Ask Away! with Jeff Smith: Why is There Wet Engine Oil on My Spark Plug Threads?

I have a 355ci small block Chevy that was rebuilt about a year ago and we included a set of Edelbrock aluminum heads, a Comp flat tappet cam, and an Edelbrock dual plane intake with a 750 cfm Holley carburetor. So far, the engine is running great. The last time I pulled the spark plugs, the porcelain area looked pretty good, with a light tan color like it should be so the air-fuel ratio is probably pretty close.

However, I noticed wet engine oil on the threads of the spark plug. Most, but not all of the plugs had threads coated with oil. I asked around and a couple of people mentioned leaking valve covers but my engines are always really clean so that’s not it. The engine does use a little bit of oil but I consider that to be normal. I might add a half-quart in between oil changes, but the engine does not smoke—so it can’t be burning oil. Can you tell me where that oil is coming from?

T.H.

It’s no surprise to our friends that most of our experience is with Chevrolet and GM engines. We’ve seen this spark plug thread situation many times on small and big block Chevys and also wondered about the origin of this oil. As you mentioned, we will assume that there are no external leaks, which leaves the source of this oil as originating inside the engine.

There are several potential sources for this oil. We will also assume, since your engine is relatively new, that the Edelbrock cylinder heads are equipped with good Viton rubber valve guide seals so we can eliminate those seals as a source of the oil. One possibility is that there is a mismatch between the top and bottom of the intake ports between the heads and the intake. We’ve seen this happen many times on lots of different engines.

Check the Intake Manifold Gasket Seal

We had a small block that burned a significant amount of oil and we eventually traced it to this poor seal at the bottom of the intake port where manifold vacuum was able to pull the oil past the intake gasket and into the port. Despite the serious oil burning problem, it did not show up as a blue haze in the exhaust since the oil was vaporized along with the incoming air and fuel.

We’ve covered how to test for this in a previous tech column, so we won’t detail that here, but this could be one source of the oil entering into the combustion chamber. Not all of this oil will burn and, over time, a small amount can be pushed up into the threads of the spark plug because the threads are not sealed on the chamber side. Over time, pressure will push this oil up into the threads where it will remain as a liquid because it’s not exposed to combustion.

You didn’t mention this, but look to see if the oil on the spark plugs occurs only on one side of the engine. It’s not unusual for this intake gasket leak problem to only occur on one side of the intake manifold. If oil is present on only one side (like the driver side for example) then this indicates you have a leaking intake gasket on that side of the engine. This will also point you toward the solution.

Inspect Your PCV Valve (& Baffle)

With oil on nearly all the spark plugs threads, the intake could still be the problem or there is another possible source. The reader didn’t mention this, but we’ll assume because this is a street engine, that it is equipped with a PCV valve.

If the PCV valve is located in the valve cover but is not protected by a baffle, this can pull quite a bit of oil into the intake manifold. Another possibility is that the baffle is incorrectly positioned and is too close to the bottom of the PCV valve. This increases velocity through the baffle and caries oil with the air. Most of this information came from our friends at M/E Wagner who make a billet adjustable PCV valve.

Another solution to minimize the oil entering the intake through the PCV is to introduce a vapor separator in between the intake manifold and the PCV valve. There are many different styles of these separators and they most often include a removable liquid reservoir and often either a built-in separator or a stainless steel mesh that helps pull liquid oil from the vapor. 

How Are Your Rings?

Of course, a third potential leak path is past the piston oil control rings. Since your engine has been recently rebuilt we will assume that this is likely not a source for the oil. However, as a test you might try running a slightly higher viscosity oil to see if the oil usage improves. If a high viscosity oil like a 20w50 is used, for example, this makes it harder for the piston rings to scrape all the oil from the cylinder walls. A lower viscosity oil will be easier to remove.

If you think you’ve found the solution to your problem, the test will be to remove the spark plugs and clean the oil with brake or carburetor cleaner and then run the engine for enough miles to allow the excess oil to return. If the plugs then look clean after 2,000 miles or so then you know you’ve solved the problem.

If the oil returns to the threads then you know you have more work to do!