AutoTips Spark Plug Wire tips page.
Plug Wires (or Spark Plug Wires, Ignition wires) connect the spark plugs to the distributor or ignition coils. Unlike most wires, plug wires are normally designed to have a lot of resistance. The resistance helps reduce radio static that the ignition system generates. DO NOT replace resistance wires with non-resistance wires unless your ignition system was designed for them. Using non-resistance wires may damage modern ignition systems that were designed with the resistance as part of the circuit. Spark plug wires also have high voltage insulation to carry the 30,000 volt or higher voltage of the ignition system. Newer vehicles often have an ignition coil mounted directly above the spark plug, and do not use traditional spark plug wires.
Tip: Keep them
clean. A thin film of dirt can form on the outside of the plug wire
and around the boots at the ends of the wires can form conductive path
to the high voltage when exposed to moisture. The conductive path can drain
away the current that should go to the spark plug. If your car misses
out or runs rough only in damp weather, you may have this type of wire
A mechanic sprayed the wires on my car around the distributor with a misting spray bottle filled with water (like one used to spray plants). The car died. He repeated the experiment a couple of times with the same result. I tried the same thing when I got home, with the same results. I cleaned the wires as well as the inside and outside of the boots (I used Armor All cleaner - use something rated safe for rubber and vinyl), and then treated the wires with Armor All "protectant". This time, spraying the wires had no effect, the car continued to run smoothly.
Testing Plug Wires: If you have access to a factory service manual for your car, you may find a resistance rating for the spark plug wires, (such as 1000 - 4000 ohms per foot). If you know how to use an ohm meter, you can measure the plug wires resistance from end to end and see if they are within specifications. Look at the connectors, check for corrosion. Insulation on plug wires can break down, and they may arc to ground or other wires. Crossed Wires: You may see guides on your car that have the plug wires crossed over each other, rather than in nice parallel lines. This is intentional to cancel out magnetic fields. Wires in parallel may cause one wire to induce a voltage in another wire, causing it to fire when it shouldn't . Plug wire problems can result in miss-firing, hard starting, and poor fuel economy. If you change plug wires, you may want to replace them one at a time to keep from mixing them up to the wrong spark plug.
Testing spark plug wire resistance with a multi-meter.
What goes bad in spark plug wires:
What makes a good replacement spark plug wire:
Tips when replacing wires: