What Causes Wet Black Spark Plugs?

The spark plugs are parts of your car’s ignition system that help start your car. Technically, should anything go wrong with them, the engine will suffer. And one of the many things that can happen to your spark plugs is being wet and black. Probably thinking of changing them?

Wet Black Spark Plugs are critical car components that offer vital indications of engine condition through their appearance. During inspection, if plugs display a wet, black appearance, it suggests volumes about what’s happening inside. This color, often accompanied by carbon deposits, points towards issues like a malfunctioning oxygen sensor or rich fuel mixture.

Unlike the desired light brown color signaling optimal combustion, black deposits hint at excessive fuel or oil entering the combustion chamber. These visual cues, combined with diagnostic codes from the engine’s computer, help decipher the underlying problems, ensuring that parts like spark plugs and the oxygen sensor work in harmony to keep the engine running with spunk.

Changing them won’t help if you don’t address whatever is causing this condition. This is true because your spark plugs are interconnected like other car components. So that brings to mind, what causes wet black spark plugs? But before we delve right in, let’s see what a wet black spark plug means.

what causes wet black spark plugs

What does a wet spark plug mean?

A wet black spark plug means that the fuel entering the combustion chamber is more than what the engine can burn during combustion. It could also mean that oil or water has found its way into the combustion chamber.

You should see black soot on spark plugs if the issue is excess fueling and a wet black spark plug if the condition results from oil. A wet black spark plug could also mean that your spark plugs are faulty or due for a change.

What causes wet black spark plugs?

We’ve established that a wet black spark plug means water, oil, or too much fuel is entering the engine. So what causes these to happen?

Rich air-fuel mixture

A rich air-fuel mixture is a mixture containing more fuel than needed. Generally, a particular amount of air and fuel is needed for combustion to occur successfully.

If, however, this mixture has too much fuel, the engine will find it difficult to burn the extra fuel, resulting in black sooty spark plugs. While this condition is rare in modern engines, it’s common in old carbureted engines.

Oil leaks

Engine oil helps in lubricating and cooling the engine. However, it has to stay in its compartment to work and outside the combustion chamber.

If a leak occurs, oil can get into the combustion chamber and burn with the air-fuel mixture. Oil is not even supposed to be in the combustion chamber so that it won’t burn. Hence, the wet black spark plug you see.

Water is entering the engine

While washing cars or driving in flood, water may infiltrate your filter area and get carried into the engine when you start your car. When this happens, you will notice wetness on your spark, making it difficult to start your engine.

Bad fuel

Fuel comes in different qualities, some with 90 octane levels, and others 80. Whichever you use, if the fuel is mixed with oil or water, you will have wet black spark plugs. This happens because the oil or water isn’t supposed to be in the combustion chamber and won’t burn during combustion.

Faulty or low-quality spark plug

Spark plugs come in different quality and sizes. If the spark plugs don’t match your engine’s specification in size and even resistance, it won’t be easy to ignite. Failure to ignite correctly will result in unburnt fuel precipitating on the plugs.

Likewise, spark plugs deteriorate over time. When the spark plugs are old with carbon built all over it, it will find it challenging to burn off the fuel-air mixture in the engine’s combustion chamber. Over time, this can result in black sooty spark plugs. 

A fouled or faulty plug would often result in many failed attempts to start your car, especially during the cold months. This often results in engine flooding, eventually soaking the plugs but not necessarily darkening them.

What to check if spark plugs are wet and black

To know what’s making your spark plug black and wet, check the following.

Check engine light

Check engine light could point to several problems. So if you have a wet black plug, check to see if the CEL is on. If on, connect a diagnostic tool to your car. This tool will throw some fault codes pointing to the problem.

Spark plugs

Check if your spark plugs are too old or if you’re using the correct spark plug for your engine. A bad or low-quality plug won’t be able to burn the fuel mixture in the engine’s combustion chamber which may result in car shaking.

Fuel injectors or carburetors

While older cars use carburetors, modern cars use fuel injectors to inject fuel into the combustion chamber. However, leaky fuel injectors or misadjusted carburetors will pump excess fuel into the combustion chamber. Check to see if they are in good condition.

Air filters

Air filters are helpful in trapping dirt, so only clean air enters your engine. However, it can get clogged and unable to supply air as needed. The car computer may get confused and try to compensate for the lack of air by pumping more fuel than needed. Check to see if the air filters are clean.

Mass airflow sensors

The mass airflow sensor measures how much air enters the engine and relates this information to the ECU. However, if this sensor is bad and unable to measure the air entering the engine, it adjusts to any condition. As a result, it may send a false signal to the car computer. The computer may think too much air is entering the system and try to compensate by adding excess fuel. 

Throttle body

The throttle body helps pull fresh air into the engine for combustion. If the throttle body is dirty, it won’t be able to supply air as needed. Hence, this causes the engine to pump more fuel to compensate for the lack of air.

Check your fuel

Some fuels are mixed with oil and water that don’t get burnt during combustion. Check to see if your gas is free of these contaminants.

Check cylinder walls, piston rings, seals, and valves

Worn piston rings, cylinder walls, or head gaskets won’t be able to seal the cylinders properly. Hence, letting oil into the combustion chamber. Check to see whether they are in good condition.

How to fix wet black spark plugs

Start by fishing out why water, oil, or excess fuel enters the combustion chamber. As these are practically why your spark plugs are wet and black. After confirmation, you can act based on the problem diagnosed, which may include fixing some of the following.

  • Clean dirty throttle body.
  • Drain out bad fuel and fill with only good quality fuel.
  • Change old or foul spark plugs.
  • Use only spark plugs that match your engine’s specifications.
  • Replace leaky fuel injectors.
  • Correctly adjust the carburetor and replace them if damaged.
  • Change leaky or blown head gaskets.
  • Replace worn-out rings and valves.
  • Change the dirty air filter.
  • Clean dirty mass airflow sensors or replace if faulty.
  • Even if the plugs aren’t the issue, ensure you clean or replace them after fixing whatever is making your spark plug black and wet.
wet black spark plug


We have explained what causes wet black spark plugs, with the leading cause being water, oil, or excess fuel entering the engine. Faulty or low-quality spark plugs can also make your spark plug black and wet. Thankfully, this condition can be fixed.

 The first step to solving this is identifying why water, oil, or excess fuel is being pumped into the engine’s combustion chamber. Once the root cause is found, swing into action by cleaning or replacing faulty components.

Solomon Osuagwu

Osuagwu Solomon is a vetted auto mechanic with over ten years of experience in the garage and five years of experience as a service writer. He prides himself in writing accurate information on professional repair guides, DIY repair guides, buyer’s guides, comparisons, and car reviews. If he’s not in his repair garage, he’s writing automotive blogs to help car owners and fellow mechanics to troubleshoot and proffer solutions to several car problems.

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