A noticeable smell of petrol in the oil – the problem may be a leaking

Sometimes it's enough to unscrew the oil filler cap or check the dipstick and a strange impression appears – the oil smells of petrol. Many drivers ignore this signal at first. The engine works, after all, the car drives, so it's easy to think it's a trifle.

The problem is that fuel in the engine oil is never a normal phenomenon. If petrol starts mixing with the oil, it means a fault has appeared somewhere in the engine's operation. Sometimes a small one, sometimes already a serious one.

In practice it looks like this: unburnt fuel ends up in the crankcase and thins the engine oil. And that means one thing – the engine's lubrication stops working the way it should. And that's when the real problems begin.

If you've ever thought: “something isn't right, the oil smells of fuel” – it's good that you noticed. This is one of those symptoms that really aren't worth ignoring.

When is a smell of petrol in the oil a warning signal – how do you recognise that fuel is mixing with the engine oil?

If you can smell petrol in the oil, it most often means that unburnt fuel from the cylinders is getting into the lubrication system. This isn't a normal situation – in a properly working engine fuel and oil should never mix with one another.

The most common symptoms confirming the presence of fuel in the oil are fairly characteristic:

the oil smells of petrol when you pull out the dipstick,
the oil becomes thinner than usual,
the oil level on the dipstick starts to rise, even though none has been added,
the engine runs less stably, especially from cold,
fuel consumption starts to rise clearly.

In practice many drivers notice it by accident. While checking the oil level it turns out that the oil has an unusual smell and a more fluid consistency. That's a sign that its lubricating properties are starting to deteriorate.

And here an important thing appears. Engine oil is responsible for lubricating all the moving components of the engine – the bearing shells, the pistons, the crankshaft and the turbocharger. If it is thinned with fuel, the oil film becomes considerably weaker, and the engine's components start working in worse conditions.

That's why even a slight smell of petrol in the oil shouldn't be ignored. In many cases it's the first signal that something is starting to happen with the injection or the ignition system.

Petrol in the oil – the causes that most often lie behind this problem in petrol engines

Most often petrol gets into the oil when the fuel isn't completely burned in the cylinder and starts getting past the piston rings into the crankcase. In a well-functioning engine combustion is almost complete, but when a fault appears, some of the fuel starts to “escape” down into the engine.

The most common causes of this problem are:

leaking or “pouring” petrol injectors,
problems with the ignition system, e.g. worn spark plugs or coils,
a mixture that is too rich,
worn piston rings,
failures of the sensors controlling the engine's operation.

In practice it looks like this: the injector delivers fuel into the cylinder, but it isn't fully burned. Part of the fuel settles on the cylinder walls and starts running down into the engine. If the piston rings are no longer perfectly sealed, the fuel gets directly into the engine oil.

Interestingly, this problem appears more often in modern petrol engines with direct fuel injection. In such units the injectors work under high pressure, and fuel is delivered directly into the combustion chamber. When a small fault appears, unburnt fuel finds its way into the crankcase more easily.

Many drivers don't realise that fuel in the oil can appear even with a minor injector fault. That's exactly why diagnostics of the fuel system and its regular cleaning are so important.

A slight smell of petrol in the oil – is that already a problem, or still a normal phenomenon in the engine's operation?

A slight smell of fuel in the oil may appear occasionally, but if it persists for a longer time, it usually signals the beginning of problems with the engine's operation. In modern engines a small amount of fuel can get into the oil, e.g. during short trips, but this shouldn't be a permanent phenomenon.

It most often happens in situations such as:

frequently starting a cold engine,
driving over very short distances,
an under-warmed engine for most of its running time,
unburnt fuel during starting,
a mixture that is too rich on a cold start.

If the car is used mainly on short journeys – a few kilometres to work or to the shop – the engine often doesn't reach full operating temperature. In such a situation the fuel delivered to the cylinders isn't always burned completely.

Then some of the petrol can get into the oil. The problem is that the oil then has no time to evaporate the fuel, which normally disappears during a longer drive.

If, however, you notice that the oil clearly smells of petrol and its level starts to rise, that's a sign the problem is bigger. It's then worth checking the injection and ignition systems, because in practice it often turns out that the culprit is a single faulty injector or dirty injector tips.

You can smell petrol in the oil – how does unburnt fuel get from the cylinder into the lubrication system?

If you can smell petrol in the oil, it means that unburnt fuel is getting from the combustion chamber past the piston rings into the crankcase. This is a mechanical phenomenon and is directly related to the movement of the piston, the pressure in the cylinder and the sealing of the engine's components.

It most often looks like this:

fuel reaches the cylinder through the injector,
part of the mixture isn't burned,
petrol settles on the cylinder walls,
it runs down along with the movement of the piston,
it gets past the piston rings into the oil.

The piston rings are responsible for sealing the cylinder and separating the combustion chamber from the engine oil. When they are worn, or when an excess of fuel appears in the cylinder, their effectiveness starts to drop.

Then a small amount of fuel starts getting into the crankcase. If the problem lasts longer, the oil gradually mixes with the petrol, and its lubricating properties start to deteriorate.

If the oil stinks of petrol, that's a sign that something abnormal is going on in the cylinders. And that is precisely why diagnostics of the injectors and of the ignition system are so important.

Ignition problems and unburnt fuel – why does a failure of the spark plugs or coils cause fuel in the oil?

One of the more common causes of fuel in the oil are problems with the ignition system, which mean the air-fuel mixture isn't burned completely. If the ignition spark is too weak or appears at the wrong moment, part of the fuel stays in the cylinder.

The faults most often responsible for this are:

worn spark plugs,
damaged ignition coils,
problems with the ignition leads,
failures of the engine control unit,
incorrect ignition timing.

In such a situation the fuel reaches the cylinder, but isn't fully burned. The unburnt petrol starts settling on the cylinder walls, and part of it makes its way down into the engine.

Over time this leads to a situation in which the oil starts to smell of fuel, and its lubricating properties gradually deteriorate. In many cases the driver then also notices other symptoms – rough engine running, difficulty starting or a drop in power.

That's why, if a smell of fuel in the oil appears, it's worth checking not only the injectors, but also the spark plugs and ignition coils. Sometimes replacing them is enough for the problem to disappear completely. In other cases a thorough cleaning of the fuel system and the injectors is needed to restore correct combustion of the mixture.

A mixture that is too rich – how can the engine sensors lead to fuel in the engine oil?

An air-fuel mixture that is too rich means that more fuel reaches the cylinders than the engine is able to burn, and its excess starts getting into the engine oil. This is one of the more common causes of the situation in which, after checking the dipstick, it turns out that the oil smells of petrol and its consistency is clearly thinner.

Most often, too large a dose of fuel is the fault of the sensors controlling the engine's operation, e.g.:

the engine temperature sensor, which wrongly tells the ECU the engine is cold,
the lambda sensor, responsible for the composition of the air-fuel mixture,
the mass air flow sensor (MAF), which determines the amount of air in the intake,
the manifold absolute pressure sensor (MAP), which influences the fuel dose,
the engine control unit, which selects the injection parameters on the basis of this data.

The failure of one small sensor can change the operation of the whole engine. In the longer term this leads not only to fuel in the oil, but also to higher fuel consumption, unstable running of the engine and the build-up of carbon deposits in the combustion chamber.

Leaking petrol injectors – the most common reason why the oil smells of petrol

Most often the situation in which the oil stinks of petrol is caused by leaking fuel injectors. In a properly working engine the injector atomises fuel into a very fine mist, thanks to which the air-fuel mixture burns almost completely.

When, however, an injector starts to wear or becomes dirty, problems appear such as:

incorrect fuel atomisation,
an excessive dose of fuel in the cylinder,
petrol running down the cylinder walls,
unburnt fuel getting into the engine oil,
rough engine running and a drop in power.

The injector, instead of atomising the fuel, starts delivering it in larger droplets or even “pouring” it into the cylinder. The engine isn't able to burn such a quantity of fuel, so its excess starts getting into the oil.

The most common cause of this state of affairs are deposits, carbon build-up and contaminants settling on the injector tips. In modern petrol engines, especially with direct injection, the holes atomising the fuel have a diameter of just a few dozen micrometres. Even slight contamination can change the way the fuel is delivered.

That's why regular cleaning of the fuel system is so important. Products used in the injection system, such as TEC 2000 Fuel Injector Cleaner, help remove carbon deposits from the injector tips, improve fuel atomisation and restore correct engine running. In many cases it is precisely such cleaning that makes it possible to stop the problem before it leads to a more serious failure.

The oil stinks of petrol – can you drive with such a problem, and what can the consequences be for the engine?

If the oil stinks of petrol, driving the car is possible, but it is definitely not safe for the engine. Fuel thins the engine oil, and that means the oil film protecting the engine's components becomes considerably weaker.

The consequences of driving with fuel in the oil can be serious:

a drop in the oil's lubricating properties,
increased friction of the engine's components,
faster wear of the bearing shells and the crankshaft,
a risk of the engine overheating,
the possibility of the engine seizing in extreme cases.

The problem develops gradually. At the beginning only a slight smell of petrol in the oil appears, later the oil starts becoming thinner and thinner. If the situation lasts longer, the engine's components start working in worse lubrication conditions.

Then further symptoms appear – rough engine running, a drop in power, higher fuel consumption or difficulty starting the engine.

The worst-case scenario is driving with fuel in the oil for a long time. In such cases serious wear of the engine's components can occur, and the repair often means reconditioning the engine or a very costly rebuild.

That's why, if you notice that the oil smells of petrol, it's best to diagnose the cause of the problem as soon as possible.

How do you remove fuel from the oil and stop the problem – what can you do before the engine is damaged?

If fuel appears in the oil, the most important thing is to remove the cause of the problem quickly and to change the engine oil. Simply topping up with new oil will achieve nothing – if the fault remains, the fuel will start mixing with it again.

The first diagnostic steps most often look like this:

checking how the injectors are working,
inspecting the spark plugs and ignition coils,
diagnostics of the engine sensors,
checking the compression in the cylinders,
changing the oil and the oil filter.

In many cases the problem turns out to be dirty injector tips, which don't atomise the fuel properly. Then a thorough cleaning of the fuel system helps.

Fuel system cleaning products, such as TEC 2000 Fuel System Cleaner (FSC), can make a big difference, especially when the engine starts running unevenly, responds worse to the throttle or higher fuel consumption appears. Agents of this type help dissolve carbon deposits, remove moisture and clean the deposits that collect throughout the fuel system.

This isn't only about the injectors themselves. A good product cleans practically the whole system – from the tank and the fuel lines, through the pump and the fuel rail, all the way to the injectors and the combustion chamber. Thanks to this the fuel is atomised better, the engine runs more smoothly and returns to its normal dynamics more easily. Regular use of such products also often helps limit the problem of jerking, poorer starting or a drop in performance.

After the cause of the problem has been removed, changing the engine oil is also necessary, because oil thinned with fuel won't regain its lubricating properties.

How do you prevent the oil smelling of petrol – prevention for the injectors and the fuel system

The best way to avoid the problem of fuel in the oil is regular prevention for the fuel and ignition systems. Most of the faults that lead to such a situation develop slowly and for a long time give only slight symptoms.

The most important preventive measures are:

regular engine oil changes,
checking the spark plugs and coils,
filling up with good-quality fuel,
periodic cleaning of the fuel system,
checking how the engine is running during servicing.

In practice mechanics often recommend preventive cleaning of the injection system around every 10,000–15,000 kilometres. Thanks to this the injector tips stay clean, and the fuel is properly atomised in the cylinder.

Regular prevention means that the injectors work properly for longer, and the risk of a situation in which the oil starts to smell of petrol is considerably lower.

FAQ

Can you drive when there is fuel in the oil

You can, but it isn't safe for the engine. Fuel thins the oil, which means its lubricating properties drop significantly. Driving for longer in such conditions can lead to accelerated wear of the engine's components.

Is a slight smell of petrol in the oil normal

A slight smell of fuel may appear occasionally, e.g. during frequent short trips. If, however, the smell persists for a longer time, it may mean a problem with the injectors, the ignition or the fuel mixture.

Does fuel in the oil increase fuel consumption

Yes. If fuel gets into the oil, it usually means the air-fuel mixture isn't burning properly, which leads to greater fuel consumption and a drop in engine efficiency.

How do you check whether there is fuel in the oil

The simplest way is to check the smell and consistency of the oil on the dipstick. If the oil is clearly thinner than usual and smells of petrol, there is a high probability that fuel is getting into the lubrication system.

How often should you clean petrol injectors

Preventive cleaning of the fuel system is recommended around every 10,000–15,000 kilometres. Regular use of cleaning products helps maintain correct operation of the injectors and limit the risk of problems with fuel in the oil.

A noticeable smell of petrol in the oil – the problem may be a leaking petrol injector!