If your oil smells like gas, there’s probably a pretty severe problem. It’s likely to be something to do with an overly rich fuel mixture, broken piston ring(s), or a problem with the PCV valve.
Oil and gasoline shouldn’t mix. It’s the oil’s job to lubricate the engine and, to some degree, keep it cool and clean it. Gas is quite simply there to explode and thus drive the motor.
The oil comes from the sump, driven by the pump, through various pipes and channels until it ends up back where it started.
In contrast, the gas is pumped by the fuel pump from the tank, down the lines, and into the combustion chambers through either the carburetor or fuel injectors.
The two should never mix beyond a small amount. Oil should never smell like gas. It should smell like oil.
In this article, I’ll explain what might cause this issue. And, most importantly, what you should do next.
Why Does My Oil Smell Like Gas?
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Why Does My Oil Smell Like Gas?
It’s most likely that you’ll notice this problem either when the engine is off (and, thus, there’s no fuel lingering in the air) when you use the dipstick or during an oil change.
When checking the motor oil level using the dipstick, smell it. If the smell of gas is noticeable, it’s best to pay attention. Another good thing to watch out for is how fast the liquids move down the dipstick. Gas is less viscous than oil, and so it’ll run slightly quicker.
When you’re performing an oil change, it’s always worth giving the oil a quick whiff, among other checks—nothing too heavy, mind. If there’s a definite, pungent smell of gas, this article will be particularly relevant to you.
If your motor oil smells of gas, then it’s highly likely that gas has somehow got into the oil system, ending up in the sump. In technical terms, this isn’t good. The oil and the gas should never mix in a properly working engine.
There are a few different places where oil and gas can jump ships, as it were.
What Are Piston Rings?
Here are the basics of how an engine works.
Piston rings sit, you’ll be surprised to hear, on the pistons. Their main job is to seal the piston against the cylinder wall. This causes a pressurized area above the piston, leading to optimal combustion.
Although they’re seldom talked about in day-to-day scenarios, the piston rings play a crucial role. Indeed, when they stop working, it leads to significant issues because the cylinder can’t hold pressure. You’ll get a misfire as a result.
Here’s Engineering Explained to take you through piston rings in more detail. I highly recommend a watch.
For the purposes of this article today, the piston rings keep the oil and gas separate. The oil in the crankcase is below it. The gas should be in the combustion chamber above it.
When gas or oil gets past the piston rings, this is known as blow-by.
A small amount of gas ends up in the oil in most engines, but we’re talking negligible amounts. Vapors rather than liquids. The temperature of the oil should be high enough to vaporize the fuel instantly. It then should get pulled back into the intake through the PCV system.
When more significant amounts of gas start getting through, there’s a more serious problem.
Does Gas Often Mix With The Oil?
When you’re driving, gas and oil are usually mixing in virtually microscopic quantities. The PCV system (to be explained in more detail shortly) is in place to deal with this kind of thing.
When it’s in small quantities, gas getting into the oil is nothing to worry about. It’s expected, actually.
The opposite is also true. Most engines are expected to burn oil as you drive, as it’s virtually impossible to seal all tiny leaks within an engine. For example, in the owner’s manual of my 2005 Vauxhall Corsa C, I’m told that the car is likely to burn 0.6 liters of oil per 1,000 miles. Since it only holds 3.5 liters when full, you can see the importance of keeping your oil topped up.
(In reality, I suspect it’s not quite that high, but it’s still good to know.)
Going back to gas mixing with oil – it’s particularly bad because the gasoline turns the oil into sludge. The sludge then clogs up passageways in precisely the same way as plaque in arteries, causing rust and generally shutting the engine down.
You can get motor oils that are more resistant to sludge, but none are entirely so. If too much gas gets into the oil, it could still turn to sludge. Thus, you should take as many steps as possible to prevent it.
What Is The PCV System?
As mentioned before, in this section, we’ll discuss the PCV valve.
The Positive Crankcase Ventilation valve controls the blow-by gases that make their way into the crankcase. It functions to remove the gases and send them back into the engine through the intake.
If these gases – made up of approximately 70% hydrocarbons (unburnt gasoline) – don’t get removed from the crankcase, they’ll lead to sludge. If the engine runs at high speeds, they can also cause increased pressure buildup within the crankcase, leading to blown gaskets.
In the past, the engine simply ventilated off the blow-by gases into the atmosphere. However, in the US, laws started coming in about 1964, meaning that you now had to recapture these gases, improving fuel economy and decreasing environmental pollutants.
The result was the PCV valve.
It’s a one-way valve. The blow-by gases can leave through it, but they can’t return the same way. From here, the gases are sent back through the combustion chamber again to be burnt. This process is continual.
For efficiency, the PCV valve also has a breather tube, sending filtered air into the crankcase, encouraging circulation. The air is either filtered by the engine’s air filter or the breather element, specifically for the PCV valve.
When you perform regular maintenance, the PCV valve is often overlooked. But it’s a vital part of your car and needs to be kept clean and replaced regularly. Looking after this may save you more expensive problems in the future.
Potential Causes: Oil Smells Like Gas
Now that we’ve seen why the oil smells like gas let’s look at what might be causing it.
In reality, the problem might be more than one of the below reasons. Usually, there’s a root cause.
It might be a good idea to use an OBD II code reader. These can read the fault codes as designed by the manufacturer. Depending on the car, it may even highlight the exact point the issue is coming from.
General Causes Of “Too Much Fuel” – Oil Smells Like Gas
In this section, I’ll split it into subcategories, each addressing a different reason as to why there may be too much fuel going into the combustion chamber. If this was to happen, it’s more likely that more significant amounts of gas could get into the crankcase.
In technical terms, this is known as a rich fuel mixture: the ratio of air to fuel is too low. You could also think of it the other way – there’s more fuel, compared to air, than there should be.
ECU Causing Rich Fuel Mixture
When you’re driving, your car’s ECU is constantly varying the air:fuel ratio of what’s going into your engine. At times, it’s more relevant to have lean fuel – at times, rich.
For example, when you start the engine on a cold morning, you’ll smell gas. That’s because the fuel mixture is richer, allowing for an easier start.
In a standard gasoline engine, the air:fuel ratio should be approximately 14.7:1 (in terms of weight). That is, to burn 1kg of gasoline as efficiently as possible, you’ll need 14.7kg of air.
The ECU uses data from many different sensors to determine how rich the mixture should be. The issues could lie with any of the following:
- Oxygen (lambda) sensor.
- MAF sensor.
- MAP sensor.
- Engine coolant temperature sensor.
- Air intake temperature sensor.
- Fuel pressure regulator.
This list isn’t exhaustive, too.
An OBD II code reader should give you the answers you need. Otherwise, you may have to do some good, old-fashioned, manual diagnosis.
Okay, “misfire” is a fairly generic term. It refers to an incomplete power stroke within the cylinder, for whatever reason. Therefore, the engine has less power output.
When a cylinder goes through a misfire, the fuel within it doesn’t get burnt. At least, not fully. Because of this, it has the potential to slip past the piston rings and end up in the oil.
As a result of this, you might notice the smell of gas within the oil.
In this case, the fuel injectors would be sending too much gas into the combustion chamber. This would, like the previously discussed section, lead to a rich fuel mixture. However, the fault would lie with the injectors rather than engine sensors and, consequently, the ECU’s expectations.
Fuel injectors use solenoids to time when to release the spray of fuel, controlled by the ECU.
Here’s another enthusiastic video from Donut Media to explain how fuel injectors work in more detail.
If there’s a problem with the fuel injector, it may be sending too much fuel into the cylinder. If there’s too much fuel, it won’t all be burnt when the spark plug fires. This increases the likelihood of some gas getting past the piston rings.
From there, it ends up in the oil.
Piston Rings – Oil Smells Like Gas
A few sections ago, I explained what piston rings are and how they function. Scroll back up the page again if you’d like to, and be sure to check out the video from Engineering Explained if you missed it.
Piston rings can become faulty. They can wear down, fray, and even completely snap.
The result? It isn’t good. The area above the piston now can’t hold pressure as effectively, meaning the engine is less efficient. There’s also space down the sides of the piston for gasoline vapors to escape into the crankcase. The reverse is also true – oil can splash up into the cylinder.
The excess fuel in the crankcase could lead to the smell of gas in your oil.
Here are a few tell-tale signs of blown piston rings.
- Smoky exhaust gases – either gray or blue. These colors of exhaust smoke indicate that you’re burning oil. The oil splashes up past where the piston ring should be sealing the chamber. It then gets burnt along with the fuel.
- More oil consumption than you’d expect – as I mentioned earlier, all cars burn oil to some degree. However, if you have broken piston rings, the car will burn oil excessively. See point 1.
- General poor performance, especially under acceleration – since the cylinder with the faulty piston ring isn’t sealing the pressure properly, the engine won’t work as well. You’ll feel a noticeable drop in performance. When you put your foot down, the car might feel sluggish and heavy. It certainly won’t have the capability of accelerating quickly. Note: if you experience this symptom but not the others, you’re probably experiencing a misfire.
PCV Valve – Oil Smells Like Gas
Over time, PCV valves become clogged up. After all, they spend all their time pulling fuel vapors back to the combustion chamber.
You should change them every once in a while and keep them clean.
If the valve stops working, the gas vapors won’t be being removed from the engine. This could lead to a buildup within the oil, and, eventually, you’ll be able to smell it.
Here are a few maintenance tips.
A few symptoms of a faulty PCV valve depend on whether the valve is stuck open or closed.
If the valve is stuck open, you might see…
- A lean fuel mixture, resulting in low power, resulting in misfires at idle and generally low power.
- Some engine oil in the valve or hose.
- A hard engine start.
- Excessive consumption of oil.
- Black smoke.
- Spark plugs contaminated with oil.
When the valve gets stuck closed, that’s more relevant to the theme of this article – why the oil smells of gas. If the PCV valve gets stuck closed, expect symptoms that represent more pressure than there should be within the crankcase. These might include…
- A buildup of pressure and temperature within the engine (above the norm).
- Seals and gaskets failing.
- Oil leaks.
- Sounds such as whistling or moaning.
- The motor might surge unexpectedly.
- Problems with sensors such as the oxygen/lambda or MAF sensors.
If you notice any of these symptoms, you should check your PCV valve and perform an OBD II check. Shake your PCV valve. If you can’t hear it rattling (opening and closing), it means it’s broken and will need replacing. The part alone is likely only to cost about $10 to $15. Factoring in labor costs, you’d probably cough out about $50 in total if you took the car to a shop.
What’s The Big Deal If My Oil Has Been Contaminated With Gas?
If your oil smells of gas, it’s probably got gas in it. Although the lists we’ve just been through don’t go through every single possible cause, it’s good to be getting started with.
If you have gas in your oil, it’s probably not going to be an immediate problem. All engines are expected to have a small amount of gas in the oil, as we’ve seen.
As the ratio of gas:oil builds up, though, you might start noticing problems. These are all related to the pressure within the engine and the state of the oil.
Sludge will begin to develop. It doesn’t look nice, but there’s much more to it than that. The sludge won’t lubricate your engine properly. It’ll also get in the way of all the good oil and make it harder for it to get around the engine. Things will begin to rust internally. It’s really not good. Over time, it can cause severe damage.
You can also end up with pressure buildup, causing internal damage. The sludge could cause this pressure, or if you have a PCV valve that’s stuck closed, that could. More pressure equals higher stresses on the engine components, leading to further, more expensive problems.
How Do I Avoid All These Problems?
Looking after your car properly will, except in unfortunate cases, generally mean it performs better. It’s also key to realize that your car will develop problems at some point. It’s just inevitable. The most important thing to do here is simply to recognize it quickly and act on it as soon as possible. The longer you leave it, the bigger the problem is likely to become.
When it comes to gas getting into your oil, it could be a quick fix, such as replacing a PCV valve or spark plug. However, it could also be more complex – something like piston ring failure or a problem with your ECU.
There’s often more than one issue at play, as I said at the beginning. For example, yes, there’s a misfire, but why is there a misfire? Is it the spark plug or the fuel injector or the ECU, or a sensor? Has that misfire caused another problem somewhere else? Or was that misfire caused by something else, like oil getting into the intake and caking the spark plug?
With cars, you often have to dig deeper to find the true cause of the issue. Otherwise, you’re treating the symptoms rather than the actual problem.
Where Do I Start?
The best place to start, in my opinion, is with an OBD II check. Take your car into your local shop and ask for a diagnosis. Doing this might throw the answer straight back at you. If not, you could consider trying to work out the problem yourself or paying a professional to do it for you.
Suppose you decide to leave your car with a trusted mechanic. In that case, I’d recommend agreeing to a reasonable price before doing so. With diagnosis, it’s complicated. It can take a very long time to find out what’s wrong and, whatever you go for, you’ll have to pay the labor rate for as long as it takes. You might decide on a flat rate and ask the mechanic to carry out a series of set tests. If those tests don’t yield the answer, you could choose to drop it at that point or pay for more extensive testing.
If you decide to work on the car by yourself, fair play to you, but it’s going to take a while. Probably. The reason for this is that you might not know if you’ve fixed the problem until you’ve driven the car for a while, with the smell of gas coming back into the oil over time.
I’d recommend getting familiar with a multimeter. When it comes to modern cars, this is an invaluable piece of kit and could save you a lot of money.
One by one, test the components: all the sensors, the fuel injectors, the spark plugs, and so on.
If they all seem fine, move onto the PCV valve, checking all the parts of that.
Once you’ve checked the PCV system – and if everything seems fine with it – only then should you move onto the piston rings. Checking these will require dismantling the engine, and so you should only attempt this if you’re familiar with them. Unfortunately, it’s reasonably likely that the piston rings could be the root cause.
Whatever the problem ends up being, make sure you do a complete oil and filter change before driving the car again.
This article has been an introduction as to why your car’s motor oil might smell of gas.
As you might have seen, a few different things might be leading to this, and it might need work from a professional mechanic to be diagnosed. There’s no real way to put a cost to this until you know what the problem is precisely.
I hope this article has been helpful – let me know how you get on in the comments.