Fuel dilution, also known as “crankcase dilution”, occurs when gasoline or diesel fuel infiltrates your engine’s crankcase and dilutes, or “waters down,” the motor oil. Technically, “oil dilution” is a more fitting term, but “fuel dilution” is industry-standard terminology.
What causes fuel dilution?
Several factors can lead to fuel dilution:
Dirty or leaking fuel injectors
Excessive idle time
Incomplete combustion
Low engine temperatures
Frequent short stops
Worn piston rings/excessive blow by
What type of vehicles are prone to fuel dilution?
Police fleets, fire trucks and other emergency vehicles are especially prone to fuel dilution. Common conditions include short bursts of driving and long periods of idling. Vehicles allowed to idle for long periods of time, like when warming the interior on a cold winter morning, are susceptible to fuel dilution.
Urban delivery trucks 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. 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 engines for greater efficiency and power.
One side effect, however, is fuel dilution. As fuel is sprayed into the combustion chamber, it can wash past the piston rings and down the cylinder walls into the oil sump.
How much fuel dilution is too much?
In a perfect world, we prefer no fuel mixes with the motor oil. But life has a way of interfering with perfection. With that in mind, the following cautionary limits are acceptable provided motor oil viscosity has not significantly changed. Fuel dilution that surpasses these guidelines requires attention:
Up to 2.4% in gas engines • Up to 3.4% in diesel engines
Why is fuel dilution a problem?
Fuel dilution reduces oil viscosity, which interferes with the formation of a durable lubricating film, inviting wear. It 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.
What if I suspect fuel dilution?
First, ensure there’s really a problem. Although it’s possible to smell fuel in a motor oil sample, oil analysis is the most accurate method of determining the severity of fuel dilution. If the 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 (API) or Diesel Injector Clean (ADFP).
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, a bad injector could be at fault, meaning it’s likely time to visit a mechanic.