How To Diagnose and Fix Oil On Spark Plugs (With 6 Common Symptoms)

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.

As a general rule, oil on spark plugs can be caused by bad valve cover gaskets, worn spark plug o-rings or valve guides, a faulty head gasket or a broken piston ring. Symptoms of oil on spark plugs include blue smoke from the exhaust, poor fuel economy and engine misfires.

There are many reasons why oil can find its way onto the spark plugs. Most of the time it can be caused by a leaking valve cover or bad spark plug o-ring that can easily be replaced. 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 plug 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 their “heat range”. Vehicle makers choose the adequate range for the engine after exhaustively testing different conditions. This is why you should only use manufacturer-recommended spark plugs in your vehicle.

As you can see spark plugs are essential to internal combustion engines in many ways. That’s why it’s crucial that spark plugs are kept in the best possible condition during their normal life cycle. Oil on the spark plugs is one of those things that drastically decreases the life of spark plugs, but more importantly, it could also cause damage to other parts of your engine.

How Does Oil On Spark Plugs Affect Engine performance?

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 Causes 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 come into contact with spark plugs ever. So how does engine oil reach the spark plugs?

1. Bad 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 role 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.

2. Degraded 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 the spark plug O-ring. Similar to the previous point, heat tends to degrade this seal over time letting the oil reaching the spark plugs.

3. 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 and causes oil on the spark plugs.

4. Broken Piston Compression Rings

A broken compression ring is also a possible cause for this condition. When the engine starts to age and compression starts 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.

5. Faulty head-gasket

Pressurized engine oil flows from the oil pan all its way up to the head of the engine via the oil lubrication system. A faulty head gasket could allow oil to enter the cooling system and/or the engine cylinder. 

6. 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 onto the spark plug.

6 Symptoms Of Oil On Spark Plugs

The most common symptoms of oil on spark plugs can include the following.

Symptom 1- Blue smoke from the tailpipe

If the spark plug tips have oil on them then this will adversely affect how efficiently the engine burns fuel.

It’s the job of the spark plugs to provide a spark at precisely the right time, in sequence with the timing of the engine valves. This should be timed to occur in between the opening and closing of the valves and at precisely the right time and in the right sequence. If the timing or the quality of the spark is off, then this adversely affects the combustion process, making it much more inefficient because the fuel no longer burns completely.

If there is oil on the spark plugs as they are no longer working as they should, then engine ECU will attempt to correct a decrease in fuel combustion efficiency by tweaking the fuel to air ratio in the cylinders. This often means an increase in the amount of fuel that is used by the engine, causing it to ‘run rich’. If there is too much fuel entering the combustion chambers, this can result in unburnt fuel leaving via the exhaust valve and entering the exhaust. This can make the exhaust gases appear blue or white.

Symptom 2 – Poor fuel economy

A very common symptom of oil on spark plugs is a noticeable drop in fuel economy. An increase in fuel consumption is primarily caused by the inability of the spark plug to create a good quality spark. Spark plugs tips can operate at temperatures in excess of 500°C when new. If oil finds its way down onto a spark plug tip, then this will quickly burn at this type of temperature and create a burnt oxidized coating on the spark plug tip. A burnt spark plug won’t operate efficiently and won’t generate a reliable spark.

The ECU will try to offset the underperformance of the spark plug by increasing the amount of fuel so that the same pressure can be created in the combustion chamber. This is why the fuel economy can plummet if one or more of the spark plugs has oil on it and is no longer working effectively.

Some studies have shown up to a 30% increase in fuel consumption due to bad spark plugs. That’s a huge increase, only comparable to bad fuel injectors

Symptom 3 – A decrease in engine performance

As explained above, a spark plug that comes into contact with engine oil won’t be able to generate a spark for very long. This is because the engine oil will contaminate the spark plug tip and prevent it from generating a spark that is capable of igniting the fuel/air mixture in the combustion chamber.

If the gas mixture in the combustion chamber is not burning efficiently or completely, then it won’t generate the level of pressure needed to push the piston with enough force when the engine requires it while under load. If you put your foot on the gas pedal expecting the car to accelerate, you will probably experience a significant lag in acceleration as the other spark plugs and the ECU try to provide the necessary power to the wheels.

Symptom 4 – Possible engine misfires

Another common symptom of oil on one or more spark plugs is engine misfires.

An engine misfire is caused by the incomplete combustion of the gas/air mixture inside one or more of an engine’s cylinders. This will feel like hesitation or shaking when you press down on the accelerator. Oil in the spark plug well or on the electrode tip of a spark plug will interfere with its ability to generate a spark that is hot enough to completely ignite the fuel/air mixture.

This will result in a lack of power from the cylinder piston and this can manifest as an engine misfire.

Symptom 5 – Engine Backfires

If the fuel in the combustion chamber is not burning up completely, then it will escape via the exhaust valves to the exhaust pipe.

An engine backfire is when the gas/air mixture in your combusts after it leaves the combustion chamber of an engine cylinder. If fuel particles don’t burn in the combustion chamber as they should they will enter the exhaust pipe with a high energy charge. That energy will be unleashed in the form of mini-explosions or backfires. 

Engine backfires are not good and will lead to damage to your car’s exhaust or intake if the cause is not fixed. It also leads to poor fuel economy and a drop in engine performance.

Symptom 6 – Gas smell from exhaust pipe

Another effect of gas not being burnt entirely in the combustion chamber is the smell of gas from the exhaust. Oil on spark plugs often cause a gas smell from exhaust fumes, especially when the engine is cold or straight after the engine is started.

The reason why is because at startup the engine will often use a slightly richer gas/air mix. This is because cold fuel is more difficult to vaporize so more fuel is needed to create a combustible air-fuel mixture. If there is oil on the spark plugs this will further compound the problem of cold fuel and will cause more fuel to be dumped directly into the exhaust causing a strong gas smell.

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 In 6 Steps

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. Below is a brief explanation of how to change them. Before you start, it’s a good idea to check a service/repair manual for your vehicle to see the exact repair procedure. Also make sure you have all of the correct parts and necessary tools before starting. I recommend using the correct torque wrench for tightening bolts during reassembly.

Step 1 – Preliminary Checks 

Ensure that the ignition switch is off and then disconnect the vehicle battery. Detaching the negative terminal is usually enough.

Step 2 – 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 will cause permanent damage to the cylinder head.

Step 3 – Remove The Spark Plugs

Carefully remove each of the spark plugs using a spark plug removal socket. Inspect each spark plug for oil leakage and general wear and tear as you remove them. If you are having trouble removing a spark plug because it is damaged or stripped, then read our article on how to remove a stripped spark plug for guidance.

Step 4 – Remove the old gasket and/or O-ring seal

Remove the old gasket, depending on the engine you may find it difficult to remove the old gasket if there was silicone applied during installation. Remove the O-ring seal from all cylinders.

Step 5 – 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.

Step 6 – 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. It’s very important to fit the correct spark plugs when after carrying out this repair. If you need help choosing the best spark plugs then check out our best spark plugs article.