What is Fuel Dilution (and Why is it Bad)?

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

What causes fuel dilution?

A number of factors, including…

  • Dirty or leaking fuel injectors
  • Excessive idle time
  • Incomplete combustion
  • Low engine temperatures
  • Frequent short-trip driving
  • Worn piston rings/excessive blow-by
Fuel injector
Bad fuel injectors can cause fuel dilution.

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.

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. This allows 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 rings and down the cylinder walls, into the oil sump.

How much 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.

Acceptable fuel dilution limits

  • Up to 2.4 percent in gas engines
  • Up to 3.4 percent in diesel engines

Why is fuel dilution a problem?

  • Reduced oil viscosity interferes with 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 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 mechanic.

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.

  • Reduce idling time as much as possible
  • Mix in longer trips to ensure the engine reaches operating temperature for several minutes
  • Use a high-quality oil that resists wear

If fuel dilution is a problem, use a high-quality synthetic motor 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 synthetic motor oils use top-grade base oils and additives to help protect components from wear.

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 motor oil label.

Updated. Originally published Oct. 29, 2018.


  1. Thankyou for sharing.
    Really I have learned more about fuel dilution. It’s so much informative and trusted. Many many thanks for sharing with us.


  2. A lot of us new generation Ford Ranger owners are experiencing fuel dilution, some to critical levels. They are experiencing lowered viscosity levels from this condition as a result. For some this happens very quickly after an oil change. Would you recommend going to a 0/40 or 5/40 from the standard 5/30 to combat the drop in viscosity. Ford seems to be having trouble coming up with a fix for this condition. Some have had injectors replaced and solved the problem while not working for others. HPFP’S have also been replaced with minimal success. Looking for an answer as many are frustrated by this condition.

    1. Hi James,

      I’m sorry to hear about the problems you Ranger owners are having.

      It’s best to stick with the viscosity the manufacturer recommends. Use the best synthetic oil you can find to combat the effects of fuel dilution. Also, we recommend oil analysis to stay on top of oil condition and ensure the oil is protecting your engine. Oil analysis is the best way to deal with this issue, outside of Ford providing a permanent fix. Check out this post on how perform oil analysis.

      Good luck,


  3. i’m having the same issues.
    Honda’s service department refusing to state that is the issues.
    They even did the software fix on my 2017 CRV to fix the oil dilution issue.

  4. Hi – We have a 2019 Honda CRV. Have done oil analysis on last two oil changes. Running 4-5% has dilution. Using AMSOIL Signature Oil. Recently installed block heater. Any suggestions?

    1. Hi Eric,

      The current-generation CRV has developed a reputation for increased fuel dilution. I’ve heard from another owner of a late-model CRV who has had the same issue. The best thing to do is to use a high-quality motor oil (which you’re already doing) that will provide good protection despite fuel dilution. It’s also a good idea to use oil analysis to monitor the level of fuel dilution and plan your oil changes accordingly. This ensures you get the most miles out of the oil without compromising engine protection.



  5. My 2007 Silverado, 6.6L Diesel runs lower oil pressure with synthetic oil. Runs good oil pressure with standard oil. I can’t get an answer as to why so I run mobile 1 with Lucas hd stabilizer. Does AMSOIL synthetic oil run a more oil pressure range than other synthetics?

    1. Hi Curt,

      Several factors influence oil pressure, including oil viscosity, the oil’s ability to flow quickly when it’s cold, the oil’s stability when exposed to heat, engine wear and more. While I can say that AMSOIL synthetic motor oil offers excellent cold-flow properties and heat stability, which help stabilize oil pressure, I can’t say with any certainty how it will affect oil pressure in your particular engine since too many variables are involved. The same holds for explaining why conventional oil offers “good” oil pressure compared to synthetic. It depends on the oil you’re using, the viscosity, your operating conditions and more. The only way to know if AMSOIL products will offer good oil pressure is to try them out.



    1. Hi Bridger,

      Our fuel-dilution standards are based on historical oil analysis data and common practice among original equipment manufacturers (OEMs). While OEMs do not publish acceptable numbers, most consider 3-7% acceptable as long as there is not a noticeable increase in wear or drop in lubricant viscosity.



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