What Causes Oil on Spark Plug Threads?

Oil on spark plugs is a fairly common problem especially in older cars. In newer cars, it is often seen in cars driven with a heavy right foot. Although there are several possible causes of an oil leak onto spark plug threads, they are all as a result of wear and tear.

The best way to avoid this problem in the first place is to inspect your car regularly so you can catch any worn parts before they begin to cause leakages. If you suspect that oil is leaking onto your spark plugs, remove all the plugs, wipe them using a clean rug then put them back into the wells and remove them again. It is possible for an oil leak to affect some and not all spark plugs.

Why Do You Need Spark Plugs?

Spark plugs are the reason you are able to start your car every time you turn the key in the ignition or press the ‘on’ button. Electrical energy is transmitted through the spark plug which then ignites the fuel and air mixture in the combustion chamber.

Plugs, as they are often referred to, also serve as heat exchangers. This means that they remove heat from the combustion chamber and transfer it to the cooling system.

motor oil

What Causes Oil to Leak into the Spark Plugs?

Engine oil is an extremely crucial part of proper functioning of the engine. The entire system has many moving parts which rub against each other. Oil helps to reduce friction which would otherwise produce too much heat and damage certain parts of the engine. Even with sufficient lubrication, there is always a lot of heat produced when the car is moving. Oil also helps to keep the entire system cool.

That said, there are certain parts of the engine where oil should be and others where it shouldn’t be. Spark plugs fall under the latter. An oil leak into spark plug wells is a serious problem which should be attended to immediately. These are some reasons why oil finds its way to the spark plugs.

Damaged Piston

Excessive heat can cause cracks or holes to form on pistons. This makes it vulnerable to oil leaks which inevitably find their way onto spark plugs. A cracked piston should not be repaired. It can only be replaced.

Leaking valve cover gasket

The function of the valve cover is to prevent leakage of oil from the engine. A gasket is placed between the cover and the engine to keep the cover tightly fixed on. High temperatures in the engine sometimes cause damage to gaskets making them brittle.  Even without excessive heat, gaskets, like any other part of the engine get worn over time. A brittle or worn gasket cannot provide effective sealing so oil leaks onto the spark plugs.

Old O-ring seals

O-rings are situated below the spark plugs and serve a function quite similar to gaskets. They are supposed to seal the opening and prevent oil being circulated in the system from leaking onto spark plugs. When these rings are damaged or worn out, oil leaks.

Effects of Oil Leaks into Spark Plugs

Your car’s not starting because of oil leaks on plugs. It would be wrong to tell you not to worry because you should. The only peace you could have is knowing that failure to start the car is a typical symptom of an oil leak into the spark plug wells.

We know that spark plugs serve to provide an electrical spark which is what starts the combustion of the air and fuel mixture. If there is a small amount of oil in the plugs, you may be able to start the car but experience rough idling and surging.

This means the electrical current needed to start the car is transmitted and the spark created, but the combustion process is not happening as it should. If the firing tip is coated with oil, the car won’t start at all.

Other telltale signs of an oil leak onto spark plugs include:

Blue smoke from the exhaust pipe.Blue smoke is always a sign of burning oil. In this case it tells you that some oil has leaked into the combustion chamber and is being burned along with the air and fuel mixture.

Engine backfiring.Backfiring happens when unburned fuel finds its way into the exhaust manifold. If oil is blocking the plugs from causing combustion, this could happen. The manifold is hot enough to ignite fuel outside of the combustion chamber and the ignition happens in what seems like a mini explosion.

Misfiring.  This can be described as hesitation in the engine when power is demanded from it. It happens when combustion fails to occur in the chamber. It could be caused by a number of failures. Lack of a spark from the plug caused by an oil leak is one of them.

Can Oil Ruin Spark Plug Wires?

Spark plug wires serve to deliver thousands of volts to the spark plugs. To be able to convey such high voltage, they must be covered in thick insulation so that the high voltage doesn’t jump off the wire before it gets to the spark plug. A rubber boot is fitted at each end to protect the metal connectors from corrosion, moisture, oil or any other source of contamination.

When there is an oil leak in the system, these wires may come into contact with hot oil which easily damages them.  It could cause a spark plug wire boot to swell and soften and eventually blow a hole in it. A damaged boot could result in a short against the engine block and could even let water into the spark plug well.


If you didn’t know what causes oil on spark plug threads, now you are well informed. This problem may not cause complete engine failure but that is not to say that it should not be addressed immediately. Continued leakage of oil onto plugs could cause broken valves and warped pistons. If the plugs happen to ignite the oil, you could end up with an engine fire.

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