Signs and Symptoms of Spark Plug Problems

closeup of a spark plug firing
The spark plug is critical to the function of your gasoline engine.

Don Farrall / Getty Images

During its lifetime, an average spark plug might fire 27.5 million to 110 million times. Each time, it vaporizes a few molecules off the spark plug electrodes. Eventually, the spark plug gap widens, and the air-fuel mixture no longer ignites efficiently. Other engine problems might contaminate the spark plug, and in certain circumstances, it may not fire at all.

How the Spark Plug Works

Fire requires three components: oxygen (O2), fuel, and heat. In your engine, every time a cylinder pulls an intake stroke, it pulls in air (≈21% O2). Port-injection engines inject fuel during the intake stroke, while direct-injection engines, gasoline or diesel, might wait until the compression stroke. Finally, heat is provided in one of two ways: one or two spark plugs for gasoline engines, or the heat of compression for diesel engines. The result, as when any time these three components are combined, is a tiny explosion. Cruising at 55 mph (88 kph), the average cylinder will fire about 1,000 times per minute, or 16 times per second!

In gasoline engines, the heat is provided in the form of a tiny lightning bolt. High voltage, 5 kV to 45 kV, depending on the vehicle, is generated in an ignition coil, controlled by a distributor or engine control module (ECM). The charge is conducted to the spark plug via a spark plug wire. The spark occurs when the charge jumps between the spark plug electrodes, just 0.01” to 0.07” (0.25 mm to 1.8 mm) across. The heat generated, between 8,500 °F and 11,800 °F (4,700 °C to 6,500 °C), ignites the air-fuel mixture, driving the piston down in the power stroke

Signs of Spark Plug Problems

How can you tell if your spark plugs need attention? Your engine is a fine-tuned machine, and performance, fuel economy, and reliability are dependent on consistent delivery of air, fuel, and spark. Here are seven signs of spark plug problems.

  • Fuel Economy – If you’re tracking fuel economy, as everyone should, spark plug problems might cause your engine to consume more fuel than usual. As the ECM has no control over spark strength or oxygen content, it adds fuel to compensate for poor combustion.
  • Misfire – A cylinder misfire occurs when the spark plug can’t ignite the air-fuel mixture. This can occur because of worn spark plugs, contamination, cracked spark plug wires, or other problems.
  • Check Engine Light – For many people, the check engine light might be the only sign your engine is having spark plug problems. The ECM is far more sensitive than many drivers and can detect a single cylinder misfire in thousands of good ignitions.
  • Hard Starting – Proper ignition is most difficult when the engine is “cold.” The ECM adds more fuel to account for poor vaporization, which can be difficult for a worn spark plug to ignite, resulting in hard starting, long cranking, or a no-start condition.
  • Rough Idle – At idle, spark plug problems might be evident as more vibration, basically a cylinder misfire that occurs only at idle.
  • Poor Performance – On acceleration, spark plug are under particular demand to deliver a strong spark to ignite more fuel and generate more power. Faulty spark plugs or weak ignition coils may not keep up with the demand.
  • Hesitation / Surging – Slightly less noticeable, hesitation on acceleration and surging might be described as the engine not “responding” immediately to driver input. Then, a “surge” in power might come unexpectedly. This slight delay in power delivery might indicate a spark plug problem.

Replace Spark Plugs Like A Pro

Depending on the vehicle, engine, and spark plug type, spark plugs generally last from 30,000 to 120,000 miles (48,000 km to 193,000 km). Spark plug wires and ignition coils tend to last a little longer, but usually no more than double the lifespan of the spark plugs they power. For most vehicles, replacing spark plugs is a simple matter, though you might need to be a contortionist to get to some of them, perhaps placed behind shields or under intake manifolds and other equipment. Here are the basic steps to replace spark plugs.

  1. Access Spark Plugs – This might require removing other parts, such as engine covers, heat shields, or the intake manifold.
  2. Remove Spark Plug Wire or Ignition Coil – Before removing wires, use tape or some other means to mark locations, or else the spark plugs may not fire in the correct order. Replace scuffed, worn, or damaged spark plugs wires or boots.
  3. Blow Out Spark Plug Channels – Debris tends to collect in unprotected spark plug tubes and channels. Compressed air is a good idea to get rid of it, preventing it from falling into the cylinder when you remove the spark plug.
  4. Remove Spark Plug – Using a spark plug socket and locking extension, remove the old spark plugs.
  5. Clean Spark Plug Threads – A spark plug thread chaser is a great investment, and a few minutes leaning the threads in the cylinder head can prevent future seized spark plug headaches.
  6. Set Spark Plug Gap – Most new spark plug gaps are preset from the factory, but it’s a good idea to check and adjust the spark plug gap according to the manual, just to be sure.
  1. Lubricate New Spark Plug – Using just a dab of anti-seize lubricant, silver or copper doesn’t matter, lubricate the spark plug threads and gasket.
  2. Install New Spark Plug – Spin the spark plug in by hand until finger tight, then torque to specification. Gasket types usually specify a quarter- to a half-turn after contact, while non-gasket types specify just a sixteenth-turn. Check the manual to be sure.
  3. Reinstall Everything Else.

Being observant is key to keeping a reliable car, and recognizing spark plug problems early can improve fuel economy and prevent you from being stranded. If you have any questions about your spark plugs, be sure to check with a trusted professional.