components and the exhaust system.

This task of replacing plug wires can be a little tricky and tedious, to be sure. Yet it's absolutely worth the effort, both in protecting other expensive components that depend on good ignition function, as well as keeping the engine operating at peak performance.