Spark plug wires are the channels by which electricity travels from the distributor or electronic ignition to the spark plugs. When everything is working in sync, the vehicle runs fine. However, if the spark plug wires are having problems, it can result in numerous issues with the rest of the vehicle from reduced power to distortion on the vehicle's radio. While a visual check will typically tell you all you need to know about your spark plug wires, sometimes that is just not enough.
Spark plug wires leaking electricity can cause the engine to misfire, resulting in hesitation when accelerating and sputtering while attempting to maintain speed. This can also cause the engine to vibrate severely enough to be felt in the interior of the car. Spark plug wire leaking can be caused by a worn place on the outer insulation of the wire. This wear can be caused by engine vibration, contact of the wire with the hot engine, friction or abrasion and even corrosion from exposure to certain automotive fluids such as battery acid.
Electro-Magnetic Interference (EMI) is a problem created when spark plug wires are not properly shielded by carbon suppression conductors. EMI is produced in the wires and can cause interference in certain electronic devices on the vehicle, including the engine management system, receiving false or erroneous signals. This can cause the on-board computers to misinterpret vehicle operation resulting in minor inconveniences such as an occasional miss in the engine to a dramatic loss of power.
Radio Frequency Interference (RFI) may not be a safety-threaten issue such as some other spark plug wire problems, but it can be an exceptional irritation. RFI from improperly shielded spark plug wires can cause the vehicle's radio to pick up undesirable signals and broadcast them as a ticking sound when the radio is played.
Spark plug wires occasionally fail internally. If the vehicle starts running rough, exhibits an erratic idle, will not accelerate properly, fails emission checks or is getting steadily decreasing gas mileage, the spark plug wires may be at fault. Even if a visual inspection can find no flaw resulting in leaking, it is possible for the internal insulation to break down. Testing the wires with an ohm meter for proper resistance can reveal whether there is internal damage to the wires.
Spark plug wires should be replaced as a set rather than individually---it is difficult to judge the internal integrity of each individual wire. Each vehicle has specific requirements so only spark plug wires designed for that vehicle should be used. Installing the wires incorrectly can result in damage to the wires, the distributor and the engine.
