Finding oil on your spark plugs can mean bad news for your engine, but don't panic yet, as it may not be as bad as you think.

There are many reasons why oil can find its way onto the spark plugs. Most of the time is can be caused by a leaking valve cover or spark plug o-ring that can easily be replace. But sometimes it's more serious and can be caused by a leaking head gasket. 

What Does A Spark Plug Do?

The spark plugs are arguably the most important component of the ignition system. To put its role on perspective, just imagine you’re deep in the woods at night and decide to make a campfire. The lighter has the same function as the spark plug, ie to start the combustion. Now imagine the consequences of a deficient lighter and translate it to the engine scenario.

But spark plugs importance doesn’t end there. They are also in charge of keeping an appropriate temperature in the combustion chamber. The combustion process is more delicate than you can imagine. A myriad of variables affects its efficiency, one of them the temperature. If the combustion chamber temperature is too low then the energy won’t propagate as intended causing a huge loss of power.

On the other hand, if the temperature is too high a phenomenon known as pre-ignition (an expontaneous ignition before the spark plug is fired) occurs. This factor is so relevant, that spark plugs are classified according to its “heat range”. Vehicle makers choose the adequate range for the engine after exhaustively testing different conditions. Keep that in mind before using a heat range different from the suggested one.

As you can see spark plugs are essential to internal combustion engines in many ways. That’s why it's crucial preventing them from deteriorating due to causes other than their normal life cycle. Having oil in the spark plugs is one of those enemies that decreases spark plugs longevity, but most importantly, could also cause severe damage to your engine that you need to avoid it at all costs.


How Does Oil On Spark Plugs Affect The Engine?

A valid question is why the oil on the spark plug is bad for your engine? Let’s greatly simplify the answer using our lighter example again. Imagine you’re trying to initiate the campfire but the lighter tip is dirty with engine oil. You’ll eventually start the campfire, but only after several failed tries. Now think of a spark plug. No spark means no ignition and no combustion. Fuel will be delivered but will exit the cylinder unburnt. 

What about the chamber temperature? Let's suppose you had a weak spark. With a combustion chamber filled with oil traces and consequently oil particles burning with the gasoline, what do you expect? As said earlier, the combustion process is a well timed delicate one and the impact of the oil will have a significant effect on engine performance. 

oil on spark plugs

A worn spark plug being removed from an engine

Having oil on your spark plugs for a prolonged period of time could potentially destroy your catalytic converter due to the unburnt fuel, and could also cause worn engine bearings due to the excess gasoline ending up in the oil pan. That’s why this condition is considered severe and should be addressed as soon as possible.


What Are The Causes Of Oil On Spark Plugs?

Spark plugs are one of the few components in direct contact with the combustion process. Engine oil, by contrast, should not be there. Then how it reaches the spark plugs in the first place?

  • Valve Cover Gaskets: spark plugs are not the only component installed on cylinder heads. Depending on the engine design (OHV/SOHC/DOHC) you may have the intake and exhaust valves, valve springs, cam(s), valve guides, etc. Most of those components require lubrication, so engine oil is pumped from the oil pan all the way to the heads. The mission of the valve cover gaskets is keeping the oil on the cylinder heads and away from the rest of the engine. Due to the high temperatures involved, valve cover gaskets tend to wear allowing oil leakages. Dripping oil can reach spark plugs in Over Head Valve (OHV) designs and hence cause the oil on plug condition.
  • Spark Plug O-Ring: on the Single Over Head Cam and Double Over Head Cam (SOHC/DOHC) designs the valve cover includes a seal often known as spark plug O-ring. Similar to the previous point, heat tends to degrade this seal over time letting the oil reaching the spark plugs.
  • Defective or Worn Valves Guides: another possible cause for this problem is excessive wear on the valve guides. The constant movement up and down of the valves literally “pumps” oil into the combustion chamber having as a direct consequence oil on the spark plugs.
  • Piston Compression Rings: a broken compression ring is also a possible cause for this condition. When the engine is aging and compression start dropping, the oil pan builds-up positive pressure. That pressure can find a way to the combustion chamber during the compression cycle if the ring is broken.
  • Faulty head-gasket: pressurized engine oil flows from the oil pan all its way up to the heads through lines and galleries part of the lubrication system. A faulty head gasket could allow oil entering the cooling system and/or the engine cylinder. 
  • Broken Piston Head: a less likely (but possible) cause for this problem is a broken piston head. Similar to the last point, positive pressure coming from the oil pan could freely pass through the piston to the combustion chamber and reaching the spark plug.

6 Symptoms Of Oil On Spark Plugs

The most common symptoms you may encounter are:

  1. Blue smoke from the tailpipe: you can tell if this condition is extreme looking at the exhaust fumes. Blue smoke usually indicates a critical condition that should be addressed immediately. 
  2. Poor fuel economy: as you may expect, this problem will increase fuel consumption dramatically.
  3. A decrease in engine performance: expect a decrease in engine performance under all driving conditions due to the reduced combustion efficiency.
  4. Possible engine misfires: the affectation in combustion energy will cause misfires that could be more noticeable depending on the driving condition.
  5. Back-fire: fuel particles won’t burn in the combustion chamber as they usually do and will enter the exhaust pipe with a high energy charge. That energy will be unleashed in the form of mini explosions. 
  6. Gas smell from exhaust pipe: you may encounter a mix of oil and gasoline coming from the tailpipe accompanied by the peculiar smell of a bad combustion. 

How To Diagnose The Cause Of Oil On Spark Plugs

For the purpose of this article, it’s assumed that you have a basic knowledge of safety precautions while working on your vehicle.

Always refer to the appropriate OEM literature when possible. Original manufacturer diagnostic procedures should always have precedence over a generic workflow.

That said, let’s start the diagnostic process!

1. Visual Inspection

> OHV Engines: perform an exhaustive visual inspection of cylinder heads exterior. Any sign of oil coming from the valve cover gaskets is enough to replace them.

> SOHC/DOHC Engines: perform the same visual inspection conducted for OHV engines and replace the valve cover gaskets if necessary. Additionally, check the exterior condition of all spark plugs. Any sign of engine oil on the spark plug wires, coil-over-plug or the spark plug ceramic coating is enough to replace all spark plugs O-ring seals.

2. Engine Tests

Approximately in the 80% of the cases, the oil on spark plugs is caused by a bad valve cover gasket or bad spark plug O-ring. The other 20% is mostly related to engine problems. 

Only perform the following tests if you already completed the visual inspection discussed earlier. 

> Engine compression test: warm-up the engine until it reaches its normal operating temperature. Remove all spark plugs and turn the crankshaft manually in the direction of its normal rotation until the piston #1 reaches its compression stroke top dead center (TDC). Using the appropriate adapter connect the engine compression gauge to the cylinder #1.

Disable the fuel and ignition system and crank the engine 6 revolutions while keeping the pedal on the wide open throttle. Take note of the gauge reading and repeat the procedure for all cylinders. Consult the OEM literature to determine if the compression is within the accepted range (usually above 100psi is fine).

A difference greater of 10% between cylinders should be considered suspicious, especially if one of the cylinders has oil on its spark plug.

> Engine differential pressure test: this test may not be as popular as the prior one, because involves a clean compressed air source, but at the end, saves a lot of time in determining the root cause of the compression problem. The procedure is basically the same, you need to warm-up your engine, rotate the crankshaft until piston #1 reaches TDC and then disable ignition and fuel systems.

Once everything is ready, connect the special differential pressure gauge to cylinder #1 as you did before. Now using the integrated air regulator adjust the air pressure to 90-80psi. Ideally, the cylinder manometer should show the same pressure. A difference greater than 15% is an indication of a leakage.

The great advantage of this procedure over the traditional compression test is that you can locate where the leak is. While the cylinder is under air pressure you can look for air leakages in the engine valves, engine coolant, or positive pressure in the oil pan.


How To Fix Oil On Spark Plugs

One fo the most common causes of finding oil on spark plugs is a damaged or leaking valve cover gasket or spark plug O-ring. Here's a brief explanation of how to change them.

> Preliminary steps: ensure that the ignition switch is off and then disconnect the vehicle battery. Detaching the negative terminal is usually enough.

> Remove valve covers: remove all decorative covers from the engine. Remove valve cover bolts and gently pull it out. You might find that the valve cover has tightly adhered to the head. If that’s the case use a proper tool (a rubber mallet for instance). Never, ever use metal tools like screwdrivers to pull the valve cover out, because that could cause a permanent damage to the cylinder head.

> Retire the old gasket and/or O-ring seal: remove the old gasket, depending on the engine you may find it difficult due to silicone applied during installation. Remove the O-ring seal from all cylinders.

> Clean the head and valve cover surface: using plastic tools (never metal tools) peel off any trace of the old gasket. Clean both surfaces (valve cover and cylinder head) using an appropriate degreaser. Be very careful while doing so, especially when cleaning the cylinder heads, avoid any substance reaching the engine internals.

> Install the new gasket and/or O-ring seal: apply the recommended silicone (only if the engine OEM literature says so) and then proceed to install the valve covers and spark plugs. Check out our article on how to choose the best spark plugs here.