Oil Fouled Spark Plugs: Causes & Symptoms

Naomi O'Colman

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Oil Fouled Spark Plugs

Oil Fouled Spark Plugs

Oil fouling, which occurs when oil enters the combustion chamber, is a typical problem that affects spark plugs. The ignition ability is harmed when spark plugs are clogged with oil. The oil elements accumulate on the surface of the spark plug, forming a coating that keeps the spark plug from arcing. In this blog, I’ll explain what causes oil fouled spark plugs and how to recognize them.

What Does It Mean When a Spark Plug Is Oily?

Oil-fouled spark plug means that you have too much oil combined with the fuel. Also, an oily spark plug indicates that the piston rings are most likely deteriorating. When the valve seals and stems wear out, this happens. Oil can leak through when these valves are open, covering the spark plug and reaching the cylinder.

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From this standpoint, an oily spark plug indicates that your engine has problems and that you should address them as quickly as possible. As a result, replace them as soon as you notice any oil on the spark plugs. Additionally, have the valves, and cylinder checked for oil leaks.

What Causes a Clogged Spark Plug with Oil?

The following are some of the most prevalent reasons for oily spark plugs:

1. Cylinders or piston rings with damage or wear

Oil will enter the combustion chamber if the cylinders or piston rings have fractured or worn, clogging the spark plugs.

2. Valve guide seals that are damaged or worn

Valve guides are critical for achieving a tight seal. Oil can enter the combustion chamber through the valve stem if the valve guide has developed a defect. Once the oil reaches the combustion chamber, it will produce black-like wet oil impurities that will settle on the spark plugs.

3. Leaks caused by a faulty head gasket

The head gasket is crucial in your car because it acts as a seal, keeping coolants from entering the combustion chamber.

Coolant can flow into the ignition chamber when the head gasket breaks its seal, causing moist, oily deposits on the spark plug. It will not be good news if the head gasket leaks, as repairing or replacing the head gasket is expensive.

4. A rich fuel mixture

The most common cause of excessive dry carbon deposits on the spark plugs is an overly rich fuel mixture. The rich fuel mixture results from extreme pressure on the fuel, which could have been caused by clogged fuel return lines or the deactivation of the fuel pressure regulator. When the fuel pressure regulator malfunctions, it allows oxygen into the mixture, making it rich.

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5. Oil system ventilation clogged

When the cause is a failure connected to the condition of the oil system, the spark plugs become fouled with oil. Here, there are only two options:

  1. The oil level is too high.
  2. A fault has caused the level to be surpassed, necessitating the disassembly of the oil vent system.

Symptoms of Oil Fouled Spark Plugs

You may always check your car for signs or consequences of oil-fouled spark plugs to see if you have them. Oil fouling occurs when oil enters the combustion chamber and contacts the spark plugs. Oil deposits can block the spark plug, preventing the spark from accessing the gap.

Here are some indicators that you may have an oil-fouled spark plug:

Spark plugs with oil deposits

The surface of an oil-fouling spark plug is shiny and black. If there isn’t enough oil in the combustion chamber, deposits can develop on the porcelain, shell, or tip.

The smell of engine oil in spark plugs

Sniff the plug if you’re unsure if the fouling is due to carbon or oil; it should smell like motor oil. The oil may come from the piston rings, valve stem seals, or the positive crankcase ventilation system.

To diagnose leaking piston rings, a leak-down test might be utilized. Furthermore, a comparative pressure check may help determine structural issues when one cylinder has oil fouling.

Positive crankcase ventilation (PCV) system problems

Faulty PCV systems are becoming a significant source of oil fouling in today’s engines. This is because these systems have progressed beyond a simple spring-loaded check valve.

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Today’s systems can separate oil from crankcase fumes and regulate when the engine consumes them electronically.

Freeze protection valve

In some PCV systems, a heater is included to keep the valve from freezing in certain conditions, such as when humidity is present.

Greater crankshaft tension may ensue if the valve freezes. Furthermore, as a result, the spark plug tip may be pushed past the valve covers.

Turbocharger failure

A malfunctioning turbocharger is another cause of spark plug oil clogging. The seals on the turbine shaft are sturdy, but they can fail due to heat or low oil quality. The shaft’s lubricating oil might enter the pressurized inlet and the cylinder head.

Oil-Fouled Spark Plug Solutions

Here are some possible solutions to the typical cause of spark plug fouling:

1. Fix bad cylinder walls and piston rings

Replace or repair cylinder walls and piston rings that are faulty, damaged, or worn. If damaged piston rings or cylinders are causing oil to enter the combustion chamber, you can do this.

2. Change the valve guides

Replace the defective valve guide if the oil penetrates the combustion chamber through it. A suitable valve guide must provide a tight seal that prevents oil from entering the internal combustion engine.

3. Replace or repair any damaged or leaking head gaskets

Although costly, repairing a faulty head gasket is necessary because it prevents coolant from entering the combustion chamber. Repairing a head gasket sealer or cooling unit sealer bottle on the cooling system is cheap to fix a leaky head gasket.

4. Examine the oxygen sensor, clogged fuel return line, and fuel pressure regulator

The substantial dry carbon fouling on the spark plug suggests a rich fuel mixture. When there is an ample supply of oxygen to a small amount of fuel, the fuel mixture becomes rich.

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In such cases, the error is always oxygen sensor failure, clogged fuel return line, and disabled fuel pressure regulator. In addition, the spark plug will foul if the jetting is improper, there is a leaky fuel inlet needle valve, or the float is adjusted incorrectly.


1. How often do oil-fouled spark plugs occur?

An oil-fouled spark plug with significant carbon buildup on the piston rings will develop in a car’s engine. Within 50,000 miles, this accumulation occurs.

On the other hand, some engines can make it to 100,000 miles before developing this issue. This will happen faster the more you drive your automobile and the older it is.

Oil may be pumped into the combustion chamber because of a flawed engine design’s excess space between the piston and the cylinder wall. This could be an issue with your timing chains or guides.

2. How can oil fouling on spark plugs be avoided?

Changing your oil more frequently is the most fantastic method to avoid oil-fouled spark plugs. However, by the time you identify an issue, it is too late.

Oil should be changed every 3,500 miles (or if you drive in extreme conditions like winter weather or off-roading). If you wait too long to replace your oil, it will dry up, and the rings will fail to seal correctly, allowing an unburned mixture to enter your exhaust system.

If the valve seals are already damaged, you’ll need to replace them. Also, make sure you obtain OEM valve seals to keep any oils out of the combustion chamber.

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3. Can I drive if my spark plugs are clogged with oil?

It would be best if you did not drive with oil in your spark plugs because of the extreme fouling effects. While it’s true that your automobile can run with oil-fouling spark plugs, it doesn’t make it safe or worry-free.

Oil in a spark plug can damage numerous sections of the engine, including bending or breaking valves and pistons and ruining the head gasket, all of which can lead to more severe problems.

It will damage engine performance, cause the engine to overheat, and potentially create a fire if not treated.

As a result, if you have oil in your spark plug, you should stop your engine or refrain from driving your car.


An oil-fouled spark plug is a bad sign that something needs to be fixed in your engine. This is because oil fouling occurs when the oil penetrates the combustion chamber and affects your spark plugs. In addition, oil deposits can clog the spark plug, making it impossible for the spark to enter the gap.

If not addressed, this situation will exacerbate in the future. As a result, it’s good to have it checked as soon as feasible. You may avoid a slew of problems with your vehicle by keeping the spark plug in good shape.