Why Do Spark Plug Threads Get Oily?

Oil is essential to vehicle operation. However, oil on spark plug threads needs immediate attention. The right place for engine oil and other lubricants in the car is important.

Oil on your car’s spark plug threads during routine maintenance is not a reason to ignore it; it can cause serious engine issues.

Let’s investigate oil leaking into spark plug threads and the solution in the following sections.

What Causes Spark Plug Thread Oil?

A vehicle’s spark plug threads leak oil for several reasons. Oil on car spark plug threads can result from the following.

Car engine oil overflow:
If you overfill your engine, excess oil will flow out and sometimes leak into the spark plug holes. Spark plugs improve vehicle ignition. The spark plugs ignite the cylinder fuel to start the car. Dirt, debris, and oil prevent the spark plug from sparking for ignition.

Oil leaks foul spark plugs. This causes lower gas mileage, rough idling, engine power loss, and misfiring. Chevy Impala and Silverado have similar oil on spark plug thread issues.

Bad O-Rings
O-rings seal spark plugs to keep oil and other contaminants out. Depending on the car’s make or model, the O-rings may be located outside the valve cover.

O-rings can also be permanently attached to the valve cover. In any case, silicone or rubber O-rings degrade over time.
High temperatures can damage O-rings, causing oil to leak into the spark plugs.

Bad valve cover gasket
Valve cover gaskets are among the most important vehicle gaskets. This part seals the valve cover and cylinder head.

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The valve cover gasket directs oil pumped into the cylinder head to the camshaft, valve springs, valve guides, etc.
The silicone or rubber valve cover gasket wears out due to heat or age. This defect causes the component to malfunction and leak oil into the spark plug.

Cracked piston
Pistons help engines convert combustion into rotational power. Pistons are in the engine combustion chamber, so they get hot.

Too much heat can crack pistons over time. It will allow oil into the combustion chamber where the pistons sit and eventually reach the spark plugs.
Bad valve guides
Valve guides align valves with valve seats. They also prevent valve stem wear.

Valve guides that break or wear out cannot seal the engine’s combustion chamber. Worn valve guides create a vacuum that leaks oil into spark plug threads.

What to Do With Oily Spark Plugs?

Oil, dirt, and debris affect spark plug performance. To fix oil on spark plug threads only, do the following.

  • Remove and clean spark plugs.
  • Spray carburetor cleaner into spark plug holes.
  • Allow oil to enter the combustion chamber while the rest dries.
  • Reinstall cleaned spark plugs

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