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 1. Key Signs That Your Spark Plugs Need Replacement Engine Hesitation or Sluggish Acceleration: If you feel your vehicle is struggling or delaying when you press the accelerator pedal. Increased Fuel Signs of Bad and Fouled Spark Plugs Autolite One misfiring plug cause shuddering while accelerating? Symptoms Of Bad Spark Plugs Champion Auto Parts