What Causes Spark Plugs To Foul? 6 Reasons & Symptoms

What causes spark plugs to foul? As they are an important component in igniting the air-fuel mixture of the combustion chamber, they can get difficulty in helping the vehicle engine run once being fouled.

Thus, you need to look for the causes of this problem and find the best solutions.

Keep reading this article to learn more about the causes and signs of fouled spark plugs.

Besides, we also give you other useful information about this component and guide you through a complete procedure to keep it in good condition.

What Are Spark Plugs?

They are electrical devices fitting into several internal combustion engines’ cylinder heads and igniting compressed aerosol gasoline with an electric spark.

A spark plug includes an insulated center electrode using a highly insulated wire to connect to the magnetic circuit or outer ignition coil.

This connection will form a spark gap within the adjacent cylinder walls at the grounded terminal of the plug base.

Internal combustion motors can come with spark-ignited machines requiring a spark plug to initiate the combustion process or compression-ignited (diesel) engines, compressing air and injecting diesel fuel into the compressed and heated air mixture.

What Causes Spark Plugs To Foul?

What Causes Spark Plugs To Foul?

What causes fouled spark plugs? Some common causes include an oil-contaminated combustion chamber, overheating engine, leaky head gasket, wrong plug gap, or injector leaks.

Oil-Contaminated Combustion Chamber

One of the leading causes of spark plug fouling is an engine oil flow to the combustion chamber.

The oil leaks can make the spark plug’s tip capture the dirt and excessive oil. This cause is also known as oil contamination, causing premature failure.

This problem often happens inside older vehicles. Thus, you must notice if your car burns oil faster than normal.

This may be a clear indicator of clogged or fouled spark plugs and lead to a shorter longevity for your engine.

Overheating Engine

If you wonder what causes spark plugs to foul quickly, an overheating engine is a typical culprit. When overheating the spark plug’s tip, it’s easy to fail.

Overheating engines often happen for various reasons, but the most popular are the failed cooling system and pre-ignition.

Pre-ignition will result in heat accumulating inside the combustion chamber leading to the failure of spark plugs. Besides, the cooling system doesn’t work properly.

It may also cause the overheating spark plugs. This overheating also makes the electrode of the spark plug wear out and foul out faster.

Fouled Carbon

It also indicates foul plugs when you see black, soft, dry carbon deposits with a large quantity on the electrodes and the insulator.

The carbon buildup degrades the insulation between the ground and center electrode, creating a leakage path through the carbon and causing misfiring.

The folded carbon causes are a filthy air filter, excess fuel mixture, idling or long periods of low-speed driving.

Besides,  retarded ignition timing, the ignition system failure, and cold spark plug temperatures also trigger this problem.

Improper Plug Gap

The wrong plug gap is one of the fouled spark plug causes you also need to consider. Normally, the proper gap may ensure the spark plug has an appropriate space amount to jump.

The gap between the center and the electrodes of the spark plug requires perfect calibration to optimize engine performance.

This calibration can be different depending on the modern vehicle type you drive.

Thus, the proper gap allows the arcing to occur at the suitable voltage to burn the fuel injector and produce the combustion helping the engine operate.

Otherwise, the spark plug tip will suffer from extra stress, leading to premature wear and erosion. You can also notice and check the strange engine performance issues.

You may check the accurate specifications in your manual service, but several spark plugs become pre-gapped.

When the crucial spark plugs get pre-gapped, you should double-check it. If not, it’s possible to gap the spark plug without a tool, as well.

Injector Leaks

Wet fouling is less popular on modern fuel-injected engines but may occur because of mechanical failures.

One typical example is a leaky cold start injector. This can result in a rich starting mixture formation in the engine compartment and spark plugs fouling.

Leaky Head Gasket

Why do spark plugs foul? If the above things are not the root cause, you can check the head gasket to determine if it is leaky.

This is because a leaky head gasket will cause the fouled spark plugs. Yet, it is quite expensive to replace and repair.

When modern coolant goes to the chamber, combustion deposits will build up on the spark plugs. Dirty spark plugs can be an early sign of a leaking head gasket.

What Are Symptoms Of Fouled Spark Plug?

When you know what causes foul spark plugs, you need to learn more about their symptoms, such as rough idle, contaminated plug, misfiring and difficulty starting the modern engine.

Below are the detailed explanations for each sign.

Rough Idling

Rough idling is one of the symptoms of faulty spark plugs. Thus, you should take your vehicle to a professional mechanic to double-check if this sign appears in your car.

Contaminated Plug

You also recognise the fouled spark plugs when finding out about the contaminated plug.

Yet, you won’t pull the plug to check it unless some problems appear. However, this is the first sign you should notice to confirm the spark plug is fouling.

Hard Engine Restart

You can know if your spark plug gets fouled when facing a difficult engine restart. So, pay attention to this problem to have a timely repair.

Engine Misfires

Misfiring will appear when the spark plugs can’t ignite the extra fuel of the normal combustion chamber. You can detect this symptom when starting to sputter or stumble.

Besides, a failing sensor is also responsible for this issue. The check engine light often comes on once the misfiring engine condition starts.

How To Fix A Spark Plug Fouled Out?

How To Fix A Spark Plug Fouled Out

After knowing the causes and symptoms of the fouled spark plugs, you need to find good solutions for fixing them.

The first important thing you must do to improve the fouled spark plug is to clean it properly. Here are the simple steps you should follow to complete this.

  1. Wipe the automotive spark plug surfaces and ignition tip with a clean and fresh cloth.
  2. Get a quick-drying cleaner such as an injector or brake cleaner.
  3. Spray the spark plug’s end and wait for it to sit for about 10 minutes.
  4. Continue using a spark plug wire brush to remove the deposit from the fouled spark plug.
  5. Blow out the spark plugs and their firing end using an air compressor.
  6. Double-check the spark plug gap and make a proper adjustment.

Besides, replacing the fouled spark plug is the ideal option with an economical cost to quickly fix this problem.

Related Questions

How Can You Read Spark Plug?

If you face the problem of fouled spark plugs, follow the guide on reading them below.

Normal spark plug often comes with grayish-tan or brown deposits on the electrode’s side.

If the spark plug gets carbon-fouled, it will have dry and black soot on the electrode and the insulator tip.

This condition originates from problems such as a rich air fuel mixture or unburned fuel, long idling, a clogged air filter, or driving at low speeds.

The oily and black deposits on the insulator tip points and electrodes also cause an oil-fouled spark plug.

For a burned spark plug, you can understand that this is a symptom of an overheating spark plug with blisters or white deposits on the melted electrodes or insulator tip.

The reasons behind include incorrect spark plug heat range, the engine overheating, improper ignition timing, a loose spark plug, or lean fuel or air mixture.

What Happens When a Spark Plug Gets Clogged?

The spark plugs will cease to work properly when they get clogged. This most obvious symptom is plug fouling.

If the spark plug in your vehicle gets fouled, the engine can consume more fuel, work rough, or lack power.


Now, you got the right answer to the question of what causes spark plugs to foul with the useful information in the above article.

Some common causes can be a wrong plug gap, leaky head gasket, overheating engine, oil-contaminated combustion chamber, or injector leaks.

When you identify one of the symptoms of a fouled spark plug, it’s best to alter the new spark plugs. With their budget price, you can fix the problems for your vehicle without the hassle.

If you can not fix the fouled spark plug yourself, you may take your car to an auto shop to get help from a professional mechanic with a thorough inspection and repair.

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