Why is there oil on the spark plug and threads?
Understanding the Issue: Oil on the spark plug and threads
On most late model engines, the spark plugs sit deep inside the 
cylinder head. A spark plug tube extends from the cylinder heat to the valve cover and a rubber seal prevent oil from leaking down into the spark plug tubes. However, over time the spark plug tube seals fail and allow oil splash to seep into the tubes, causing oil on the spark plug. If your spark plugs are soaked in oil, worn, aged, shrunken spark plug tube seals are the cause.
This article will explore the common causes of oil on the spark plugs and threads and the effects this can have on vehicle performance, and the steps you should take to address the issue.
Leaking Valve Cover Gasket Spark Plug Tube Seals
The most common causes of oil on the spark plugs is a leaking valve cover gasket and spark plug tube seals. The valve cover sits at the top of the engine and covers the valve train, keeping oil inside and preventing it from spilling into other areas. The gasket acts as a seal between the valve cover and the engine head, and over time, it can wear out due to heat, pressure, and age.
When the valve cover gasket fails, oil can escape and seep into surrounding areas, including the spark plug wells, which house the ignition coils. The oil accumulation in these wells can submerge the ignition coils in oil, causing them to misfire or fail completely. Ignition coil oil soaking is often a telltale sign of this issue.
Symptoms of a oil on the spark plug
Ignition coil soaked in oil
Oil pooling around the ignition coil or spark plugs.
Misfiring or rough idling.
Burning oil smell from under the hood.
Oil leaks visible from the top of the engine.
If you notice any of these signs, you should have the gasket inspected and replaced promptly. Neglecting to do so could lead to severe engine damage as well as compromised ignition coil performance.
How to replace spark plug tube seals
How to clean an ignition coil soaked in oil
Use aerosol brake cleaner to wash the oil off the ignition coil. 
Then replace the ignition coil boots.
Replace the valve cover gasket and spark plug tube seals
Remove the retaining bolts from the valve cover
Remove the old valve cover gasket and tube seals
Clean the mating areas on the cylinder head
Place the new gasket into the groove on the valve cover and install the valve cover. Torque the bolts to spec. Do not over-tighten.
On some vehicles, the individual spark plug tube seals are pressed into the valve cover. As they age and harden, they may be difficult to remove. Use a pry bar or flat blade screwdriver to pry the rubber seals out of the valve cover. Then press in the new ones.
©, 2024 Rick Muscoplat