Why is There Moist Engine Oil on My Spark Plug Threads?

Why is There Moist Engine Oil on My Spark Plug Threads?


I’ve a 355ci small block Chevy that was rebuilt a couple of yr in the past and we included a set of Edelbrock aluminum heads, a Comp flat tappet cam, and an Edelbrock twin airplane consumption with a 750 cfm Holley carburetor. To date, the engine is operating nice. The final time I pulled the spark plugs, the porcelain space appeared fairly good, with a light-weight tan colour prefer it must be so the air-fuel ratio might be fairly shut.

Nevertheless, I observed moist engine oil on the threads of the spark plug. Most, however not the entire plugs had threads coated with oil. I requested round and a few folks talked about leaking valve covers however my engines are all the time actually clear in order that’s not it. The engine does use a bit of little bit of oil however I contemplate that to be regular. I’d add a half-quart in between oil modifications, however the engine doesn’t smoke—so it could actually’t be burning oil. Are you able to inform me the place that oil is coming from?


It’s no shock to our mates that almost all of our expertise is with Chevrolet and GM engines. We’ve seen this spark plug thread scenario many occasions on small and massive block Chevys and in addition questioned in regards to the origin of this oil. As you talked about, we’ll assume that there aren’t any exterior leaks, which leaves the supply of this oil as originating contained in the engine.

There are a number of potential sources for this oil. We will even assume, since your engine is comparatively new, that the Edelbrock cylinder heads are outfitted with good Viton rubber valve information seals so we are able to eradicate these seals as a supply of the oil. One risk is that there’s a mismatch between the highest and backside of the consumption ports between the heads and the consumption. We’ve seen this occur many occasions on plenty of completely different engines.

Examine the Consumption Manifold Gasket Seal

We had a small block that burned a big quantity of oil and we finally traced it to this poor seal on the backside of the consumption port the place manifold vacuum was in a position to pull the oil previous the consumption gasket and into the port. Regardless of the intense oil burning downside, it didn’t present up as a blue haze within the exhaust for the reason that oil was vaporized together with the incoming air and gas.

We’ve coated methods to take a look at for this in a earlier tech column, so we gained’t element that right here, however this might be one supply of the oil getting into into the combustion chamber. Not all of this oil will burn and, over time, a small quantity might be pushed up into the threads of the spark plug as a result of the threads should not sealed on the chamber facet. Over time, stress will push this oil up into the threads the place it’ll stay as a liquid as a result of it’s not uncovered to combustion.

You didn’t point out this, however look to see if the oil on the spark plugs happens solely on one facet of the engine. It’s common for this consumption gasket leak downside to solely happen on one facet of the consumption manifold. If oil is current on just one facet (like the driving force facet for instance) then this means you could have a leaking consumption gasket on that facet of the engine. This will even level you towards the answer.

Examine Your PCV Valve (& Baffle)

With oil on practically all of the spark plugs threads, the consumption may nonetheless be the issue or there may be one other attainable supply. The reader didn’t point out this, however we’ll assume as a result of this can be a road engine, that it’s outfitted with a PCV valve.

If the PCV valve is situated within the valve cowl however just isn’t protected by a baffle, this could pull fairly a little bit of oil into the consumption manifold. One other risk is that the baffle is incorrectly positioned and is just too near the underside of the PCV valve. This will increase velocity via the baffle and caries oil with the air. Most of this info got here from our mates at M/E Wagner who make a billet adjustable PCV valve.

One other answer to reduce the oil getting into the consumption via the PCV is to introduce a vapor separator in between the consumption manifold and the PCV valve. There are various completely different kinds of those separators they usually most frequently embody a detachable liquid reservoir and infrequently both a built-in separator or a stainless-steel mesh that helps pull liquid oil from the vapor. 

How Are Your Rings?

After all, a 3rd potential leak path is previous the piston oil management rings. Since your engine has been lately rebuilt we’ll assume that that is seemingly not a supply for the oil. Nevertheless, as a take a look at you may attempt operating a barely greater viscosity oil to see if the oil utilization improves. If a excessive viscosity oil like a 20w50 is used, for instance, this makes it more durable for the piston rings to scrape all of the oil from the cylinder partitions. A decrease viscosity oil shall be simpler to take away.

If you happen to suppose you’ve discovered the answer to your downside, the take a look at shall be to take away the spark plugs and clear the oil with brake or carburetor cleaner after which run the engine for sufficient miles to permit the surplus oil to return. If the plugs then look clear after 2,000 miles or so then you already know you’ve solved the issue.

If the oil returns to the threads then you already know you could have extra work to do!

Placement of the baffle contained in the valve cowl for the PCV valve is necessary. If the baffle is just too near the inlet for the PCV valve, this creates a excessive velocity air motion that can pull the oil together with the vapor. So the right baffle provides not less than 3/8 inch of hole and ideally extra like 3/4 inch of house between the PCV inlet and the baffle itself. (Picture/Jeff Smith)

Jeff Smith has had a ardour for vehicles since he started working at his grandfather’s gasoline station on the age 10. After graduating from Iowa State College with a journalism diploma in 1978, he mixed his two passions: vehicles and writing. Smith started writing for Automotive Craft journal in 1979 and have become editor in 1984. In 1987, he assumed the function of editor for Sizzling Rod journal earlier than returning to his past love of writing technical tales. Since 2003, Jeff has held varied positions at Automotive Craft (together with editor), has written books on small block Chevy efficiency, and even cultivated a formidable assortment of 1965 and 1966 Chevelles. Now he serves as an everyday contributor to OnAllCylinders.


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September 2022