Gasoline Engine Fuel Dilution Trends

I’ve been getting many questions lately regarding fuel dilution in automotive applications particularly related to gasoline engines.

What is fuel dilution?
To put it plainly, fuel dilution is the percentage at which fuel has diluted engine oil, typically reported as a percentage. Under certain operating conditions, fuel may leak past the piston rings and mix with engine oil.

What does fuel dilution do?
Fuel dilution, when mixed with engine oil, has two basic effects. The first is the immediate thinning of the oil. This results in a drop in viscosity. When not designed for, the drop in viscosity reduces the film strength of the oil in question and compromises its ability to protect engine internals, particularly under high stress conditions. Second is the long-term effect of varnish on engine oil internals.

What causes fuel dilution?
There are a number of root causes for fuel dilution. The two most common are the presence of direct injection (particularly when turbocharged), and long idle times. Frequent short trips may also contribute the way long idle times would, especially in the winter.

It is important to understand the difference between port injection and direct injection as it pertains to fuel atomization. Always keep in context the position of the crank relative to TDC and the “time” it takes for the piston to reach TDC. Normally, a port injected engine will spray fuel into the intake ports as the piston is performing its intake stroke. This allows nearly 360 degrees of rotation during which the fuel can vaporize. Generally speaking, automotive engine oils don’t burn liquid fuel; it has to vaporize in order to burn. In a port injected engine, we have 180 degrees of crank rotation between TDC and the end of the intake stroke, when the valves close. Then, we have another 180 degrees of crank rotation (compression stroke) between the end of the intake stroke and TDC minus whatever our spark advance is for that particular combustion; typically between -40 degrees before TDC and 10 degrees after TDC. That’s a long time for the fuel to atomize.

In the case of a direct injected engine, the fuel is sprayed directly into the combustion chamber during the compression stroke or sometimes even after the piston has reached TDC. As a result, the fuel may not have time to completely vaporize. Furthermore, spraying liquid fuel on a compression stroke, will result in the cylinder walls being “washed” by the fuel, and the fuel seeping past piston rings and mixing with the oil.

In port injected engines, we also have fuel dilution during long idle times due to the engine’s low operating temperature (particularly during the winter, when the engine may take much longer to reach full operating temperature).

A Technical Evaluation
More than likely, if you found this article, you already know most of what I said above, and this is the section you came here to read. I want to make clear, what I’m describing here is my expert analysis and is based on many years of oil analysis testing, review, interpretation, and tribology research. If any of this information becomes outdated (or incorrect), please contact me so I can make the appropriate revisions.

First and foremost, fuel dilution does not increase linearly with miles. I’ve seen it discussed on many occasions that due to fuel dilution, people need to shorten their drain intervals. This is only partially accurate, if you’re using a low quality oil with additives that are unable to address the acidic effects of fuel dilution for the duration of the OEM-specified service interval. In general, however, it is a mislead conclusion, as you really can’t escape fuel dilution in direct injected turbo engines. While some OEMs are able to better tune their engines to reduce fuel dilution under normal operating conditions, others still allow a substantial amount of fuel and simply factor that into the engine’s bearing design with respect to viscosity shear. Back to my primary point though, fuel dilution does not increase linearly with miles. In other words, if your oil analysis report shows 3% fuel dilution at 4,000 miles, that doesn’t mean you’ll have 6% fuel dilution at 8,000 miles.

You have to understand that fuel dilution can increase or decrease as a function of a environmental conditions. There are contributors such as those described above, but there are also subtracters. These subtracters include long trip durations, normal or high operating temperatures, and light load driving. Fuel is what’s known as a low pressure vapor liquid; in other words, it vaporizes under low pressure. It does this in pure form, but also does this when mixed with engine oil. The fuel will vaporize from the engine oil through the PCV system to be burned again by the combustion process.

To tie this all together, the balance of contributors and subtracters for fuel dilution will a point of equilibrium. There is a point at which the rate that fuel is added to the oil meets the rate at which fuel is removed from the oil, based on a specific set of driving conditions. As you can guess, once you reach this equilibrium point, fuel dilution will not increase or decrease by a consequential amount assuming your driving conditions and environment remain constant. If you go from summer to winter, however, your warm-up times increase, and so may your fuel dilution as well. Inversely, going from winter to summer will show a reduction in fuel dilution.

Regarding Oil Change Intervals
In most cars, I’ve noticed that the point of fuel dilution equilibrium is reached within 1,000 miles, if not sooner! If we were to use fuel dilution as a metric for oil service intervals, we would be changing our oil at prohibitively short intervals. See below, an oil analysis showing 4.4% fuel dilution in a direct injected 2.0L FA20DIT Subaru engine (2015 Subaru WRX). Note also that fuel dilution is higher at 950 miles than it was under the previous oil analysis report at 4133 miles (3.7%).

Oil Analysis Fuel Dilution
Oil Analysis Fuel Dilution


If you’d like to get your oil tested for fuel dilution, I strongly recommend using Oil Analyzers Inc. They are a private label for Polaris Labs, which uses Gas Chromatography to actually test for fuel dilution accurately. Polaris Labs is an ASTM certified lab.

If you have any other lubrication topics you’d like me to write about, please let me know!