Ever pop the hood, pull out a spark plug, and find it drenched in oil?
It’s the kind of discovery that makes any car owner’s heart skip a beat.
But don’t panic just yet!
Whether you’re a seasoned grease monkey or a curious car enthusiast, grasping the causes, symptoms, and fixes for oil on spark plugs is key to keeping your engine humming at its best.
Let’s examine why oil may be fouling your spark plugs and why you must remedy this issue immediately.
This Article Contains:
- Why Is There Oil on Your Spark Plugs, and Is It Serious?
- 8 Common Causes of Oil on Spark Plugs
- What Happens When You Have Oily Spark Plug?
- How to Diagnose and Fix Oil-Fouled Spark Plugs?
- What is the Cost of Fixing Oil-Fouled Spark Plugs?
- How Can You Prevent Oil on Spark Plugs?
Why Is There Oil on Your Spark Plugs, and Is It Serious?
When oil has reached your spark plugs, it’s likely due to a leaking seal or an engine malfunction.
Where exactly you find oil on the spark plugs can help pinpoint the cause:
- If there’s oil pooled in the spark plug wells, it means there’s a compromised seal, like the valve cover gasket. When this happens, you’ll also notice oil in the corresponding ignition coil (spark plug wire).
- If you spot oil only on the tip of the spark plug, along with ash deposits, but the rest of the plug body and the spark plug wells are dry — it means oil has entered the combustion chamber. This points towards a more serious engine issue.
Now, a small quantity of oil isn’t an immediate concern as the engine will burn it along with the air-fuel mix. But if left unresolved, it can lead to a major engine breakdown, requiring expensive repairs.
So, let’s dig a little deeper into how oil can leak on your spark plugs.
8 Common Causes of Oil on Spark Plugs
Here are some possible sources of oil leakage on the spark plugs:
1. Leaking Valve Cover Gasket
The valve cover gasket prevents oil from leaking by sealing the gap between the valve cover and the cylinder head. But time, heat, pressure, and age can wear out the cover gasket. A leaky valve cover gasket will allow oil to seep into the spark plug wells.
Note: While a leaking valve cover gasket allows oil to seep externally, a worn valve guide seal (valve stem seal) can cause oil to enter the combustion chamber.
2. Worn Spark Plug Tube Seals or O-rings
The spark plug tube seal (O-ring) is found at the top of the spark plug tube where the spark plug sits. It maintains a tight seal between the valve cover and the spark plug tube to keep engine oil from leaking into the spark plug wells.
Like the valve cover seal, the spark plug seal can degrade too, allowing oil to seep into each spark plug hole.
Note: Although rare, sometimes a spark plug tube in the cylinder head can also crack, allowing oil to contaminate the spark plug threads.
3. Worn-out or Broken Piston Rings
There’s an oil control ring located at the bottom of each piston. It prevents oil from entering the combustion chamber where the spark plug is located.
When these rings become worn or stuck, oil can escape into the chamber, leading to wet oil and ash deposits on the spark plug. You’ll also notice increased oil consumption and visible smoke from the tailpipe.
4. Worn Cylinder Wall
Fuel combusts at extremely high temperatures inside the combustion cylinders. Over time, friction and heat can cause the cylinder walls to become worn or scored. A worn cylinder wall can compromise the tight seal between them and the piston rings, resulting in oil leakage.
5, A Blown Head Gasket
The head gasket sits between the engine block and the cylinder head, creating a seal that keeps the combustion chamber, oil passages, and coolant passages separate. If the head gasket fails, oil can leak into the combustion chamber.
Once that happens, you’ll notice oil deposits on the spark plug tips.
Note: A blown head gasket is a serious issue that you should address ASAP. If you keep diving with a blown gasket, your engine will continue to overheat until it ceases completely.
6. Clogged Crankcase Ventilation System
The crankcase ventilation system (PCV system) helps regulate and remove blow-by combustion gases that leak past the piston rings into the engine’s crankcase.
A clogged PCV valve or its associated hose can prevent these gases from escaping and increase pressure inside the crankcase. The built-up pressure will force engine oil past the valve cover gasket or spark plug tube seals.
7. Overfilled Engine Oil
Excess oil can increase pressure within the crankcase. This pressure can force oil past seals and gaskets, resulting in the oil entering the spark plug wells or combustion chamber.
Likewise, low oil in the oil pan can increase friction and wear out seals and gaskets, resulting in oil on plugs down the line.
8. Cylinder Deactivation and Variable Valve Timing Problems
Several modern vehicles have variable valve timing (VVT) engines and cylinder deactivation features to improve engine performance.
However, these systems can sometimes lead to excess vacuum in the engine, pulling oil past the piston rings and fouling the spark plugs.
If the leak is minor, your vehicle will seem to run fine. But no matter how tempting it may be, you shouldn’t put off this repair.
Let’s understand why.
What Happens When You Have an Oily Spark Plug?
You’ll likely notice the following issues when oil has made its way to the spark plugs:
- Engine misfires or runs rough: Oil on plugs can prevent the engine from firing correctly, hampering the combustion process. This can result in misfires in one or multiple cylinders. Consequently, your vehicle will run rough, or you may experience engine backfire.
Note: When there’s a misfire, your vehicle’s ECU will trigger the check engine light and register a related code such as P0300, P0301, P0304, etc. - Blue smoke from exhaust: When oil leaks into the combustion chamber, it burns along with fuel, manifesting as blue smoke from the exhaust.
- Increased emissions: Oil on spark plugs will hamper combustion efficiency and engine performance, leading to an increase in exhaust emissions coupled with more fuel consumption.
The longer you ignore these signs, the more likely they are to cause long-term engine damage and affect other critical parts, like the catalytic converter.
Your best bet is to identify the underlying cause quickly to prevent hefty engine repairs.
But how do you do that?
Keep scrolling to find out!
How to Diagnose and Fix Oil-Fouled Spark Plugs?
Diagnosing and fixing oil-fouled spark plugs can be tricky. If you’re not comfortable with car parts, let a mechanic handle this job for you.
Here’s how a professional would go about it:
A. Diagnosis
First, they’ll perform the following checks to identify the oil leak source:
- Visual Inspection: Remove each spark plug and check for wet oil or black, tar-like buildup on the electrodes, insulator, and spark plug thread. Oil in the spark plug well indicates a leaking spark plug seal or cover gasket. Oil on the spark plug tip means a bigger trouble related to the combustion chamber.
- Check engine codes: DTC codes related to misfires are common when oil fouls spark plugs. This will help identify the cylinder number and the possible reason for a misfire.
- Perform a leak-down test: To check if the valve seals or the head gasket are the oil leak source.
- Compression test: Low compression in one or multiple cylinders can mean worn piston rings or damaged cylinder walls. The mechanic may also inspect the cylinder head for cracks or warping.
- PCV valve check: Check the PCV valve for any clogs that could be causing excessive crankcase pressure.
B. Fixes
Based on the diagnosis, the mechanic will perform one of the following repairs:
- Replace the worn seals
- Replace the head gasket
- Clean or replace the PCV valve
- Disassemble the engine to repair piston rings or cylinder walls
Once they fix the root cause, they’ll clean the old plugs (if they still have considerable life left) or install a new plug set.
Wondering how much these repairs would cost?
Let’s find out.
What is the Cost of Fixing Oil-Fouled Spark Plugs?
Some mechanics may initially charge a basic diagnostic check fee of $50 to $100, depending on the car’s make and location.
Based on the results, they’ll suggest the required repairs. Here are the cost estimates (including labor) for different repairs and replacements:
- O-rings replacement: $20-$220
- Comprehensive diagnostic testing: $100-$170
- Valve cover gasket replacement: $110-$340
- PCV valve replacement: $185-$220
- New spark plug set: $100-$500, depending on spark plug type
- Valve guide seal (valve stem seal) replacement: $950-$1800
- Piston ring replacement: $1,800-$3,500
- Piston replacement: $1000-$5000
Some of these repairs can burn a hole in your pocket unless you’re vigilant and perform preventive maintenance regularly.
How Can You Prevent Oil on Spark Plugs?
Here are some preemptive measures you can take:
- Perform a regular oil change and monitor oil levels in the oil pan to keep your engine well-lubricated.
- Use the correct oil grade to prevent it from bypassing the piston rings.
- Avoid overfilling the engine oil to keep internal pressure from getting elevated.
- Ensure that the crankcase ventilation system is functioning correctly to prevent pressure buildup.
- For turbocharged engines, inspect the turbocharger for oil leaks or excessive wear.
- Get routine engine compression tests to ensure the internal components are in good condition for optimum engine performance.
Keep Your Spark Plugs Sparkling Clean with AutoNation Mobile Service
Oil on spark plugs can result from relatively minor issues like a leaking valve cover gasket to more severe problems like worn piston rings or cylinder walls. You should address this as soon as possible.
If you notice oil on spark plugs, get a thorough diagnosis from experts at AutoNation Mobile Service.
We are available seven days a week and can perform the repairs right from your driveway!
Get a quote today.