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.
On coil-on-plug (COP) ignition systems, there are no plug wires because the coils are mounted directly on top of the spark plugs. But in coil-near-plug (CNP) systems, there are short wires that connect the coils and plugs. On some applications, these wires are part of each coil assembly and cannot be replaced separately.
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 and fewer misfires.
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. Most late model import and domestic vehicles come factory-equipped with mag style plug wires.
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 older European imports.
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.
The insulation around the core may also be surrounded by additional reinforcement such as braided fiberglass, and there may be an outer covering or jacket on the wire such as EVA (Ethylene Vinyl Acetate), EPDM or silicone to provide additional thermal protection and abrasion resistance. Economy wire sets typically use less expensive insulation that may not be multi-layered or as thick, and does not provide the durability and reliability of a premium wire set.
The terminals on the spark plug wires are also important, and must fit properly to maintain reliable ignition performance. The terminals that connect the wires to the distributor cap or coil must be tight and fit correctly. The spark plug boots must also fit the plugs tightly to keep out moisture and dirt that could cause arcing and misfire. Spark plug boots are usually molded silicone rubber, but many import applications may be phenolic or even metal tubes.
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.
CAUTION: Never touch the plug wires while the engine is running. The high voltage can give you a severe electrical shock!
If the vehicle has an intermittent misfire, you can artificially increase the coil's firing voltage with a spark plug tester to produce a hotter than normal spark. This should cause any leaks in the plug wires to reveal themselves.
You can also use an ignition scope or digital storage oscilloscope (DSO) to detect bad plug wires. Look at the secondary firing pattern. A bad plug wire with excessive internal resistance will typically cause an intermittent or steady misfire that is most noticeable under load. This will cause an increase in the affected cylinder's firing voltage. An open plug wire or spark plug may cause the firing voltage for that cylinder to spike to the coil's maximum output.
If you see a higher than normal firing voltage for a particular cylinder, turn the engine off and measure the plug wire's resistance end to end with an ohmmeter. Refer to the manufacturer specifications, and if resistance exceeds specifications (generally no more than 12,000 ohms per foot with carbon-core wires, or over 1000 ohms per foot with mag wire), the wire is bad and must be replaced.
If the spark plug wire set is more than seven or eight years old, and one of the wires is obviously bad or has excessive resistance, you should replace the entire wire set. Also, on newer vehicles that were factory-equipped with long-life platinum 100,000 mile spark plugs, if it is time to change the spark plugs (and your engine has a distributor or DIS or Coil-Near-Plug (COP) ignition system with plug wires), it is a good idea to replace the plug wires too.
A shorted plug wire or grounded spark plug will cause a drop in the firing voltage when the ignition pattern is observed on a scope. The engine will also have a steady misfire in the affected cylinder.
Another trick for finding weak spots in the wire insulation is to rub a grounded probe along the length of each plug wire while the engine is idling. If you suddenly get a misfire or see arcing, it indicates a crack or leak in the plug wire insulation. Replace the wire.
When one cylinder shows a higher firing voltage than the others on a scope, and a shorter spark duration, high secondary resistance is indicated. High secondary resistance may be caused by bad plug wires, or by worn spark plugs or a lean fuel mixture (dirty injectors or an air leak).
To further isolate the cause, the KV (kilovoltage) demand for the affected cylinder should be compared to the other cylinders. If the required firing voltage is 20% or higher than the rest, the problem is either too wide a plug gap or a lean fuel condition. But if the firing voltage is roughly the same as the other cylinders, the most likely cause is high resistance in the plug wire or spark plug.
Replacement spark plug wires are usually sold in complete sets for four cylinder, six cylinder or eight cylinder engines. You should buy the highest quality plug wires for your engine so they will fit and perform the same as or better than the original plug wires. If you have an older vehicle with carbon core or fixed resistor style plug wires, consider upgrading to mag plug wires for improved durability, longevity and performance.
Spark plug wires come in different lengths. When you open the package compare the lengths of the new plug wires to the original plug wires on the engine to make sure they match and which wires go to each spark plug.
Replace each wire one 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.
Replacement wires should be routed exactly the same as the originals, and supported by looms or clips so they are kept away from the hot exhaust manifold(s). Mounting the wires in looms also prevents them from rubbing against sharp edges that might damage the insulation or cause a misfire.
Plug wires should NOT be routed parallel to one another for a long distance because this can cause ignition crossfire. The magnetic field around one wire can induce a voltage surge in an adjacent wire if they are too close and right next to each other. If long wires must be routed next to each other, make sure they crisscross at some point to prevent crossfire problems.
Make sure each spark plug boot is pushed all the way down onto each spark plug and snaps in place. This will prevent the plug wire from vibrating loose, that would result in a misfire. Likewise, make sure the distributor end of each plug wire is inserted all the way into its terminal on the distributor cap and is firmly locked in place. The same goes for the coil wire that connects the ignition coil to the center high voltage terminal on the distributor cap.
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).