What are Spark Plugs and What do They Do

What do spark plugs do?

Think of spark plugs as the tiniest bolt of lighting. Small but mighty, the spark of electricity that the plug emits across a small gap creates the ignition for the combustion needed to start your car. By putting the engine’s pistons in motion, your car can power up, stay powered up and produce a smooth burn of the compressed air-fuel mixture. Mind you, spark plugs like it hot: they withstand extreme heat and pressure within your cylinders and are built to burn off deposits from fuel additives or other contaminants.

What does it mean for you?

Well, without a spark, your car won’t start—or go anywhere. And because spark plug health is directly linked to engine performance, it stands to reason weak or bad spark plugs lead to problems, be it issues with cold-starting or misfires during acceleration. Besides—without healthy ones—your ride can’t sustain maximum power, and your vehicle can see a drop in fuel economy. Now, nobody wants that.

Does Firestone Complete Auto Care carry spark plugs that meet my vehicle’s specifications?

Meets or exceeds, my friend. Meets or exceeds. That’s the mantra around here. That’s why we only install Bosch brand spark plugs. For over 100 years, Bosch has been a leader and innovator in spark plug technology. Consider this: Bosch was making car parts before Henry Ford ever unveiled the first Model T. They’ve also been the spark plug found in the last 17 Indy 500 winners*.

*Indy 500 is a registered trademark of Brickyard Trademarks, Inc. Used with Permission.

Make an appointment to get your spark plugs checked out at your Firestone Complete Auto Care.


What Are Symptoms Of Worn Spark Plugs?

Once a spark plug becomes sufficiently worn, it will affect the performance of your engine. Replacing your plugs at the recommended intervals written in your vehicle owner’s manual schedule will virtually eliminate these problems, but a fouled or faulty plug can cause issues.

These are the signs of worn spark plugs:

  • Misfiring/Rough Idle: When your spark plugs are worn, you might notice unusual vibrations and noises like rattling or knocking coming from your engine while you’re idling.
  • Misfiring/Slow Acceleration: Another sign that your engine is misfiring as a result of worn spark plugs is slow acceleration. When a spark plug doesn’t operate optimally due to being fouled or dirty, it doesn’t spark effectively which causes your car to feel sluggish. You might feel your engine stop, stall, and then stumble to start again.
  • High Fuel Consumption: Worn spark plugs can cause your engine to work inefficiently, ultimately leading to increased fuel consumption and emissions.
  • Hard Starting: Vehicle struggling to start? A dead battery isn’t always to blame. In order for your engine to start, the spark plugs need to produce a sufficient spark. If your vehicle is particularly difficult to start, worn spark plugs could be the culprit.         

If you spot any of these signs, it’s best to ask a professional to check the spark plugs to see if they need to be replaced. You should do this as soon as possible to prevent further damage to your vehicle.

Heat rating and heat flow path of NGK Spark Plugs

Some basic structural factors affecting the heat range of a spark plug are:

  • Surface area and/or length of the insulator nose
  • Thermal conductivity of the insulator, center electrode, etc.
  • Structure of the center electrode such as a copper core, etc.
  • Relative position of the insulator tip to the end of the shell (projection)

The major structural difference affecting the heat rating is the length of the insulator nose. A hot-type spark plug has a longer insulator nose. The insulator nose of a hotter spark plug has a longer distance between the firing tip of the insulator and the point where insulator meets the metal shell. Therefore, the path for the dissipation of heat from the insulator nose to the cylinder head is longer and the firing end stays hotter. The insulator nose of a hotter spark plug also has a greater surface area that is exposed to more of the ignited gases and is easily heated to higher temperatures. A colder spark plug functions in an opposite manner.

The heat range must be carefully selected for proper spark plug thermal performance. If the heat range is not optimal, then serious trouble can be the result. The optimal firing end temperature is approximately 500°C (932°F) to 800°C (1472°F). The two most common causes of spark plug problems are carbon fouling (< 450°C) and overheating (> 800°C).

How long do your Spark Plugs Last?

Your spark plugs, thankfully, are one of the most durable parts of your engine, as they are housed in the ignition chamber, right in the middle of your engine block. Spark plugs are typically replaced 50,000km to 80,000 km, given proper engine conditions, however, the spark plugs in newer cars can last even longer. Most new cars also come with a lifetime warranty on spark plugs.

4. How Often Should I Replace My Spark Plugs?

Clearly, spark plugs are crucial to the health of your vehicle’s engine and car, but how often should you be replacing them? Well, there’s obviously a lot of variables when it comes to how quickly your spark plugs go through enough wear-and-tear to the point where a replacement is necessary. A modern Toyota model with spark plugs that are platinum-made should last at least 50,000 miles while iridium spark plugs are good for up to 100,000 miles and sometimes more. Consult your owner’s manual to learn what kind of spark plugs your model has and the recommended lifespan of them. If you notice some problems before either benchmark then it’s always better to replace them sooner rather than later to avoid getting stranded as well as potential damage to the vehicle.

Tip Temperature and Firing End Appearance

There are many external influences that can affect the operating temperature of a spark plug. The following is a brief list to consider in avoiding reduced performance and/or expensive engine damage.

Engine Speed and Load

  • If the engine is to be operated at high RPM, under a heavy load or at high temperatures for long periods, a colder heat range may be needed.
  • Conversely, if the engine is to be operated at low speeds or at low temperatures for long periods, a hotter heat range might be needed to prevent fouling.

Air-Fuel Mixture

  • Excessively rich air-fuel mixtures can cause the plug tip temperatures to decrease and carbon deposits to accumulate, possibly causing fouling and misfires.
  • Excessively lean air-fuel mixtures can cause the cylinder and plug temperatures to increase, possibly resulting in knock and/or pre-ignition. This may cause damage to the spark plug and/or seriously damage the engine.
  • If an air-fuel ratio meter or gas analyzer is not available, it will be necessary to visually inspect the spark plugs frequently during the tuning process to determine the proper air-fuel mixture.

Fuel Type/Quality

  • Low quality and/or low octane fuel can cause knock which will elevate cylinder temperatures. The increased cylinder temperature will cause the temperature of the combustion chamber components (spark plug, valves, piston, etc.) to rise, and will lead to pre-ignition if the knock is uncontrolled.
  • When using an ethanol blend fuel with high ethanol content in high-performance applications, a colder heat range may be necessary. The spark timing can be advanced further because ethanol blend fuel has a higher resistance to knock (higher octane). Due to the decreased knock, there will be less audible “warning” from knock before the spark plug overheats and pre-ignites.
  • Some types of fuel additives in lower quality fuels can cause spark plug deposits that can lead to misfires, pre-ignition, etc.

Ignition Timing

  • Advancing ignition timing by 10° will cause the spark plug tip temperature to increase by approximately 70° to 100°C.
  • A colder heat range spark plug may be necessary if the ignition timing has been advanced to near the knock level. Higher cylinder temperatures near the knock level will bring the spark plug firing end temperature closer to the pre-ignition range.

Compression Ratio

  • Significantly increasing the static/dynamic compression ratio will increase cylinder pressures and the octane requirement of the engine. Knock may occur more easily. If the engine is operated near the knock level, a colder heat range spark plug may be necessary due to the resulting increased cylinder temperatures.

Forced Induction (Turbocharging, Supercharging)

  • A colder heat range spark plug may be necessary due to the increased cylinder temperature as boost pressure (manifold pressure) and subsequent cylinder pressure and temperature increase.

Ambient Air Temperature/Humidity

  • As the air temperature or humidity decreases, the air density increases, requiring a richer air-fuel mixture. If the air-fuel mixture is not properly richened, and the mixture is too lean, higher cylinder pressures/temperatures, knocking, and the subsequent increase in the spark plug tip temperatures can result.
  • As the air temperature or humidity increases, the air density decreases, requiring a leaner air-fuel mixture. If the air-fuel mixture is too rich, decreased performance and/or carbon fouling can result.

Barometric Pressure/Altitude

  • Air (atmospheric) pressure and cylinder pressure decrease as altitude increases. As a result, spark plug tip temperature will also decrease.
  • Fouling can occur more easily if the air-fuel mixture is not adjusted to compensate for the altitude. Higher altitude = less air = less fuel.

What is the scoop on platinum spark plugs?

Platinum is much harder metal than nickel alloy and has a higher melting point. Since platinum is harder, it holds its sharp edge much longer than a conventional spark plug, up to 100,000 miles. Longevity is a key advantage of platinum spark plugs.

Another advantage of platinum spark plugs is that they run a little hotter, which burns deposits off the spark plug better and helps prevent fouling. Platinum also handles high heat, enabling the spark plug to wear better.

Platinum spark plugs come in two varieties – single and double platinum. A single platinum plug is much like a copper spark plug with a platinum disc welded to the center electrode; a double platinum spark plug has a platinum disc on both the center and side electrodes.

How Much Does It Cost to Replace Spark Plugs?

A lot can go wrong during a DIY spark plug replacement, which is why it’s recommended to let a professional replace your spark plugs instead of trying to handle the task on your own.

The cost of a spark plug replacement will vary depending on several factors, including the type of vehicle you drive. Some vehicles require more expensive spark plugs than others, and some vehicles might have spark plugs that are more difficult to access. These are some of the factors that can increase the overall cost of repairs.