What I Learned Today With Jeff Smith — Oil On The Spark Plug Threads

Over the years, we’ve seen many street engines with a significant amount of oil on the threads of the spark plugs. Our first thought was usually some type of external leak, like weeping valve cover gaskets, which might cause this. But, further evidence suggests this oil almost always enters the engine on the combustion chamber side. The test then becomes locating the source of the oil. Let’s look at the possibilities

This scenario is not unusual. The oil enters the chamber and finds its way around the exposed end of the spark plug. Under cylinder pressure, this oil is pushed further into the threads until the oil hits the sealed end of the threads. Insulated from high combustion temperatures, the oil just sits there in liquid form ready to be discovered when the spark plugs are removed. It’s also possible that not all the cylinders will be affected.

oil on spark plug threads

Oil on the spark plug threads is present on all the cylinders in this photo. This engine suffered from detonation problems, which were at least partially caused by oil in the combustion chamber. So oil on the plugs can lead to more serious problems if not repaired promptly.

Of course, it is possible that oil can find its way past the piston rings, entering the combustion chamber and squeezing past the threads in the spark plug hole. For the purposes of this tech treatise, we will assume the rings and short-block assembly are in good shape and the rings are doing their job.

This leaves the intake side of the engine as the source of this unwanted oil. We see three potential avenues where oil could enter the chamber before the combustion event. The first (in no particular order) is a broken, leaking, or bad intake valve guide seal. This will allow oil to travel along the intake valve stem with manifold vacuum pulling it into the chamber. This is where only selective cylinders may be affected but this does not rule out the possibility that all the intake seals are bad.

oil on spark plug threads

Worn valve stem seals can be a source of oil getting into the combustion chambers. They are inexpensive and easy enough to change if you are already in the engine.

The next avenue of approach is poor positive crankcase valve (PCV) operation. Some PCV valves can actually pull a serious amount of oil out of an engine. The early LS truck engines were especially vulnerable to this. We won’t get into all the possibilities here as this subject is covered in other online stories. Often the solution involves using a separate oil separator tank that will prevent pulling liquid oil into the intake manifold through the PCV valve.

A final avenue for this errant oil is found on engines that use the intake manifold to seal the lifter valley like a small-block Chevy or Ford. In this situation, the intake manifold is sealed properly at the bottom of the intake ports and intake manifold vacuum pulls the oil past the intake gasket and into the intake port where it enters the combustion chamber. In some cases, the engine is affected only on one side of the engine, the side where the intake is leaking disclosing an obvious clue to the repair. A leaking intake gasket can pull a tremendous amount of oil through the engine in a relatively short time.

oil on spark plug threads

The Positive Crankcase Ventilation (PCV) valve can actually be a cause of oil on the spark plug threads, so it should be on the troubleshooting checklist.

About the author

Jeff Smith

Jeff Smith, a 35-year veteran of automotive journalism, comes to Power Automedia after serving as the senior technical editor at Car Craft magazine. An Iowa native, Smith served a variety of roles at Car Craft before moving to the senior editor role at Hot Rod and Chevy High Performance, and ultimately returning to Car Craft. An accomplished engine builder and technical expert, he will focus on the tech-heavy content that is the foundation of EngineLabs.
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