Bad Spark Plug in Car? Signs & How to Fix It 7 Symptoms of a Bad or Worn Spark Plug If your car is not running smoothly, a worn spark plug could be the reason. 1 Hard Starting When the spark plug is I did an oil change and spark plugs and now my car is shaking at 45 50 mph what could it be ? The spark plugs are gapped and I changed the coils Causes of Car Engine Vibration While Driving and Stopping* : *Primary Causes of Engine Vibration:* 1. *Spark Plug Damage*: Worn out or fouled spark plugs Improper gap spacing Spark Engine Shaking Diagnoses Japanese Auto Repair do spark plugs make a difference in acceleration? if they look like this vs new, don't want to replace this if it doesn't need to be : r MechanicAdvice