Click on a thumbnail to see the full-size image
Diagnosing Faulty Spark Plug Wires
Ignition systems have changed a great deal in recent years, with coil-on-plug (COP) ignition systems being the most common setup on many late model engines. COP ignition systems have a single coil for each spark plug mounted on top of the plug. This setup eliminates the need for a spark plug wire and the troubles it can cause. Most other distributorless ignition systems (DIS) still have plug wires, as do older vehicles with distributors.
By Larry Carley
systems have changed a great deal in recent years, with coil-on-plug
(COP) ignition systems being the most common setup on many late model
engines. COP ignition systems have a single coil for each spark plug
mounted on top of the plug. This setup eliminates the need for a spark
plug wire and the troubles it can cause. Most other distributorless
ignition systems (DIS) still have plug wires, as do older engines with
Plug wires are necessary to carry high voltage from the ignition coil
to the spark plug. We’re talking firing voltages that can range from
5,000 volts to as much as 50,000 volts! That’s a lot of electrical
energy to keep contained inside a wire.
Spark plug wires are vulnerable to heat, vibration, aging, moisture and
physical damage from mishandling during installation or removal. Issues
Plug wires may burn through and short out if they lay against a hot
exhaust manifold (depending on the heat resistance of the outer jacket
around the wire).
Unsupported wires can vibrate and rub against sharp edges, causing the insulation to wear through and short out the wire.
The carbon conductor that carries voltage inside some types of wire
can degrade after years of use, causing an increase in resistance that
may cause misfires.
Boots that seal the ends of the wires can be deformed or pulled loose,
allowing moisture to sap away voltage before it reaches the spark plug.
The metal clips that attach the ends of the wires to the spark plugs
and coils or distributor cap can also loosen or be damaged by vibration
or improper handling.
A different type of misfire called “crossfire” may occur if two plug
wires are routed parallel to each other and the two cylinders follow
each other in the firing order. The magnetic field around one wire
induces a current in the second wire, causing the other spark plug to
Misfires are bad news because it causes a loss of performance and fuel
economy, and a big jump in emissions. On late model OBD II vehicles, it
will also turn on the Check Engine light and misfire codes. So if wire
problems are causing misfires, it’s time to replace the wire set.
Most late model original equipment spark plug wire sets now use
spiral-wound stainless steel mag wire. This type of construction has
less internal resistance than carbon-core wires (only about 500
ohms/foot versus 5,000 ohms/foot with carbon-core wire). It uses
inductance rather than resistance to suppress radio frequency
interference (RFI). The result is a hotter spark with less voltage load
on the ignition system. Mag wire can be recommended as an upgrade for
older carbon core wires.
Some European imports use Fixed Resistor plug wires, which have a steel
or copper metallic core with a fixed resistor in the plug boot to
Other differences in plug wires include the thickness and type of
insulation around the wire. Wire sizes may be 7mm, 8mm or larger.
Thicker is typically better because it reduces the risk of voltage
leaks. Wires with thicker insulation are typically used on high-output
electronic ignition systems or on engines where the spark plugs have
wider electrode gaps.
Premium wire sets typically use silicone or EPDM (Ethylene Propylene
Diene Monomer) insulation around the core. There may be an inner layer
of EPDM surrounded by an outer layer of silicone, or a double layer of
silicone around the core. EPDM has a high dielectric rating that
resists voltage leaks, and silicone can withstand high under-hood
temperatures and retain its flexibility unlike some other materials
that can melt or become hard and brittle as they age.
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.