* Increased Heat: Higher RPM under load generates significantly more heat in the engine. The ignition coil, already operating at high temperatures, is pushed further beyond its thermal limits. This increased heat can cause insulation breakdown, leading to arcing or short-circuiting, which manifests as misfires. At idle, the engine runs cooler, and the coil doesn't experience the same thermal stress.
* Higher Voltage Demands: Under load, the engine requires more energy for combustion. This translates to a higher demand on the ignition system, forcing the ignition coil to generate stronger and more frequent sparks. A weakened coil might be able to handle the lower demands of idle but fail under the increased stress of higher RPMs and load.
* Vibration: Higher RPMs and load often correlate with increased engine vibration. This constant shaking can contribute to the failure of a coil that's already weakened by age or internal damage. The vibrations can fatigue the coil's internal components, causing cracks or breaks in the windings or connections.
* Insufficient Cooling: The coil might not be adequately cooled under high-demand conditions. The airflow around the coil might be restricted, and heat dissipation becomes less effective at higher RPMs.
Therefore, a coil's failure being RPM-dependent is a strong indicator of heat-related or vibration-related stress exceeding its operational capabilities. The coil is likely weakened internally, and the increased demands of higher RPM and load push it over the edge.
