Fuel Dilution Can Wreck Havoc On An Engine

Gasoline and diesel fuel are great solvents.

Their solvency isn’t so great when gas or diesel fuels infiltrates your engine’s crankcase and contaminates the motor oil, a condition known as fuel dilution.

Of course, vehicle owners prefer no fuel mixes with the motor oil. Fuel finds a way to mix with the oil. With that in mind, a cautionary limit of up to 2.4 percent fuel dilution in gas engines and 3.4 percent in diesel engines may be acceptable, provided viscosity has not significantly changed. Fuel dilution that surpasses these guidelines requires attention.

A number of factors can cause fuel dilution, including the following:

Leaking fuel injectors
Excessive idle time
Incomplete combustion
Low engine temperatures
Frequent short-trip driving
Worn piston rings/excessive blow-by

Some applications are especially prone to fuel dilution given their operating conditions. Police fleets, fire trucks and other emergency vehicles are great examples. Common conditions include periods of idling punctuated by short bursts of driving. 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.

Gasoline direct injection (GDI) can be another cause of fuel dilution. GDI technology locates the fuel injectors directly in the cylinder as opposed to the manifold. This arrangement allows for greater control over the injection event, allowing engineers to fine-tune engine for greater efficiency and power. The added efficiency is the reason GDI is expected to be a staple of most new vehicles built by 2020.

One side-effect 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.

However it occurs, fuel dilution can cause a number of problems.

Reduced oil viscosity interferes with formation of a durable lubricating film, inviting wear. Combustion zone parts are especially prone to wear, including the piston, rings and liner. Reduced viscosity also negatively affects the oil’s ability to function as a hydraulic fluid, which is critical in engines with variable valve timing.
Fuel can wash oil from the cylinder wall, causing higher rates of ring, piston and cylinder wear.
Reduced effectiveness of detergency additives limits the oil’s ability to guard against deposits.
Increased oil volatility results in higher oil consumption, requiring more frequent top-offs.
Accelerated oxidation reduces the oil’s service life and requires more frequent oil changes.

It is sometimes possible to smell fuel in a motor oil sample; oil analysis is the most accurate method of determining the severity of fuel dilution. Visit www.oaitesting.com to see the options available through Oil Analyzers Inc. If the oil analysis report reveals abnormal fuel dilution, the next step is to determine the cause.


Start with the easiest and least expensive fix. Use a fuel system cleaner, such as Performance Improver (Pi) or Diesel Injector Clean (ADF). 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 treatment of 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 mechanic.

Sometimes, driving conditions are to blame for fuel dilution, reducing your idling time and, of course, use a high-quality synthetic motor oil to guard against accelerated wear. 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 synthetic motor oils use top-grade base oils and additives to help protect components from wear.

Vehicles prone to fuel dilution typically operate under severe service. For maximum protection, customers should use AMSOIL Signature Series Synthetic Motor Oil, which has extra protection for added peace of mind.

Diesel Injector Clean

    Have a Question?

    If you're not sure which product is right for you, just ask.