A faulty spark plug may be the root of many engine problems. There are a few telltale indicators that it's time to replace your spark plugs. The most common Signs of bad spark plugs in V8 engines include: Trouble starting the car  The engine check light is on Misfires cause the engine to shake Increased emissions  Decreased fuel economy  To keep your car running smoothly and efficiently, check the spark plugs if you've noticed any of these issues, and replacing spark plugs in V8 can avoid premature engine damage.