These coil boots are attached to the bottom of the ignition coil, between the spark plug and coil body. Their purpose is to isolate spark from 'leaking' to ground between spark plug and ignition coil. Over time they may become saturated with oil causing them to swell, or crack from age causing misfires, weak spark, or other poor running conditions. Z1 recommends inspecting and replacing these where necessary whenever the spark plugs are serviced.