What Is Spark Plug?- 7 Symptoms of Bad Spark Plug

What is a Spark plug?

Spark plug, also called Sparking Plug, device that fits into the cylinder head of an internal combustion engine and carries two electrodes separated by an air gap, across which current from a high-tension ignition system discharges, to form a spark for igniting the air-fuel mixture.

The electrodes must be able to resist high temperatures, and the insulator separating them must withstand high temperatures and also electric stress up to several thousand volts. Spark-gap length affects the energy of the spark, and the shape of the insulator affects the temperature of operation.

When too cool, operation leads to carbonization and short-circuiting of the gap; when too hot, there may be preignition.

How do spark plug works?

Electrical energy is transmitted through the spark plug, jumping the gap in the plugs firing end if the voltage supplied to the plug is high enough. This electrical spark ignites the gasoline/air mixture in the combustion chamber. The plug is connected to the high voltage generated by an ignition coil or magneto.

A spark plug is an electrical device that fits into the cylinder head of some internal combustion engines and ignites compressed aerosol gasoline by means of an electric spark. Spark plugs have an insulated center electrode connected by a heavily insulated wire to an ignition coil or magneto circuit on the outside, forming, with a grounded terminal on the base of the plug, a spark gap inside the cylinder.

Internal combustion engines can divide into spark-ignition engines, which require spark plugs to begin combustion, and compression-ignition engines (diesel engines), which compress the air and then inject diesel fuel into the heated compressed air mixture where it auto ignites. Compression-ignition engines may use glow plugs to improve cold-start characteristics.

The spark plug has two primary functions: To ignite the air/fuel mixture. Electrical energy is transmitted through the spark plug, jumping the gap in the plugs firing end if the voltage supplied to the plug is high enough. This electrical spark ignites the gasoline/air mixture in the combustion chamber. To remove heat from the combustion chamber.

Spark plugs cannot create heat, they can only remove heat. The temperature of the end of the plug\’s firing end must be kept low enough to prevent pre-ignition, but high enough to prevent fouling.

The spark plug works as a heat exchanger by pulling unwanted thermal energy from the combustion chamber and transferring heat to the engine’s cooling system. The heat range of a spark plug is defined as its ability to dissipate heat from the tip.

Spark Plug Construction:

Spark Plug

1. Insulator

Insulates the terminal, center shaft and center electrode from the housing, preventing escape of high voltage from the electrodes.

Since the bottom of the insulator projects into the combustion chamber, high purity alumina with superior heat-proof characteristics, mechanical strength, excellent insulation and thermal conductivity at high temperature, etc. is used.

2. Terminal

The terminal is connected to a high-tension cord through which a high-voltage current from the ignition system flows. A terminal nut is installed so this type can support almost any high-tension cord in the world. For vehicles not requiring a terminal nut, the terminal can be removed.

3. Ring, packing washer

Makes the insulator and the housing fit tightly to each other and maintains the airtightness.

4. Center shaft (stem)

Center shaft connecting the terminal and the center electrode. This shaft is made of steel and has the role of allowing high-voltage current to flow from the terminal to the center electrode without loss.

5. Housing

The housing forms an outer shell that surrounds the insulator supports the insulator and installs the spark plug in the engine. At the bottom, the ground electrode is located, so current can flow through the engine itself to the center electrode over the gap.

6. Glass Seal

Mounted between the center shaft and insulator to maintain airtightness. Denso uses the glass seal method. A special mixture of glass powder and copper powder is charged in the installation section for the insulator and center shaft and center electrode and melted at high temperature. This bonds the center shaft and the center electrode and fuses the insulator and the metal.

The sealing for both is good and the thermal ratio of expansion is appropriate, so even under harsh conditions gaps do not occur and good airtightness can be secured.

7. Gasket

Makes the housing and the engine fit tightly to each other and maintains the airtightness of the combustion chamber. There is a procedure for tightening and the appropriate tightening margin must be secured.

8. Electrode with copper

Special nickel alloy is used for the center electrode to reduce electrode wear. Copper is sealed into the center section to improve thermal conductivity.

9. Center Electrode

A new iridium alloy tip with a diameter of 0.4 mm is laser welded to the tip of the center electrode to make the center electrode. This lowers the spark voltage, secures reliable spark, reduces the quenching effect, and improves ignition performance.

Iridium, like platinum, is a precious metal and has extraordinarily superior properties for a spark plug electrode, for example high-temperature withstand, high strength, and low resistance. In order to further improve oxidation resistance at high temperatures, Denso developed a unique new iridium alloy containing rhodium.

10. U-groove ground electrode

Nickel chrome material used for the ground electrode and various measures is taken with the shape to improve ignition performance.

One of these measures is the U-groove.

  • The surface contacted by the air-fuel mixture is large,
  • There is much edge section, and sparks occur easily.
  • The flame core (flame size) widens easily.

There are many other features as well and large ignition energy can be obtained. Denso obtained patents for spark plug U-grooves from 1975 to 1992.

11. Tapered cut ground electrode

The ground electrode has a shape in which the electrode tip is cut to a finely tapered shape. This reduces the quenching effect, so it improves ignition performance.

What Do Spark Plugs Do?

Your engine is a pretty remarkable machine, designed to turn an energy source (gasoline) into actual movement. But how does it do this? The answer is the principle known as internal combustion. In order to turn the fuel in your car from a source of potential energy into a source of kinetic energy, your engine needs to find a way to release it, and it does so through the combustion process.

The engine cycle is what makes this process happen. In your engine cycle, your valves fill your cylinder with a mixture of air and fuel, which are highly-explosive when combined. As the piston in your engine moves upward, it compresses this mixture until it’s in an extremely small space, creating even more potential energy.

At the peak of this compression, your engine ignites this mixture with a small spark, creating an explosion that forces the piston back downward, turning the crankshaft in your engine and creating the power which makes your car move forward.

Your spark plugs are what supply the spark that ignites the air/fuel mixture, creating the explosion which makes your engine produce power. These small but simple plugs create an arc of electricity across two leads that are not touching, but close enough together that electricity can jump the gap between them. Your spark plugs, along with the electrical and timing equipment which powers them, are part of what’s known as your ignition system.

Generally, your spark plugs made from extremely durable material, and capable of withstanding millions and millions of explosions before wearing out or needing to replace. But it’s true that over time, the explosions and corrosion lead to smaller or weaker sparks, which leads to reduced efficiency in your engine, and could lead to other issues including misfiring, or failure to fire.

What are the symptoms of bad spark plugs?

1. Engine Has A Rough Idle

If your Spark Plugs are failing your engine will sound rough and jittery when running at idle. This can cause vibrations to resonate through the vehicle which could cause further costly damage.

2. Trouble Starting

Car won’t start and you’re late for work… Flat battery? Low fuel? One often overlooked cause is Spark Plug failure. Without that crucial spark for ignition, you’ll be stuck where you are.

3. Engine Misfiring

When your engine misfires it momentarily cuts out and feels jerky because the cylinders are not firing correctly. This can lead to an uncomfortable ride and higher emissions.

4. Engine Surging

Again, this can lead to an uncomfortable – and potentially dangerous – ride. The engine is working inefficiently, sucking in more air than is usually needed in the combustion process, causing continual stop-start motions.

5. High Fuel Consumption

Are you filling up more than you used to? Failing Spark Plugs can really take a bite out of your fuel efficiency. A simple change of Spark Plugs could be the solution.

6. Lack Of Acceleration

If your vehicle feels unresponsive and the accelerator has lost its sensitivity you might consider a service with a change of Spark Plugs to get things up to speed again.

What are the costs of replacing your spark plugs?

The cost depends entirely on the make of your vehicle. Some have more cylinders than others do. One way to count the number of spark plugs in your car is to count the cylinders. However, some cars have a dual spark systems. An example is a V4 engine that has four cylinders. Each cylinder has two spark plugs.

Regardless of the number of cylinders, your car has, replacing your spark plugs is usually cheap. One spark plug piece costs less than 10 dollars. Replacing all plugs plus labor for the mechanic should be between $40 -$150. It is not a complicated process; hence, it should take the mechanic an hour or less.

When Do I Replace My Spark plugs?

Spark plugs will serve you for many years and miles without the need to change them. However, motor manufacturers recommend you change them after every 30,000 miles.

How long a spark plug lasts depends on the type of plug. Copper spark plugs have a short life compared to platinum or iridium.

Always replace spark plugs with the manufacturer’s original choice. If you cannot find the same spark plug, then replace it with high-quality spark plugs. Copper plugs are substandard and will wear off fast.

How to Change Spark plugs?

Changing spark plugs takes about an hour (for a four-cylinder engine) and will save you at least a hundred bucks in labor if you do it yourself. In most cases, it’s a simple job that will help to maintain peak performance and the highest possible gas mileage.

Tools Required

  • gap gauge
  • Needle-nose pliers
  • Rags
  • Socket/ratchet set
  • spark plug wire puller
  • swivel socket.
  • Torque wrench

1. Confirm the Issue

  • Many car owners are unsure about how often to change spark plugs, but as a rule of thumb, they should be replaced every 20,000 to 40,000 miles. This number will vary depending on the make and model of your car and how often you drive each day. Consult your owner’s manual to be sure.
  • If your check engine light appears, and you have trouble starting the car each time, the spark plugs are likely the issue.

2. Disconnect the Wires

  • Open the hood of your vehicle and disconnect the wires from the spark plugs. Follow your owner’s manual for instructions on where the wires are located.
  • Carefully inspect the plugs to make sure there is no water or debris around them.
  • Wipe them clean, if needed, to make sure nothing falls into the engine cylinder upon removal.

3. Remove the Spark Plugs

  • Use a spark plug socket paired with a 3/8 ratchet to loosen and pull the first plug from its housing.
  • These sockets are designed to securely grip the spark plugs for effortless removal and may require a socket adapter to use with your current drive tool.
  • Discard the old plugs after inspecting for corrosion and confirming they need to be replaced.

4. Prepare the New Spark Plugs

  • Consult your owner’s manual to make sure you purchase the correct spark plugs before installing new ones.
  • Some plugs have a special plating to prevent corrosion and ensure the engine cylinder does not seize up. Others will need to be coated with an anti-seize lubricant to ensure smooth installation and cranking.
  • Additionally, a pre-gapped model is the best option for those who do not know how to gap a spark plug.
  • Use a spark plug gap gauge to make sure the gap specifications match the manufacturer requirements outlined in your owner’s manual.

Tip: If your new spark plugs are not pre-gapped, refer to the included instructions to learn how to set the gap on your new spark plugs.

5. Install the New Spark Plugs

  • Make sure the cylinder head is cool to the touch and begin by hand-tightening the plug to reduce the risk of damage.
  • Use a torque wrench to finish installing the new spark plugs. Check the instructions included with the new spark plugs to confirm the correct torque specifications.
  • Add a bit of lubricant to the inside of the spark plug wire and reattach it to the spark plugs.


  • Make sure your car’s engine is cold before you start. Under hood environment can be very warm – only remove the spark plugs wires when the engine is cool to the touch. It can take a few hours for an engine to cool.
  • Wipe off the spark plug insulator and distributor cap towers before installing new wires. Route the plug wires exactly as they were originally. To avoid arc-over, do not route wires of consecutively firing cylinders next to each other.
  • A couple of things to keep in mind if you have more radio noise upon completion – (1) Metallic wire sets can radiate the ignition noise like an antenna. (2) Always check radio noise with the hood closed and latched. (3) Ensure the spark plug wires aren’t routed parallel to a sensor wire.
  • Never clamp spark plug wires tightly or allow them to be pinched between the air cleaner and the distributor. The high voltage stresses will be increased at these points and hasten wire failure.
  • Bundling of spark plug wires and wrapping them with tape, pulling them through metal tubes or dressing them tightly against the engine, may make them look neat but can cause serious voltage losses and/or cross firing. Changing the routing from the original OEM routing can lead to early spark plug wire failure.

Tips for maintaining your spark plugs

Here you should consider preventative maintenance. Monitor your car closely to note changes. Keep looking out for the symptoms. It does not end at this point. Run a further diagnosis using these simple methods.

1. Inspect wires

Find a place with sufficient lighting and open under the hood of your car. Locate the spark plug wires and do a visual inspection. Look for damages to the cables. Scorch marks and cuts. Check for signs of corrosion.

2. Run your engine

Look out for electrical sounds. Listen to the sound from your engine. Noises may be from a high voltage leak.

3. Use water to spray the wires

Spray the areas where boots attach to spark plugs. If arcing occurs or the presence of mist switch off the engine. Remove the spark plug from the boot and look inside the boot for traces of carbon tracking. The presence of carbon tracking means that you will need to replace the spark plug.

4. Use a spark plug tester

A spark miss or spark misfire can make your engine lose its power and traces of black smoke from your exhaust. Run this checklist to know if you have a plug misfire.

  • Disconnect the spark plug from the spark plug lead
  • Connect your spark plug lead with your spark plug tester
  • Connect the spark plug to the spark plug tester and create a link from the spark plug to the lead.
  • Rerun your engine and listen for a gap of inconsistent timing.

5. Confirm if the wires to your spark plug are routed well.

Cross-cutting drains power from your car. If your vehicle has a manual, use it to determine which wire leads to which port.

6. Clean your spark plugs.

Disconnect the spark plug from the spark plug lead. Clean the spark plug thoroughly to avoid debris getting into the combustion chamber. Do not force or remove the spark plug with your hands. Use the spark plug socket wrench.

Use a spray and wire brush to remove any deposits. Never use an abrasive to clean your spark plugs.