Engine Oil Level Rising? Here Is How To Fix Fuel Dilution!

I have recently written about my personal experience with increasing engine oil levels and consequently, fuel dilution as it is commonly referred to. The increased engine oil levels are just a consequence of fuel dilution. You can check out that article and how I fixed these problems on my OM642 diesel engine HERE. Today, however, I want to do a deep dive on what fuel dilution is, why it happens, what are the symptoms of fuel dilution and how to fix it for good. I will also discuss what kind of engine oil you should use to best combat fuel dilution. 

This topic is currently gaining traction even according to Google Trends analytics. Wondering why? It is because most of the new cars on the market today, both diesel and petrol, come with direct fuel injection. We will explain how to direct fuel injection systems impact the possibility of fuel dilution later on, so stay with me!

What Is Fuel Dilution And Why Does It Happen?

Fuel dilution is a term for when diesel or petrol fuel leaks in the engine oil sump and dilutes the engine oil. Car owners usually recognize this issue as they are checking their engine oil level and see that their engine oil level is rising. The increase in engine oil level is due to the fact that engine oil has been diluted with fuel. 

Fuel dilution can occur in both diesel and petrol engines, especially in modern directly-injected petrol engines. If your oil level is too high and you also notice a smell of petrol in the engine oil, there is a good chance that your petrol engine has an oil dilution.

Why and how does it happen?

There are several ways in which fuel dilution can happen, especially in diesel engines. Here is a breakdown.

Petrol engine fuel dilution

In petrol engines, the most common cause of fuel dilution is something called “wet stacking”. Wet stacking is a process that occurs when the engine is not at its working temperature and it does not burn up all the fuel that is injected into the combustion chamber. The excess petrol fuel leaks past the piston rings, straight into the oil sump where it dilutes the engine oil. This is why it is important to always drive easy as your car is warming up to its optimum working temperature. Flooding the engine with fuel while it is cold is not something you should be doing. 

Another reason for fuel dilution in petrol engines can also be poor functioning and leaky fuel injectors or a leaky high-pressure fuel pump. Leaky fuel injectors can flood the combustion chamber with excess fuel which sticks to the cylinder walls and slowly seeps past the piston rings and into the engine oil. Direct-injection petrol engines operate at much higher pressure, making fuel dilution even more common than it used to be with port-injected petrol engines.

To avoid fuel dilution we also recommend following the regular service schedule and avoiding engine modifications that could also lead to increased engine oil levels.

Diesel engine fuel dilution

Just like petrol engines, diesel engines can suffer from fuel dilution that comes as a consequence of wet stacking and excessive idling when the engine is not warmed up. One of the common reasons is also leaky fuel injectors and poor maintenance. However,  additional components of modern diesel engines are also DPF filters or diesel particulate filters. These filters are made to capture all the harmful soot and particles that diesel engines blow out. Because this filter would clog pretty quickly without some sort of a regeneration program, diesel vehicles trigger a process called “DPF regeneration”. As the regeneration process happens, the engine will inject a larger amount of diesel engine into the combustion which will burn out in the exhaust system, and as it burns out, it also burns out all the foul particles in the DPF. 

Cars with a diesel engine and DPF that are mainly used for short trips can have an increased oil level because the DPF regeneration process gets cut short during those short trips. The use of biodiesel only exacerbates the problem. I recently wrote about this exact issue from my own experience on my OM642 Mercedes diesel engine and you can read all about my fuel dilution experience here.

Symptoms Of Fuel Dilution And Increased Engine Oil Levels

The worst thing you can do in regards to fuel dilution and increased engine oil levels is to ignore it or not notice it at all. As dangerous as it is, your car should run just fine for a while, even with heavily diluted engine oil (trust me, I’ve been there). To avoid this, be on the lookout for these symptoms.

Increased engine oil levels

The first and most common is the ever-increasing engine oil level. If you regularly check your engine oil, you should have a rough idea of the engine oil level in your car. By doing that, you should immediately detect if your car is either burning too much engine oil or if your engine oil level is increasing. If you see engine oil level above the max. mark and you are sure, that your engine was not overfilled with engine oil, then fuel dilution of engine oil is the only possible reason for it. If you know-how, we recommend getting rid of the excessive engine oil (either by draining it or sucking it out through a tube) and scheduling an appointment with your mechanic as soon as possible.

Engine oil smells like diesel or petrol fuel

Knowing how to clean engine oil should smell is a useful skill to have. Especially when you are dealing with fuel dilution. We are sure most of you reading this, have an idea what diesel or petrol smells like. If there is a strong odor of fuel as you check and smell the oil dipstick, it is very likely that your engine oil is mixed with fuel.

Your engine oil level fluctuates

All engines, even new or those in perfect condition burn a certain amount of engine oil. If there is a big percentage of fuel dilution, however, you might notice quick drops in engine oil levels. Quick drops of engine oil levels usually happen after a long journey on the highway which gives your engine the chance to burn off all the fuel that is diluting the engine oil. Check your engine oil before and after a long journey if you suspect fuel dilution problems.

White exhaust smoke

As we already mentioned, every engine out there burns a certain amount of engine oil. If you have fuel dilution problems it is entirely possible that a big cloud of white smoke is rolling behind your car as you accelerate. This happens when an engine burns off large or better said fails to burn off the excess amount of diesel fuel. The engine will do everything to burn off the excess fuel in both the combustion chamber and the oil sump which results in clouds of white exhaust smoke.

How Dangerous Is Fuel Dilution?

If the engine oil is diluted with diesel or petrol fuel, the viscosity of the oil drops – it becomes thinner and is no longer able to lubricate the engine, especially at higher temperatures. But it does not stop there, not only does the viscosity of the oil drop, the overall quality of the oil drops as well. There is an increased risk of acids forming, which can lead to corrosion inside the engine and long-term engine damage. Due to a lowered flash point of engine oil due to fuel dilution, there is also an increased risk of fire. Premature wear of internal engine parts (mainly the rod bearings and the crankshaft) and even a complete engine failure can be the result. The only safe countermeasure for drivers is to resolve the underlying reason for fuel dilution or to change the oil more frequently.

Should You Drive With Engine Oil Level Above Maximum?

No, you should not drive a car that exceeds the maximum oil level. Not only does that create additional pressure in the engine which puts stress on all the gaskets and seals, it can also lead to the crankshaft dipping into the oil which leads to foaming and frothing of the oil which can get sucked into the positive crankcase vent. This can lead to sudden oil loss, spark plug damage, and catalytic converter failure.

Whether you have overfilled your engine with oil or you have a fuel dilution problem that caused the engine oil level to rise above the maximum mark, you should absolutely not drive a car with too much oil.

How Much Fuel Dilution Is Too Much?

These values are normally determined by the manufacturer of the engine. There is however some 3rd party laboratory research by OelCheck, a German oil analysis company specializing in fuel and grease dilution. You can see their recommendations in the table below.

Type of fuelNormal fuel dilutionElevatedCritical
Petrol< 1%1 – 2.5%> 2,5%
Diesel< 3%3 – 5%> 5%

Unfortunately, car manufacturers do not make any precise statements as to the degree of dilution at which there is a risk to the engine. In general, experts assume that a degree of dilution of the engine oil of up to 10% does not pose a problem for the diesel engine. 

Does Fuel Dilution Only Happen With Diesel Engines?

No, fuel dilution is not something that only happens with diesel engines. Plenty of petrol engines, especially those with direct-fuel injection suffer from the same problems. One of the recent cases where a petrol engine had big issues with fuel dilution was actually Honda’s 1.5 liter turbocharged directly injected petrol engine, you can read more about it here.

How To Prevent Fuel Dilution And Engine Oil Level Increase?

The best way to get the oil dilution under control is to avoid just driving short distances – especially in winter. Only when the engine reaches its intended operating temperature does the fuel in the oil have a chance to evaporate. Checking the oil after long (and fast) drives can give clues as to whether an oil dilution problem exists. There are also other things you can do:

  • Avoid stop-and-go traffic if you can. If you can’t, make sure you replace your engine oil at a shorter interval.
  • Inspect your vehicle’s fuel injection system. Find an experienced professional to do the job.
  • Avoid big accelerations on short distances to avoid overflowing the engine with fuel.
  • Have your engine inspected. Let the mechanic inspect your valve guides, piston rings, and the state of other engine internals (this is the last resort).
  • Avoid using fuels with a high percentage of biofuels.
  • Avoid excessive idling and warming up of the engine.
  • Use high-quality full synthetic engine oils that handle fuel dilution better.
  • Replace engine oil at a regular interval. Adjust the interval if needed.
engine oil replacement

What Is The Best Engine Oil For Fuel Dilution?

To tackle this problem to the best of their ability, engine oil manufacturers started producing engine oils that better tackle the increasing occurrences of fuel dilution that came with the introduction of directly injected engines. 

The best engine oil to battle against the dreadful effects of fuel dilution is a high-quality fully synthetic engine oil with additives that are designed to withstand diluted oil.

How To Test For Fuel Dilution?

There are three ways you can identify fuel dilution problems. Two are at home and not a 100% reliable option, the third one however is based on laboratory tests that are reliable and proven.

Monitor your coolant and oil levels regularly

If you suspect there is fuel in your engine oil, because engine oil levels are clearly rising, first make sure that it is not the coolant fluid that is mixing with engine oil. Monitoring both the coolant and the engine oil level regularly do rule out all options but fuel dilution.

Smell your engine oil

This option is not as reliable but a good option for many people who have a good sense of how their engine oil should smell. If you detect a strong diesel or petrol scent on your engine oil dipstick, it is clear that you have fuel mixed with your engine oil. 

Send a sample of your engine oil to an oil lab

The third, the most reliable and the most expensive option that guarantees reliable results is lab testing. You can extract a sample of your engine oil and have it professionally analyzed in a laboratory setting. They will provide you with the exact % of fuel dilution which will give you the only reliable insight into what is going on with your engine. Read more about this process of oil analysis.


Fuel dilution is one of those silent killers that can without serious symptoms or none at all cause a complete engine failure. I have dealt with these problems myself and even though I had a severe case, my car was not showing any symptoms. If it was not for my regular oil checking, I would not have any idea at all. If you do not know how to check your engine oil, please see our guide on how to check your car’s fluids. Most owners usually fear that their engine is burning engine oil, boy, how times have changed.

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