Spark Plug Wire Testing
Worn spark plug wires can be tough to diagnose as they may function normally upon inspection, but as cylinder pressure rises under load the resistance at the spark plug goes way up. A faulty wire allows spark energy to "leak" out of the wire resulting in a miss and reduced output. If the wires are brittle and difficult to bend, they are in need of replacement. If you have access to a volt ohm meter, you can also test the resistance of each wire. A good wire should have 1500 to 3500 ohms resistance. Replace any wires with resistance greater than 30,000 ohms which indicates that the internal conductor is failing.
When handling spark plug wires, never bend the wires at sharp angles as this can cause an internal break in the wire. When removing spark plug wires, do not pull on the wire itself. This is a common mistake that can ruin the wire by breaking the internal connection to the spark plug connector. Pull only on the boot by hand or with a spark plug wire removal tool. Spark plug wires typically last 30 - 50K miles.