When you have your regular tune-up and find oil on the spark plugs, you may wonder how that happened. Oil on the spark plugs often shows that there is a mechanical problem, but how do you know what’s wrong?
In this article, we will review the top seven reasons that there is oil on the plugs, and how to fix the problem along with some answers to questions you might have.
Reasons for Oil on Spark Plugs
Spark plug oil typically comes from leaking valve covers or a clogged PCV valve. It can also originate from clogged crankcase ventilation, a malfunctioning turbocharger, and worn intake valve seals. The oil can come from the pistons from worn piston/Oil control rings or bad pistons.
If this situation goes unresolved you will eventually face larger bills. Let’s discuss some possibilities of the situation.
#1. Leaking Valve Cover Gasket.
If there is oil in the wells or on the threads of the spark plugs (ignoring any oil from changing the spark plugs), this could be due to a leaking valve cover gasket. The valve cover gasket keeps oil sealed inside the engine but as time passes the gasket becomes brittle and breaks down to easier leak.
Oil will leak into the well surrounding the body of the spark plug. Depending on how bad the leak is, it may be a small amount on the threads or a pool of oil that the plugs are sitting in.
#2. Bad or Clogged PCV Valve.
All modern vehicles are equipped with a positive crankcase ventilation (PCV) system which is meant to reduce emissions and keep the environmental impact as low as possible. The PCV system not only vents engine crankcase vapors but also helps , reroute the vapors to be re-burned in the engine.
When the PCV valve is clogged or has failed, the system can siphon oil. This issue leads to higher amounts of vapors being present and can lead to oil fouling the spark plugs.
#3. Clogged Crankcase Ventilation.
The crankcase ventilation system is effectively set up to relieve crankcase pressure from blow-by gases. It also brings those gases to the intake manifold for the engine to use as fuel. However, if crankcase ventilation pressures reach excessive levels, oil can leak out.
If a crankcase ventilation system has failed, the eventual symptoms will be similar to a bad PCV valve. The crankcase pressures will become higher because of a clog in many cases. You may also notice a reduction in engine performance as well as a higher idle than normal.
#4. Bad Turbocharger.
A turbocharger provides extra power during a drive, so it’s a great addition to any vehicle. However, in a lot of cases, the turbocharger is the source of oil getting on the spark plugs. The seals on the turbine shaft were originally designed for extreme conditions, but they, like any mechanical seal, will eventually fail.
As the heat takes its toll, any oil that is lubricating the shaft makes its way into the intake and combustion chamber then onto the spark plug. Once the turbocharger has blown, a replacement is inevitable, which can be costly.
#5. Worn Intake Valve Seals.
Intake valves are located in the cylinder heads and allow air to enter the engine and help exhaust gases out. Each valve has a stem that fits into the cylinder head. The valve seals keep the oil from getting past the guides into the combustion chamber.
If the seals or guides start to wear, oil can enter the system and cause the plugs to foul. The engine will begin using more oil and you may notice smoke coming out of the exhaust, especially on start-up.
#6. Bad Piston Rings.
The piston rings of a cylinder keep the oil from leaking into the combustion chamber and loss of compression. Furthermore, they are made to last the entire life of the engine, but that is not always the case.
If a damaged or worn-out piston ring allows the oil to leak through that barrier, it will foul the spark plugs.
#7. Bad Pistons.
A possibility is that you may have a piston issue. Once the either a piston is compromised or cracked, oil enters from the crankcase and into the combustion chamber of the compromised piston.
Oil begins to foul the plug and you are left with some big issues to address. If the vehicle is worth spending money on, you are looking at a costly engine repair.
How To Fix Oil On Spark Plugs?
To fix spark plugs with oil on them, figure out where the oil leak is. You can clean the crankcase ventilation hoses and possible PCV valve.
If you do a leak test but have still run into trouble, it might be time to ask for a mechanic.
#1. Check If It’s Leaking Inside the Cylinder or from Above.
The first thing you have to do is figure out where the oil is coming from. You don’t want to just swap out new spark plugs and hope the issue goes away. You must fix the problem first to ensure another set of plugs doesn’t get fouled up.
Plus, if you keep running the engine with leaking oil you will cause even bigger damage. This fix could potentially cause you even permanent damage to the engine, depending on the issue.
#2. Clean Crankcase Ventilation Hoses & PCV Valve.
To clean the crankcase ventilation system:
- Remove the oil filler cap. If the cap has ventilation holes, dip it in parts cleaner and then shake it clean before putting it back. If the oil cap has no ventilation holes, be sure to check the rubber ring around it for sealing.
- Locate the PCV ventilation valve. It is at the end of a hose that connects the crankcase to either the inlet manifold or the carburetor (depending on how old your vehicle is.)
- Remove the valve assembly.
- Clean it off with an old paintbrush that you dip in solvent.
- Remove the hose and look for blockages.
- Clean out the hose with stiff wire and a piece of cloth.
- Put it all back together and check for air leaks after you are done.
If you find anything defective, this is the time to take care of the repairs!
#3. Leak-Down-Test (Test For Blow-By).
While most mechanics will perform a leak-down test, you can do one yourself if you have experience. Here are a few basic steps you should understand.
- First, gather your equipment. You will need the leak-down tester, air compressor, sockets, ratchet, extension, spark plug socket, and breaker bar.
- Second, while the vehicle is off, remove the spark plug that is causing issues. If you want to do a leak-down on all the cylinders, you must remove all of the spark plugs.
- Third, find top dead center.
- Next, put the tester into the spark plug hole or holes with the appropriate adapter.
- In the next step, connect your compressed air to the tester.
- Finally, read the results to see if there’s any leakage.
Once you can determine where the leak is coming from, you can properly fix it.
#4. Contact A Professional.
These are tricky steps, even for professional mechanics. We often have trouble figuring out where the oil is leaking from on spark plugs. This has the ability to take time.
If you are not sure, please consult a professional. You want to make sure you do not cause further damage.
Can oil make spark plugs fail?
Yes, oil on the tip of the spark plug will cause premature failure.
Oil is not supposed to leak into the combustion chamber, meaning there’s a problem that you have to fix. Fix the problem before installing new plugs.
Can a bad PCV valve cause oil on spark plugs?
Yes, if the PCV valve gets stuck closed the crankcase pressure buildup will push oil into the combustion chambers.
That oil lands on the tip of your spark plug and fouls the plug. You need to repair the issue before placing the spark plugs, otherwise, you’ll run into more issues.
What is the cost of replacing a valve cover gasket?
In general, valve cover gasket replacement will cost about $100-$350. The parts alone are cheap, generally only running $10-$50.
The majority of cost is labor on average $90 – $300.
If you are handy enough to do the work yourself, it can save you a good bit of money.
What causes black spark plugs that are wet?
The issue is oil leaking, most likely caused by leaking from the valve cover gasket or a clogged PCV valve.
It could also be from clogged crankcase ventilation, a bad turbocharger or worn intake valve seals. The spark plugs can also oil leak from bad piston rings or bad pistons.
How do you clean oily spark plugs?
If your spark plugs have oil on them, the first thing is to fix the issue which caused oil to get on the spark plugs.
After the issue is fixed, you can spray some carb cleaner into the hole and rag out oil, then do the same with the plug. Do that as many times as needed until you get rid of all the oil.
Oil on the spark plugs generally for sure means you have a major issue. You can tell, with proper diagnostics and knowing a little bit about mechanics you can get it fixed.
You should act promptly to avoid further damage. If you are uncertain about solving the problems or fixing them, take the issue to your mechanic.