Carbon Fouled Spark Plug one Cylinder: Causes, Symptoms & Fixes

Carbon-fouled spark plug one cylinder or on all cylinders are the common causes of ignition system problems. Spark plugs are critical components of the ignition system and should be in good condition for the engine to function correctly. The fouling of carbon on one spark plug can cause issues like bad gas mileage, engine stalling, rough idling, and vibration.

A lousy fuel injector or damaged oil seal in that cylinder is the common reason one spark plug keeps fouling. If there’s a defective oil seal or damaged injector in any head cylinder, the spark plug will keep fouling until you track and fix the root cause.

In this article, I’ll outline the possible causes, symptoms to watch out for, and the best ways of addressing this issue. Sit back, relax, and have a five-minute read.

black spark plug on one cylinder

How do you identify the signs of carbon fouling in the spark plug?

The insulator of any carbon-fouled spark plugs is usually covered with soft, dried, black carbon on the plugs. Other notable signs of carbon-fouling spark plugs include:

  • Difficulty starting the engine.
  • A misfiring engine.
  • Poor fuel economy.
  • Contaminated plugs.
  • Strange exhaust smell.
  • Rough idling.
  • Acceleration defect.

Here’s a detailed look at what fouled spark plugs look like.

Unusual exhaust smell

Carbon-fouled spark plug causes an unusual exhaust smell. If spark plugs are fouled, the fuel in the combustion chamber will not burn correctly. The unburnt fuel will exit the vehicle through the exhaust tailpipe. As a result, the exhaust gas will have an unusual gasoline smell.

Contaminated spark plugs

If you put a new spark plug side to side with a fouled one, no matter how small the fouling is, it won’t look the same. The difference will be clear.

However, since we only remove the spark plug when we suspect it isn’t functioning as it should, contaminated plugs aren’t the first sign you’ll notice. But it’ll help you know for sure if the plugs are fouled.

Hard engine starting

Without the spark plugs, gasoline-powered engines won’t start. And if the plugs are worn, damaged, or fouled, the engine will take longer before starting. It may take multiple trials before coming on.

At first, the engine may take a few seconds longer to start. But as the fouling continues, it takes multiple tries before starting the engine. And if you don’t rectify the problem, it may lead to a no-start situation.

Engine misfiring

If the engine eventually starts, the plugs will not burn the fuel properly. This will invariably cause engine misfiring. At the early stage, the vehicle will start with a stutter or stumble and worsens as the fouling continues.

In any case, it’ll be unfair if I fail to mention that several factors can cause engine misfiring. So, it is essential to properly diagnose the vehicle before pointing accusing fingers at the spark plugs.

Bad gas mileage

The vehicle will consume more fuel whenever there’s a misfire or the engine is running poorly. Since fouled spark plug won’t burn the gas properly, the vehicle will use more gas than usual. Other spark plug issues like too close or too open plug gaps can also cause bad gas mileage.

Acceleration defect

One of the notable carbon-fouled spark plug symptoms is acceleration defects. Typically, cars should run faster once you depress the gas pedal. But if the spark plugs are fouled, the car won’t run as it’s supposed when you step on the gas pedal.

Like other symptoms I outlined above, several factors can cause acceleration defects. Therefore, you have to run a proper diagnosis before accusing the plugs of being fouled.

Rough idling

When an engine runs as it should, it will maintain a consistent RPM and a specific sound. You should know where your vehicle idles and the sound it gives. If the plugs become fouled, the RPM will start fluctuating, and the sound may change as the powertrain control module tries to compensate for the failure.

What causes carbon fouled spark plug one cylinder

Before proceeding, you need to understand that why all spark plugs foul and why one spark plug keeps fouling could be caused by different things. Several factors like wrong spark plug gap, carbon buildup, and oil leaks or bristle valve seals are the common reasons one cylinder spark plug keeps fouling.

Carbon buildup

Carbon buildup is one of the common causes of fouled spark plugs. If the electrode or tips of the plugs are fouled, it could be carbon buildup. Several factors can cause carbon build-up on plugs. But where the carbon buildups are only happening on one cylinder, you likely have a lousy or dirty injector in that cylinder.

Damaged oil seal

One of the common causes of carbon fouling on one cylinder is bristled or dry oil seal on that particular cylinder. This problem is common with old-model cars. The only feasible solution is replacing the damaged, dried, or bristled seal and cleaning or replacing the fouled plug.

Wrong plug gap

The spark plug gap is essential and should be gapped according to the manufacturer’s specifications. If any plug is not sized correctly, it can cause carbon to foul the spark plug, leading to other issues like engine misfire, rough idling, and unresponsive acceleration.

Lousy injector

Another notable cause of carbon fouling on one cylinder spark plug is a lousy, damaged, or leaky fuel injector. Each engine cylinder has one fuel injector that sends fuel into the cylinder and one spark plug that ignites the fuel in the cylinder head.

If the injector on that cylinder is bad, it’ll not supply the required fuel in that cylinder. This will cause the spark plug to run hotter, leading to overheating in that cylinder. This will typically foul the spark plug on that cylinder.

How do you fix a carbon-fouled spark plug in one cylinder?

The first step to addressing a carbon-fouled spark plug is to clean it. Sometimes, that could be all you need to fix the underlying problem. The most interesting part is that it’ll cost you nothing. So, what’s stopping you from giving it a try? If that doesn’t fix the underlying problem, follow the below carbon-fouled spark plug fix procedures.

Replace damaged oil seal

If the plug on the same cylinder fouls again after cleaning it while others are in good condition, you have a severe issue that needs to be tracked and fixed. Firstly, you have to remove the spark plug and examine it.

If the carbon on the plug is oily, you likely have a damaged or bristled valve oil cover seal. Take off the valve cover and inspect the valve cover oil seal. Pay more attention to the O-ring that covers that particular cylinder. Replace the entire seal if it is bristled or broken.

Re-adjust plug gap

If the carbon on the spark plug is dry, check the gap and see if it is too close or too open. Take your plug gap tool and measure the plug gap and see if it is at the manufacturer’s specifications. Adjust the plug gap if needed.

Replace lousy fuel injectors

Inspect the fuel injector in that cylinder and ensure it functions as it should. If it is not, replace or clean it as needed. However, you don’t want to waste cash by replacing good components. So, you have to properly diagnose the root cause of the carbon fouling before replacing any part.

carbon fouled spark plug symptoms

Frequently Asked Questions – FAQs

What is the problem if the spark plug tip is carbon fouled?

The most common reason for a carbon-fouled spark plug tip is an incorrect plug gap and rich fuel mixture. It is hard to see a rich fuel mixture on only one cylinder. However, it can occur if you have a leaky fuel injector. Factors like damaged oil seals, carbon buildup, and fuel injectors can cause carbon-fouled spark plugs.

Do carbon fouled spark plugs need to be replaced?

If the carbon deposit on the plug electrode is not too much, cleaning it will address the issue, and if it doesn’t, you have to consider replacing the plugs. If the carbon deposit is much, replacing the plugs with new ones would be the best solution.

Will a hotter spark plug stop carbon fouling?

Manufacturers design hotter spark plugs with more heat insulation to transfer heat slowly. This typically helps it maintain high temperatures, burn off carbon deposits, and prevent unnecessary plug fouling. It invariably extends the shelf life of the spark plug.

Will carbon fouled plugs clean themselves?

On some occasions, fouled spark plugs can clean themselves. For instance, if you replace a leaking oil seal responsible for fouling the spark plug, the plug will burn off the carbon deposits as you continue cruising the vehicle.

Can you drive with a fouled spark plug?

Driving a fouled spark plug will cause several drivability issues like rough idling, poor acceleration, engine misfiring, reduced gas mileage, car losing power while driving, and other issues. So, it is better to clean or replace the fouled plugs before hitting the road.

Final Words

Issues like leaking fuel injector, bristled oil seal, and wrong plug gap can cause carbon fouled spark plug one cylinder. A single fouled spark plug can prevent your car from showing its glory on the road.

I don’t want your car to be a clunker on the road, and that’s why this article has outlined the possible signs and how to fix a black spark plug on one cylinder. However, if you are not a DIYer, have your mechanic diagnose and resolve the problem.


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