Oil on spark plugs and threads — The Most Common Cause

What Causes Oil on the spark plugs and threads

In my experience, when you find oil on the spark plugs or on the spark plug threads on an overhead cam engine, the cause is always failed spark plug tube seals. On late-model engines, the spark plugs sit deep inside the cylinder head inside a spark plug tube. A rubber spark plug tube seal fits into the valve cover and is designed to prevent oil from leaking into the spark plug tubes.

Over time, these spark plug tube seals degrade, allowing oil to seep into the tubes, leading to oil on the spark plugs. The instant you loosen the spark plug, the oil seeps down onto the spark plug threads. That usually freaks DIYers out, making them think they’ve got a huge oil consumption issue when the real cause is worn-out spark plug tube seals.

The presence of oil on spark plugs can have serious consequences for engine performance, in some cases causing spark plug misfires where the spark shoots to ground, using the oil as a conductor.

Wait, Isn’t Oil Non-Conductive (Dielectric)?

I’ve met many highly experienced technicians who vehemently disagree that oil in the spark plug tubes can cause misfires. They cite the fact that oil is nonconductive and is even used as a nonconductive coolant in transformers. That’s true, but has nothing to do with this situation.

Pure oil is nonconductive, but motor oil can be conductive at high voltages

The anti-wear/anti-corrosion additives in new oil and the wear metals, acids, and water in used oil can absolutely conduct electricity in the oil when exposed to high voltages. Just conduct an oil analysis on the used motor, and you’ll see the trace wear metals present in the report. So yes, motor oil can conduct high voltage and can cause misfires.

Here’s a more in-depth explanation from an oil manufacturer

“Under normal circumstances, the conductivity of lubricating oil is very low so that it can be used as insulating oil or transformer oil. However, in addition to the type of base oil, additives also affect the conductivity of the lubricant. Organometallic additives will increase the conductivity of the lubricant. Common organic metal additives, such as ZnDTP antiwear agents, are widely used in engine oil and hydraulic oil. It is a multifunctional additive that can resist wear, oxidation, and metal corrosion.

For the same lubricating oil, as the temperature increases, the conductivity of the lubricating oil increases. However, the relationship between lubricating oil’s conductivity and temperature is not a purely proportional relationship. When the temperature rises, different lubricating oils have different increasing trends in conductivity.”

Source: Baoxing Lubrication Oil

The motor oil’s temperature also affects its dielectric properties. Read this from Machinerylubrication.com:

“Engine oils in power systems are electrically insulating liquids with electrical conductivities in the range of less than 1,000 picosiemens (pS) in normal ambient conditions. The value will depend on how pure the oil is and whether it has been altered with additive surfactants. For most liquids, the product of their viscosity and electrical conductivity is constant. As the temperature goes down, the oil’s viscosity increases exponentially, and its electrical conductivity decreases exponentially.”

Leaking Spark Plug Tube Seals Are The #1 Cause Of Oil On Spark Plugs

One of the primary culprits of oil on spark plugs is a failing valve cover gasket and spark plug tube seals. The valve cover is responsible for sealing the top of the engine, preventing oil from escaping. Over time, exposure to heat and engine pressure causes the valve cover gasket and spark plug tube seals shrink, allowing oil to leak into areas it shouldn’t, including the spark plug tubes. When the spark plug tube seals fail, they allow oil to accumulate in the wells, leading to oil on spark plugs and oil on spark plug threads as you loosen them and break the seal between the spark plug gasket and the cylinder head or the open a gap between the spark plug’s tapered seat and the cylinder head.

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 oil on the spark plugs

• Ignition coil boot is 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

I always recommend replacing the valve cover gasket and the spark plug tube seals. In some engines, the seals are built into the valve cover gasket, while in other designs, they’re ordered as separate parts. Whichever design you have, I suggest replacing both at the same time

1) Remove the retaining bolts from the valve cover
2) Remove the valve cover and then remove the valve cover gasket and tube seals
3) Clean the valve cover gasket mating areas on the cylinder head
4) If your engine has separate spark plug tube seals, remove the old ones from the tubes and replace with new seals. If the seals are mounted in the valve cover, pry out the old seal, lubricate the new ones and press them into the valve cover.
5) 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.

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.
©, 2024 Rick Muscoplat