What Causes Oil on Spark Plugs?

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Expert-approved advice on spotting and diagnosing oily spark plugs
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Your spark plugs are responsible for providing the spark that ignites the fuel and air in each of your engine’s cylinders (so if you have a 4-stroke engine, you have 4 spark plugs). These spark plugs should always be dry, so if they’re coated in oil it’s a big sign that you have an oil leak—normally as a result of a leaking valve cover gasket. To help, we spoke to auto repair and design specialist Hovig Manouchekian to get the lowdown on how to diagnose and fix whatever’s causing oil to appear on your spark plugs.

Why Is There Oil on My Spark Plugs?

In most cases, oil on the spark plug threads indicates that you have a leaking valve cover gasket. This gasket sits between the cover of the engine and the spark plug well, so oil entering the spark plug chambers must come through this gap. See a mechanic to have them inspect and repair the issue.

Section 1 of 5:

Common Causes of Oil on Spark Plugs

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  1. This gasket sits between the engine cover and the spark plug well, so if there’s oil on a spark plug it probably seeped into the spark plug compartments from the top of the engine where the valve cover sits. [1] In rare cases, oily spark plugs may also be due to:
    • Worn valve guides or intake seals.
    • A blown head gasket.
    • A clogged crankcase vent.
    • Overfilled oil.
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Section 2 of 5:

Symptoms of Oil on Spark Plugs

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  1. Manouchekian points out that the spark plug provides the spark (via the ignition coil) that each cylinder in the engine needs to combust the fuel and air. Since every engine needs a combination of air, fuel, and spark to start, an oily spark plug will make it noticeably harder to turn the engine over.
    • You may still get the engine to start, it will just take longer and feel more violent than it normally would.
  2. Manouchekian explains, “The most common culprit to engine misfire is lack of spark. And the apparatus that controls that, if you will, or delivery is the spark plug, very common, and that's usually lack of maintenance or age of the spark plug.” Since oily spark plugs won’t spark efficiently, you’ll notice a lot of misfires .
  3. The spark plugs fire at a stable pattern when a car is idling. So, if your spark plugs are oily and struggling to fire properly, it’ll be especially noticeable when you’re idling. Your car may shake more than normal and your idling power will be greatly diminished as the cylinders fire without their required spark.
    • The loss of idling power will be even more noticeable when the engine is actively misfiring.
  4. Without the necessary spark from the oily spark plugs, Manouchekian explains there won’t be enough combustion to burn away all of the fuel entering the engine. This will lead to a lot of wasted fuel that isn’t actually helping to power the engine. As a result, you’ll end up filling the tank more often than you normally would.
  5. Whenever there’s an oil leak in the engine bay, the oil will burn away as it heats up when you drive. A bad valve cover gasket leak is no different. You’ll likely smell the burnt oil and you may even see smoke coming out of the engine when your car is running. [2]
    • If you can see smoke coming out of your engine bay, it is extremely important that you do not drive. An oil fire could break out at any point if there’s a big enough leak to cause a smokescreen.
  6. If the problem gets bad enough, your engine may experience a phenomenon known as hydrolocking. This occurs when a liquid (in this case, oil) enters the engine and begins messing with the volume of air in the cylinders. This will eventually cause the engine to seize up and the car will stall out or shut down. [3]
    • Hydrolocking is normally caused by vehicles driving through deep water and sucking up water through the air intake.
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Section 3 of 5:

How to Fix Oil on a Spark Plug

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  1. Oil leaks require a professional to begin with, but since these oil leaks are inside of your engine, it’s a tall task for a DIY gearhead. See a professional mechanic—they’ll find the source of the oil leak, replace the gasket if necessary, and clean or replace your spark plugs. [4]
    • If you do want to try your hand at this, purchase a replacement gasket designed specifically for your make and model. Remove the spark plugs, clean them, and then remove the cover on top of the engine. Swap out the rubber gasket surrounding the entire housing for the new gasket.
    • Expect to spend $500-800 resolving issues that cause oil on your spark plugs.
Section 4 of 5:

What causes a leaky valve cover gasket?

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  1. The valve cover gasket sees a lot of trauma of the life of an engine. It absorbs heat, pins flowing oil inside of the engine, and experiences all kinds of vibrations as the engine runs. Even with all that, the valve cover gasket doesn’t normally fail! It’s rare you run into problems unless you do not take care of the car. To prevent leaky valve cover gaskets, be sure to:
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Section 5 of 5:

Can I drive with oil on my spark plugs?

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  1. Oil may not combust as easily as fuel does, but it will catch fire eventually if it’s leaking into the combustion chambers or spark plug well. If you notice oil on your spark plugs, have the vehicle towed to a mechanic if you can. [5]
    • If you do drive with an oily spark plug, go slow, stay in the righthand lane whenever possible, and shut the engine off if you see smoke or notice the engine is running hot.

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      References

      1. ↑ https://www.jdpower.com/cars/shopping-guides/what-is-a-valve-cover-gasket
      2. ↑ https://www.dispatch.com/story/opinion/cartoons/2016/04/22/car-talk-valve-cover-leak/23741635007/
      3. ↑ https://www.carthrottle.com/news/how-can-hydrolocking-occur-and-why-it-potential-engine-killer
      4. ↑ https://www.abcbalancingbeads.com/will-oil-spills-catch-fire/
      5. ↑ https://www.abcbalancingbeads.com/will-oil-spills-catch-fire/

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