What is Fuel Dilution and Why is it Bad?Posted by Amsoil, 5th November 2018

Fuel dilution occurs when petrol or diesel fuel infiltrate your engine’s crankcase and dilute, or “water down,” the car engine oil. 

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What causes fuel dilution?

A dirty or leaking fuel injector is one cause of fuel dilution.

A number of factors can cause fuel dilution, including…

Some applications are especially prone to fuel dilution. Police fleets, fire trucks and other emergency vehicles are great examples. Common conditions include short bursts of driving and long periods of idling. Delivery trucks, too, can suffer from fuel dilution given the constant stop-and-go conditions that prevent the oil from warming enough to evaporate any accumulated fuel.

Petrol direct injection can be another cause of fuel dilution. Petrol direct injection technology locates the fuel injectors directly in the cylinder as opposed to the manifold. This arrangement allows for greater control over the injection event. This allows engineers to fine-tune engines for greater efficiency and power. That’s one reason Petrol direct injection is expected to be a staple of most new vehicles built by 2020.

One side-effect, however, is fuel dilution. As fuel is sprayed into the combustion chamber, it can wash past the rings and down the cylinder walls, into the oil sump.

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How much is too much?

In a perfect world, we prefer no fuel mixes with the car engine oil. But life has a way of interfering with perfection. With that in mind, the following cautionary limits are acceptable provided car engine oil viscosity has not significantly changed. Fuel dilution that surpasses these guidelines requires attention.

Acceptable fuel dilution limits


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Why is fuel dilution a problem?

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What if I suspect fuel dilution?

First, ensure there’s really a problem. Although it’s possible to smell fuel in a car engine oil sample, oil analysis is the most accurate method of determining the severity of fuel dilution. If your oil analysis report reveals abnormal fuel dilution, the next step is to determine the cause.

There’s an old adage when troubleshooting: start with the easiest and least expensive fix. In this case, try a fuel-system cleaner, such as P.i.® Performance Improver or Diesel Injector Clean.

Sometimes a piece of debris can lodge in an injector tip and prevent it from closing, allowing fuel to trickle into the cylinder and wash into the crankcase. A fuel-system cleaner may be all that’s required to solve the problem.

If that doesn’t work, it’s likely time to visit a garage.

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Adjust driving conditions

Sometimes, driving conditions are to blame for fuel dilution. In this case, there isn’t much you can do except change how you drive, if possible.

If fuel dilution is a problem, use a high-quality fully synthetic oil to guard against accelerated wear. (You should do this anyway.) Although oil film thickness is related to viscosity – and excessive fuel dilution reduces viscosity regardless of oil quality – film strength is a function of base oil and additive quality. AMSOIL fully synthetic oils use top-grade base oils and additives to help protect components from wear. Find out how to switch from conventional to synthetic oil.

Finally, make sure you change oil according to the proper recommendations. Vehicles prone to fuel dilution typically operate under severe service, so follow the severe-service guidelines in your owner’s manual or on the car engine oil label.

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