Will a Bad Spark Plug Make My Car Sputter?

Spark plugs have a central core of copper encased in a steel jacket and outside protective shield made up of a ceramic insulator. An electrode is fashioned at the bottom of the spark plug, or engine side of a plug body, and it has two components: a hot firing tip, commonly called the electrode, and a ground strap prong that sits over it in a curved or straight fashion. A gap exists between the electrode and strap, which when activated, receives a high voltage spark. The spark jumps the gap via an arc, thus providing an electrical charge that ignites the air-fuel mixture.

Spark plug sputtering can be defined as a miss, or non-firing condition of the spark plug. Sputtering, also known as missing, happens when the electrode fails to ignite, or pre-ignites out of regular firing sequence. The sputter or miss results from a cylinder that does not fire and produce a compression stroke. A sputtering failure will sound like a consistent pinging, knocking or "plapping" noise, or a sporadic misfire during different driving conditions. The end result produces less horsepower and engine rpm (revolutions per minute).

Spark plug wet fouling results from an early induction (fuel pre-delivery) or excessive amount of fuel that enters the combustion chamber, rapidly cooling the spark plug electrode. If the electrode becomes too cold due to flooding, it cannot reach the needed ignition temperature to fire the air-fuel mixture. Narrow or closed spark plug gaps, improper fuel injection or carburetor settings, colder heat range plugs, or a total lack of voltage from the primary and secondary ignition, will cause a noticeable sputter or misfire. Wet foul sputtering will decrease gas mileage, reduce horsepower and cause cold hard-starting. Fuel-soaked or black spark plug electrodes show signs of wet fouling.

Carbon deposit fouling will cause a spark plug to sputter. Carbon deposits, produced from unburned hydrocarbons, collect on or in between the electrode contacts when a temperature of approximately 450 degrees Fahrenheit or below exists. The colder temperature allows carbon deposits to form, blocking or diluting the high ignition voltage required for firing. Large deposits can create hot spots, causing pre-ignition, which causes sputtering symptoms. Overly rich fuel, excessive oil consumption, retarded ignition timing and a colder spark plug heat range will cause carbon deposits.

If the gap between the electrode tip and ground strap is too great, set incorrectly or worn from age, the voltage required to fire the plug increases. If the ignition system is weak and not putting out a high-enough voltage, large-gapped plugs can miss or sputter. Wide-gapped plugs will sputter particularly under high-speed or heavy engine load. Plugs that have a narrow gap will show signs of sputtering or misfire during cold driving, low speed and frequent start and stop driving. The spark plug electrode tip will also wear faster with a colder heat range.

Spark plugs with the improper heat range can cause sputtering. Heat range is determined by the length of the electrode insulator and its ability to transfer heat. Hotter heat ranges remain at higher temperatures longer than colder heat ranges. The higher heat range burns hotter, and performs better than the colder heat range under low-speed, heavy load and colder temperature driving. However, if the heat range is too high it can cause blistering of the electrode, high engine temperature and pre-ignition. A colder than normal heat range will promote a weaker or colder spark and will load up and foul, particular under overly rich fuel-air conditions. Colder heat range plugs have more trouble with hot, self-cleaning firing.
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Structural damage to the spark plug case, connector or insulator can cause a sputter or misfire. Some spark plug connectors have screw-on tips, and if they become loose, the voltage signal is lost. A cracked insulator body on the plug will allow voltage to escape from the inner core and ground out against metal, causing a continuous or sporadic sputter or miss. A broken electrode or ground strap, usually due to excessively high temperature, will cause a no-fire condition, a hot spot inside the head or cylinder, or piston or valve damage.