Diagnosing Faulty Spark Plug Wires - Engine Builder Magazine

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 engines with
distributors.

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
include:

• 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
fire prematurely.

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
control RFI.

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. 

You May Also Like

Going the Extra Mile with Cylinder Head Porting

It’s not just the port work alone that creates spectacular cylinder head performance. The most critical areas of a cylinder head are those which pass the most air at the highest speed and for the longest duration. Your bowl area, the valve job, the throat diameter, and combustion chamber are all crucial parts. 

Every engine builder is after the ultimate in performance gains. When it comes to cylinder head performance, we’re all trying to maximize airflow to create more power. In order to accomplish that, builders have to know what to manipulate in order to make improvements. Those improvements to airflow quality and quantity come from porting and removing areas of restriction.

Tight Tolerances and Building Power

As you ascend Mt. Everest, you reach an area called the death zone. Once you climb high enough, the margin of error becomes perilously thin. That death zone also applies to engines. As the horsepower per cubic inch and rpm increase, the margin of error decreases. 

CNC Update: Features and Automation

Precision is key when it comes to automotive parts; the complex designs of connecting rods, pistons and rings, blocks, cylinder heads, and other parts require super tight tolerances that are getting more and more difficult to be met by hand or with other machining processes outside of CNC.

All Things Media Blasting

Engine building is a segment of the automotive industry that has always been ahead of the curve in media blasting, and no matter the engine shop, cleaning equipment is a common bond.

Engine & Hub Dynos: Necessary Tools and Additional Revenue

Being able to see the horsepower and the direct correlation to what is lost in the driveline is invaluable – dynos offer a myriad of benefits for the modern engine shop.

Other Posts

November 2022 Shop Solutions

November tricks and tips for the shop!

Shop Solutions October 2022

When machining on the CNC mill, it’s necessary to blow the flood coolant and chips off the parts for inspection. I tried a tool holder mounted fan, but it wouldn’t get all the chips and coolant out of the deeper areas.

Could Engine Oil Soon Contain No Oil?

The trend towards ever thinner engine oils is an effort to reduce fuel consumption and emissions. How far can it go?

Building Big Block Marine Engines

Find out what it takes to build a formidable marine engine.

Marine Engines