Final answer:
Defective spark plugs will cause the engine to run rough at idle speed.
Step-by-step explanation:
Defective spark plugs will cause the engine to run rough at idle speed.
Spark plugs are a crucial component of the ignition system in a car's engine. They create a spark that ignites the fuel-air mixture in the combustion chamber, allowing the engine to run smoothly. When spark plugs are defective, they may not produce a strong spark or may misfire, resulting in an uneven running engine. This uneven running is particularly noticeable at idle speed, when the engine is not under load.