Spark plug wires in an MX5 NB (1998 - 2000) conduct high ignition voltage from coil to spark plug, right at the moment when the mixture should ignite in the cylinder. Where heat, vibration and age come together, you often see that cable insulation hardens or internal resistance slowly increases, causing the spark to appear less powerful. This is reflected in idling, throttle response or an engine that picks up less nicely under load. When the rest of the ignition system around spark plugs and sensors is also functioning properly, the combustion remains nicely stable.
