10 Symptoms of a Bad Spark Plug | How to Replace Spark Plugs?

The engine is one of the most important parts of the vehicle. It contains multiple parts, and the spark plug is one of them. The spark plug is usually used in petrol engines. For the efficient working of the engine, your spark plug must work efficiently. As the spark plugs go bad, they produce different signs. This article explains the bad spark plug symptoms, causes, working, and its replacement cost.

What is a Spark Plug?

A spark plug is a mechanical device that generates an electric spark to ignite the compressed fuel-air mixture inside the combustion chamber. It gets power from the ignition system.

Spark plugs are most commonly used for gasoline engines. However, in diesel engines, combustion of the air-fuel mixture occurs due to high compression of the air. Therefore, diesel engines don’t require spark plugs to ignite the air-fuel mixture.

Construction of Spark Plug

Spark plugs have a threaded metal shell. The ceramic insulator is used to insulate the shell from the central electrode electrically. This central electrode usually contains a resistor. It connects to the output terminals of the magneto or ignition coil through a highly insulated wire.

Spark Plug construction

The metal shell of the spark plug is electrically grounded. It also connects to the cylinder head of the engine. The center electrode protrudes into the combustor through a porcelain insulator.

Symptoms of Bad Spark Plugs

The efficient working of the spark plug is very important to ensure the optimum performance of a gasoline engine. When the spark plug goes bad, it produces different symptoms such as poor acceleration, engine stalling, poor fuel economy, misfiring, engine knocking, or hard starting issues.

The following are the most common symptoms of a faulty or clogged spark plug:

  • Check engine light illumination
  • Rough idling
  • Poor acceleration
  • Reduced fuel mileage
  • Engine won’t start
  • Hard starting conditions
  • Black smoke
  • Engine knocking
  • Engine misfiring
  • Smell from the exhaust system

1) Check Engine Light Illumination

The check engine light illumination is one of the clear symptoms of a faulty spark plug. This light usually comes on when your engine has running issues due to a bad spark plug, oil contamination, or overheating.

check engine light, symptoms of bad spark plug

However, your vehicle check engine light may also activate due to many other issues such as engine overheating, faulty fuel pressure sensor, low engine oil, low fuel, low transmission fluid, damaged spark plug wires, or bad catalytic converter. Therefore, when your vehicle computer triggers the check engine light, you should properly inspect all the parts to find the main issues.

Read More: Causes of Check Engine Light Illumination

2) Rough Idling

If the spark plug doesn’t generate sparks according to the requirements, the engine starts rough idling.

Rough Idling

A delayed spark doesn’t allow the engine to idle properly, especially after a few hours of idle time. Cold, rainy mornings make it difficult for even faulty spark plugs to function as intended if condensation goes into the combustor. Replacing the faulty spark plugs and wires improves the idling and engine operation.

Read More: Why Car Shakes at Idle?

3) Poor Acceleration

The damaged spark plugs highly affect the acceleration ability of the engine. As the spark plugs are damaged, your engine doesn’t run the vehicle according to your requirements.

poor acceleration due to bad spark plug

Dirty spark plugs don’t have the ability to catch sparks effectively and can slow the engine, especially when changing gears.

4) Reduced Fuel Mileage

A quick reduction in gas mileage is one of the clear symptoms of a clogged or bad spark plug. If your spark plug is clogged or damaged, it will not work efficiently, which can negatively affect fuel economy.

Poor fuel economy due to a faulty spark plug

5) Engine Won’t Start

If your car’s engine won’t start, it is one of the main symptoms of a fouled spark plug. The car engine won’t start due to some other possible causes, such as a bad alternator, a bad ignition, or a dead battery.

If the battery check reveals that it is not fully charged while the battery terminals or cables are not corroded, the next best idea is to inspect the spark plugs (the alternator and ignition are difficult to diagnose on their own). You must properly maintain the spark plug and replace it on time.   

Read More: Different Types of Engines

6) Black Smoke

The bad spark plugs may lead to higher emissions of exhaust gases and black exhaust smoke. As you notice this symptom, you must immediately contact a specialist to diagnose the problem. The engine may emit black smoke due to engine oil entry in the combustion chamber or low coolant.

Black Smoke from the Exhaust Pipe

7) Hard Starting Conditions

If the car does not start after attempting multiple times, your car spark plugs may be defective. In order to ignite the air-fuel mixture of all cylinders in time, the engine requires a powerful ignition spark.

If one or more plugs are not working efficiently, your vehicle engine will not get the spark according to the requirements. This can cause one or more cylinders to malfunction and make it difficult to start the engine.

Hard Starting Conditions, symptoms of clogged spark plug

If your car is not starting, then check the spark plugs. If the plug is damaged, replace it with a new one.

8) Engine Knocking

As the spark plug goes bad, you may hear a knocking noise from the engine. This is a very serious problem that may cause the complete failure of the engine.

Knocking Noise

The damaged spark plugs may also damage other engine parts such as pistons, valves, compression rings, piston heads, and the engine cylinder.

9) Engine Misfiring

Engine stalling or misfiring is one of the common symptoms of a dirty spark plug. When the spark plugs are damaged, they don’t properly inject fuel into the engine cylinder, which leads to engine misfiring.

engine Misfiring

Following are some other reasons for engine misfiring:  

  • The engine is not getting the desired amount of fuel
  • The engine is not getting sufficient oxygen to burn the fuel.
  • The engine combustion chamber is not getting sufficient spark to ignite the air-fuel mixture.

The “spark misfire” is another problem that occurs due to a damaged spark plug that disrupts the combustion process. As your engine starts misfiring, immediately check your plugs and replace them.

10) Smell From Exhaust System

When the spark plugs do not burn the air/fuel mixture efficiently, unburnt fuel enters the exhaust system of your vehicle. As a result, a smell like unburnt gasoline starts coming from the exhaust pipe.

smell of unburnt fuel

Causes of Bad Spark Plug

Your spark plug may go bad due to one or more of the below-given causes:

  • Coolant leak
  • Pre-ignition
  • Engine overheating
  • Improper spark plug gap
  • Carbon accumulation
  • Engine Oil in combustion chamber
1) Coolant Leak

The coolant leak is one of the most common causes of your spark plug failure and causes misfiring. The latest coolants have very low zinc, phosphate, and other minerals. Therefore, a short coolant leak usually doesn’t damage the spark plugs. However, prolonged problems with coolant leaks can cause the spark plugs to start fouling.

Coolant Leaks, causes of bad spark plug

The coolant leaks usually occur around the head gasket or intake manifold. If coolant enters the combustion chamber, it starts to burn and form ashy-white deposits on the spark plugs.

Read More: Symptoms and Causes of Coolant Leaks

2) Pre-ignition

You should always prevent your vehicle engine from pre-ignition. Pre-ignition causes heat to build up in the combustion chamber and can lead to spark plug failure. In addition, your spark plug and the engine may also overheat if the cooling system is not working properly.  

3) Engine Overheating

Engine overheating is one of the common causes of a bad spark plug. This overheating leads to premature wear of the spark plug electrodes.

engine overheating

However, your engine may overheat due to many other reasons, including insufficient water in the radiator, insufficient coolant, insufficient engine oil, low fuel, or cooling system failure.

Read More: Causes and Symptoms of Engine Overheating

4) Improper Spark Plug Gap

In order to improve engine performance, the gap between the side electrodes and the center of the spark plug must be perfectly adjusted. Proper spacing allows the arc to occur with the correct voltage to burn the gasoline and produce combustion to keep the engine running.

Spark Plug Gap

An improperly adjusted gap (the gap between the side electrodes and insulator) puts extra stress on the plug tip, which may cause it to corrode and wear out the spark plug tip early.

5) Carbon Accumulation

Black-dried soot on the spark plug insulator and electrode tip specifies carbon deposits on the spark plug. These carbon deposits may shorten the service life of your spark plug and cause the check engine light illumination, engine stalling, engine misfiring, poor fuel economy, poor acceleration, and hard engine starting conditions.

Exterior Carbon Buildup on the spark plugs

The carbon accumulates on your spark plug for multiple reasons, including a dirty fuel injector, too rich fuel-air mixture, driving too slowly, a dirty air filter, or an engine idling for too long.

6) Engine Oil in Combustion Chamber

The leakage of engine oil in the combustion chamber is one of the major causes of the spark plug failure.

When the engine oil enters the combustion chamber, the tip of the spark plug becomes oily and can lead to its early failure. This problem is most common in old vehicle models. Therefore, if you have an old vehicle and you observe that it is burning engine oil, you should immediately inspect the spark plugs.

However, engine oil may leak due to a damaged valve stem seal, piston ring, head gasket, or other parts of the PCV system.

Read More: Low Engine Oil Symptoms and Causes

How to change a Spark Plug?

The replacement of the spark plug is not a tough job. If you have some technical knowledge, you can change it yourself. Follow the below-given steps to change your spark plug:

  • Cool the Engine: First of all, make sure that the engine is cold. If your engine is still hot, let it cool down and then start working. 
  • Locate the spark plug: Spark plugs are usually located on the top of your engine and are connected to the ignition coil or the distributor through a thick wire.
  • Disconnect the Battery: Now, disconnect all the battery wires connected to the spark plugs. 
  • Remove the faulty spark plug: Take a spark plug ratchet and socket to detach the faulty spark plugs. Loose it by rotating counterclockwise and remove it from the engine.
  • Check the electrode gap of the new spark plug: Check the new spark plug gap and ensures that it is within the vehicle’s specified range. You can check your vehicle manufacturer’s manual to confirm this information.
  • Install new spark plug: After removing the faulty plug, thread the new spark plug on the engine by hand. Use the spark plug ratchet and socket to tighten it properly. Don’t overtight the spark plugs because this may damage your engine threads.
  • Connect the spark plug wires: After properly tightening, connect the battery wires to the new spark plug.
  • Repeat for the other spark plugs: If replacing more than one spark plug, repeat the above steps for each spark plug.
  • Engine Test: Now, put the ignition key into your ignition switch and turn on the engine and make sure it runs smoothly. If there is a problem, ensure that the spark plugs are installed properly, and the wire is securely connected.

Spark Plug Replacement Cost

The replacement cost of the spark plug depends on your vehicle model, labor cost, and the type of brand.

The average replacement cost of the spark plug is from $200 to $270. In this cost, the cost of the parts is from $70 to $100, while the labor cost is from $130 to $170.

How to Clean a Spark Plug?

If your spark plug is dirty and you want to clean it, follow the below-given steps to clean it:

  • Gather the needed tools: You must have all the necessary tools to perform this job.
  • Check Engine Temperature: Check the temperature of your vehicle engine. If its temperature is still high, then wait for a few minutes and let it cool.
  • Locate the spark plugs: Spark plugs are usually located on the top of your engine. However, the location of the spark plug varies for each vehicle model. Use the manufacturer’s manual for this step.
  • Remove the spark plugs: Use a spark plug ratchet and socket to detach the faulty spark plugs from your engine.
  • Check Spark Plugs: Inspect your spark plugs to determine if they can be cleaned or need replacement. If the electrodes are badly worn, or the ceramic insulator of your spark plugs is cracked, we recommend replacing the spark plugs.
  • Clean the spark plug: Clean the ceramic insulator and electrodes by using a spark plug cleaning tool or wire brush. Clean it carefully so as not to damage the insulator or electrodes. If the electrodes are badly corroded, you can clean them with a small file.
  • Check gap: Check the gap between the electrode and the insulator by using a gap measuring tool. If this gap is not according to the vehicle specifications, then adjust it.
  • Reinstall the spark plugs: After cleaning the dirty ones, install them back in their original place and tighten them using a spark plug socket and ratchet. Don’t overtight the spark plugs.
  • Engine Test: Start the engine and check its performance. Your engine shouldn’t misfire and run smoothly after cleaning the spark plugs. Double-check that the spark plugs are installed correctly if there is a problem.

FAQ Section

What is the function of the spark plug?

The spark plug is one of the most important parts of the gasoline engine. The main function of the spark plug is to produce an electric spark to ignite the air-fuel mixture in the combustion chamber. It takes power from the vehicle battery to generate a spark.

Where is the spark plug located?

The location of the spark plugs depends on the vehicle model and type of engine. In a six-cylinder engine, the spark plugs are typically installed on the top or side of the engine head. However, the spark plugs in the 4-cylinder engine are usually located on the side or top of the engine in a row.

In the V8 and V6 -cylinder engines, the spark plugs are generally split evenly across either side of the engine.

What is spark plug fouling?

The spark plugs of your car have the capability to tolerate millions of controlled explosions, but they may start to wear with time. Spark plug fouling is a famous form of wear. Spark plugs are somewhat self-cleaning, but they may foul with time, especially if you drive your vehicle on dirt roads.

With the passage of time, a small amount of fuel residue and ash can accumulate on the spark plugs. If these contaminations don’t burn during driving, they start to spoil the working of the electrodes that create the electric spark. In such conditions, the spark plugs will be unable to work properly and may fail completely over time.

How often do spark plugs need to be replaced?

The spark plugs ultimately wear out and usually fail after 100,000 miles under normal driving environments. However, this can be significantly reduced in adverse conditions such as oil leaks, coolant leaks, or poor driving conditions.

How to tell if a spark plug is bad?

When your spark plug produces one or more of the below-given symptoms, it means your spark plug is bad and you need to replace it:

  • Your engine is misfiring 
  • Your vehicle is stalling
  • A clear reduction in the fuel economy
  • The check engine light is illuminating on your car dashboard.
  • Acceleration issues
  • Engine starting issues
  • Black smoke from the exhaust pipe
  • Engine knocking
  • Unburnt fuel smell from the exhaust system

What are the components of the spark plug?

  • Threads
  • Excellent
  • Metal Body
  • Insulator
  • Ribs
  • Central Electrode
  • Spark Park Electrode Gap
  • Seal Washer / Gasket
  • Insulator Tip
  • Electrode Gap
  • Plating
  • Ground Electrodes

How to check a spark plug without removing it?

Follow the below-given steps to check your spark plugs without removing them:

  • Connect one end of the spark plug tester to the ignition cable. The other end of the tester should be connected to the desired spark plug.
  • Turn on the ignition switch.
  • Run the engine; if your engine won’t start, turn it over and look at the bright side of the tester.

Why do diesel engines not have a spark plug?

The spark plugs are used to ignite the air-fuel mixture. In the case of a petrol engine, when the air-fuel mixture enters the combustion chamber from the carburetor, the spark plug instantly produces a spark to start the combustion process. However, in diesel engines, the ignition of the air-fuel mixture occurs due to the high compression of the air. Therefore, diesel engines don’t have spark plugs.

What are the signs of bad spark plugs?

  • Smell from the exhaust system
  • Check engine light illumination
  • Knocking noise
  • Reduced fuel mileage
  • Engine won’t start
  • Hard starting conditions
  • Poor acceleration
  • Black smoke from the exhaust pipe
  • Engine misfiring
  • Rough idle

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