Hobbies Cars & Motorcycles Fixing Oil-Filled Spark Plug Wells Does This Problem Need an Immediate Solution? Share PINTEREST Email Print Jamiekop/Flickr Cars & Motorcycles Cars How Tos Basics Reviews Classic Cars Corvettes Mustangs Tires & Wheels Motorcycles Used Cars Trucks ATVs & Off Road Public Transportation By Matthew Wright Matthew Wright Matthew Wright has been a freelance writer and editor for over 10 years and an automotive repair professional for three decades specializing in European vintage vehicles. Learn about our Editorial Process Updated on 11/01/18 When a spark plug well fills with oil, it means that the O-ring that seals the well to keep out oil and debris has deteriorated and started to leak. Although the leak can sometimes be fixed by tightening the valve cover bolts, more often than not the valve cover gasket and well seals will need to be replaced. Oil in the Well If this leak is left untreated, the spark plug boot could swell, which in turn will lead to a misfire in the engine. It's import to fix this leak as soon as you discover it. Oil in a spark plug can do extensive damage to different areas of the motor, such as warping or breaking the valves and pistons or destroying the head gasket, which could lead to bigger problems. If left unresolved, this will not only hurt engine performance, it might even cause the engine to catch on fire. Classic cars are especially prone to oil leaking into the spark plugs, so it's important to know what to look for when assessing where a leak is coming from. The best place to start is usually by checking whether or not the valve covers need to be replaced. How to Fix the Leak It's important to keep in mind that it's not always necessary to replace your spark plug wells entirely. Oftentimes, just replacing parts of the valve, especially the cover, will suffice for fixing an oil-filled spark plug well. If the gasket is not well sealed around the spark plug, it is likely to allow oil to leak into that area, which will pool and eventually cause problems for the engine as outlined above. Although this is the most common cause of the issue, a failing piston or worn valve guides can also cause this issue, so your mechanic should investigate those as well. Ideally, you or a mechanic should check your valve cover gaskets, O-ring seals, pistons, piston compression rings, and valve guides to rule out any possibility of further damage than the surface-level valve seal. Other Issues that Effect Spark Plug Output Although oil-filled spark plug wells are a common issue, there are a variety of different factors that can contribute to misfires in the engine, especially as they relate to the spark plugs and their associated parts. The spark plug wires, for instance, can break down in a variety of ways that will cause a vehicle's Check Engine light to come on. You should inspect your spark plug wires for breaks in the insulation regularly as this can lead to arcing and a weak spark or no spark at all, which ultimately affects your gas mileage. Oftentimes, this will require replacing your spark plug wires, which should happen every 30,000 miles regardless of engine performance—if you're changing the spark plugs, consider changing the wires at the same time. If you don't feel comfortable making these inspections and/or corrections at home, your mechanic will be able to do a thorough check to assess the problems and make any necessary repairs.