Spark plug wires (also called "ignition cables") carry high voltage current from the ignition coil(s) to the spark plugs. On older engines with distributors, the wires run from the distributor cap to the plugs, and come in different lengths to reach the closest and furthest spark plugs. The wire set also includes an extra wire that connects the center terminal on the distributor cap to the ignition coil (unless the application is a GM HEI distributor where the coil mounted inside the top of the cap).

On engines with distributorless ignition systems (DIS), plug wires are also used to connect the coils to their respective spark plugs. On some engines, there is one coil per spark plug, while on others with "waste spark" ignitions, two plugs share each coil.

TYPES OF SPARK PLUG WIRES
Spark plug wires come in three basic types:

Distributed Resistance wire. This type has a fiber glass core impregnated with latex graphite. This type of wire provides the maximum amount of radio frequency interference (RFI) suppression. RFI occurs when high voltage passes through the plug wires. Creating a controlled amount of resistance in the wire (3,000 to 12,000 ohms per foot) suppresses RFI and prevents sensitive onboard electronics from picking up false signals that could cause driveability problems.

One of the drawbacks of carbon core suppression wires is that internal resistance creates internal heat. Over time, this ages the carbon core causing resistance to increase. And as resistance goes up, so does the chance for ignition misfire.

Prior to 1980, 95% of all vehicles were equipped with carbon core suppression wires. But concerns over emissions and long term reliability led many of the Japanese OEMs to switch to "mag" style spark plug wires.

Inductance (mag) wire This type has a spiral wound core of copper/nickel alloy wire. RFI is suppressed primarily by the magnetic field formed by the loops of wire wrapped around the core rather than the resistance of the wire itself. Mag wire has less total resistance (only about 500 ohms/foot) than suppression wire, so it reduces the current needed to fire the plugs. The main advantage is improved durability over the long run.

Mag style spark plug wires have been used on Honda and Acura engines since 1971, most Nissan and Infinity applications since 1980, and many Toyota and Lexus applications since 1984.

Fixed Resistor wire This type of wire has a steel or copper metallic core with a fixed resistor in the plug boot to control RFI. This wire is used on many European imports.

SPARK PLUG WIRE INSULATION
Spark plug wires also use different types of insulation. Premium spark plug wire sets typically use silicone or EPDM (Ethylene Propylene Diene Monomer) insulation, with some having an outer covering of EVA (Ethylene Vinyl Acetate) for added temperature resistance and tensile strength.

Under the outer silicone, EPDM or EVA jacket is fiberglass braiding for strength and flexibility, and under that is a layer of EPDM insulation that prevents arcing and voltage leaks. Mag core wire is typically surrounded by a latex silicone bonding layer that provides additional stability and support to hold the spark plug wire in place.

SPARK PLUG WIRE PROBLEMS
Regardless of the type of ignition system or the type of spark plug wires used, good plug wires are absolutely essential for reliable ignition performance and trouble-free operation. A bad spark plug plug wire may create so much resistance that the voltage never reaches the plug, or a break in the insulation may allow the spark to arc to ground. Either way, a bad plug wire will cause a spark plug to misfire.

Bad spark plug plug wires can cause hard starting (particularly during wet weather), poor fuel economy, rough idle, hesitation when accelerating and increased hydrocarbon (HC) emissions. On 1995 and newer vehicles with OBD II Onboard Diagnostics, misfiring due to bad plug wires may also set a fault code and turn on the Check Engine light.

Spark plug wires should always be inspected if any of these symptoms are present, and when the spark plugs are changed. If wires show any obvious damage such as burned or cracked insulation, chaffing, loose plug boots or terminals, the wires should be replaced. Also, if visible arcing is present new wires are needed. Wires should also be replaced if their resistance measured end to end with an ohmmeter exceeds OEM specifications.

As a rule, if more than one spark plug wire has excessive resistance, the entire set should be replaced.

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SPARK PLUG WIRE REPLACEMENT TIPS
Spark plug wires come in different lengths, so match up the lengths with the original wires so the wires fit properly and do not droop or rub.

Replace ONE WIRE at a time to avoid mixing up the firing order (very important!). Start with the longest plug wire(s) and go to the shortest or vice versa. If you mix up the firing order, the engine may not start or it may pop and backfire. This may damage the engine so always double-check the firing order if you are unsure. Refer to the firing order in a service manual or markings on the intake manifold, plug wires or distributor cap (if used). Note: Different vehicle manufacturers number they cylinders differently so make sure you know which plug is number one and how the cylinder banks are numbered.

The replacement wires should be routed exactly the same as the originals, and supported by looms or clips so they do not contact the exhaust manifold(s).

Additional items you might need when replacing spark plug wires include spark plugs, a spark plug boot puller (makes removal and installation easier), replacement looms or wire supports for original parts that might be missing, and dielectric grease for tight-fitting boots (makes removal easier next time, and helps keep out water).