Advance Essentials: Understanding Spark Plug Wires


Spark plug wires are the lifeblood of your automotive ignition system. If they don’t perform well, engine performance and gas mileage suffer. You can go as far as spending extra cash on the best iridium spark plugs and install them expertly, but if you don’t change spark plug wires at the same time, you’ve wasted your time and money. Old, worn out spark plug wires leak voltage, cause misfires, and reduce acceleration. Worse yet, those misfires can damage your expensive catalytic converter, and replacing that part will cost big bucks. That’s why we recommend installing premium spark plug wires at every tune up. 

When to change spark plug wires You can test some types of spark plug wires with an ohmmeter. Basically, there are three spark plug wire types, but more on that later. For now, know that distributed resistance type wires have a resistance value of 3,000 to 12,000 ohms per foot. MAG and fixed resistor spark plug wires have much lower resistance per foot. But the actual resistance per foot isn’t important. It’s the consistency of the readings between each wire that determines if the spark plug wire is good or bad. So, measure the length and resistance of each spark plug wire and write it down. Then compare the resistance values. A bad spark plug wire will have much more resistance than another wire of equal length. That’s your signal that it’s time to change spark plug wires.

Spark plug wire types

Spark plug wire Cutaway

As mentioned earlier, there are three spark plug wires types: distributed resistance, inductance (MAG), and fixed resistor. Many American vehicles use distributed resistance spark plug wires. They’re made with fine strands of fiberglass impregnated with carbon. The carbon core creates very high resistance. Carmakers rely on that high resistance to reduce radio frequency interference (RFI) “noise” generated by normal ignition system operation. RFI not only ruins radio reception, but in severe cases it can interfere with the sensors and computers used to control the engine. That’s why it’s so important to buy the best spark plug wires you can afford. If your vehicle came equipped with distributed resistance wires, replace them with the same type.

Japanese manufacturers like Honda, Acura, Toyota, and Lexus prefer inductance (MAG) style spark plug wires. MAG wires are made from a spiral wound core of copper/nickel alloy wire. The spiral winding creates a magnetic field that reduces RFI. MAG wires also have less electrical resistance, so they need less current to fire the spark plug. If your automotive ignition system was designed for MAG wires, you must use buy a set of premium spark plug wires of the same type when it’s time to replace them. Don’t install low resistance MAG wires on a vehicle designed for high resistance wires.

Finally, some European cars use fixed resistor spark plug wires. These wires are made with steel or copper wire and include a resistor inside the spark plug boot. Once again, if you vehicle came with this style spark plug wire, replace them with premium spark plug wires of the same type.

What’s the difference between a cheap spark plug wire set and a premium spark plug wire set?

So far we’ve only discussed the differences in the core of the spark plug wire. But it’s the outer insulation that keeps all that voltage inside. Premium spark plug wire sets are made with expensive silicone or Ethylene Propylene Diene Monomer (EPDM) insulation. Some premium spark plug wire set brands even add an extra coating of Ethylene Vinyl Acetate (EVA) for better temperature resistance and strength. A cheap spark plug wire set, on the other hand, uses cheaper insulation and less robust spark plug boot materials. Also, they use less of those materials, so these spark plug wire sets are thinner. Economy spark plug wires are also built with cheaper coil connectors and may not exactly match the length and spark plug boot construction of your factory wires. You may like the price of cheap spark plug wires, but they won’t last as long as a premium spark plug wire set. If you’re just trying to squeak a few more years out of an old clunker, go ahead and buy a cheap spark plug wire set. However, if you don’t want to deal with spark plug wires until the next tune up, pony up the extra change by then for a premium spark plug wire set.

How to change spark plug wires

Here’s the single most important advice we can offer on how to change spark plug wires: Remove the old spark plug wires one at a time. Then match it with a new wire of the same length. Install it along the exact same route as the original wire. Snap the new wire into the same clips on the plastic wire looms as you run the new wire. If you misroute the spark plug wire you can create misfires due to “induction.” That’ll engage your check-engine light (and no doubt generally drive you crazy.)

Next, apply a dollop of dielectric “tune-up grease” to the inside of the coil and spark plug boots on each end of the new spark plug wire. The grease prevents voltage leakage and makes future removal easier. Finally, replace any broken wire loom clips with new ones. Never leave a spark plug wire dangling in a spot where it used to be supported by a plastic clip. The wire can touch metal engine parts and wear away, ultimately causing misfires.

Before you change spark plug wires, always consult your owner’s manual first. Follow the manufacturer’s recommendations to ensure warranties are not voided.

What tools are needed when you change spark plug wires?
From a spark plug socket to spark plug boot grease, we’ve compiled a basic list that covers all you’ll need the next time you change spark plug wires.

Spark plug socket with swivel locking extension
A GearWrench spark plug socket with built in spark plug socket extension provides maximum turning angle and the most clearance for tight spaces. The spark plug socket and the swivel locking extension are permanently locked together so no need to worry about it disconnecting from the spark plug socket as you remove the plug. It’s a real time saver and frustration reducer.

Spark plug boot puller pliers
Spark plug boots tend to “weld” themselves to the plug. Pulling directly on the wire can damage the connection and separate it from the boot. Use a pair of spark plug boot puller pliers to grab the boot and break it loose. Then pull the boot off the wire with the pliers.

Spark plug wobble sockets
Buy a spark plug socket with built-in wobble to fit into tight spaces. The advantage of this type of spark plug socket is that it can easily take up less room than an ordinary spark plug socket with an added wobble extension. That alone could mean the difference between getting the old plug out — or having a very frustrating day under the hood.

Spark plug boot grease
Squirt a dollop of spark plug boot grease into the boot. Then install the spark plug boot on the spark plug. This dielectric grease prevents high voltage “flashovers” (voltage leaking out of the spark plug boot to ground) and makes future spark plug boot removal easier the next time you need to change spark plug wires.