See Figures 1 and 2
Spark plug wires do a critical job for your vehicle's ignition system. They transmit the very tiny amounts of current that fire the spark plugs at an extremely high voltage. Voltage is high at the plugs because the spark must actually leap the plug gap, something which requires tremendous electrical pressure or voltage . Electricity has great difficulty in traveling through air or air/fuel mixture. In fact, the mixture will not transmit electricity at all in its natural state. The tremendous voltage at the plug's center electrode actually pumps electrons into the air, ionizing it before the spark can travel to the ground electrode. The required voltage is especially high because of the pressure generated in the cylinders during the compression stroke.
Since the coil's ability to increase the voltage in the ignition system is traded in direct proportion for decreased amperage or current flow, there is little room for loss of current due to poor ignition wires. To make matters worse, the high voltage existing throughout the wires naturally tends to create a corona effect and loss of both current and voltage.
All this is worth knowing for one simple reason, it pays to buy quality ignition wires with a metallic conductor and it pays to inspect wires frequently for either cracked or brittle insulation. A most common symptom of ignition wires that are starting to give out is poor performance or difficult starting in wet weather that improves after the engine warms up or when the weather clears. You can confirm this problem by watching the engine run in the dark. If you see a lot of blue sparking around the wires, replace them. At this time, it is also a good idea to inspect the distributor cap and rotor, replace them, if there is any sign of cracking or carbon tracking. These are paths of burnt plastic, usually beginning at a crack or groove, where sparks may have been leaping from the center of the cap to ground.
If you have an ignition problem that seems to come and go, yet the insulation on the outside of the ignition wires is still good, it may pay to have your vehicle run on an ignition scope or diagnostic system. The scope readings will sometimes reveal high resistance in the conductor at the center of each wire. You can also use an ohmmeter to measure the resistance of the wire from end to end; it should not exceed 25,000 ohms.
Ignition wires are most easily replaced one at a time. Make sure to very carefully insert wires all the way into the cap and onto the ends of the plugs; make sure all insulator boots are installed all the way onto the cap or plugs.