What do Spark Plugs do, and How Can They go Bad? Blog AutoTechIQ Sparkplugs were causing misfires and hard shifting like feeling while driving. At least I think this was the real problem. : r 4thGen4Runner Worn spark plugs can cause poor performance, misfiring, and higher fuel consumption. Replace them when you notice rough idling, slow pickup, or engine vibration. Thank you for choosing #ELZBrunei . Visit your Engine Vibration Testing Before and After Spark Plug Change