Your Engine Will Thank You: Don’t Mix Gas and Diesel

Fuel Properties
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It may seem obvious, but many drivers are still unclear on whether gasoline and diesel fuel can or should be mixed. Given their distinct roles in engine combustion, it is imperative to understand how these two fuels interact, the risks of mixing them accidentally or intentionally, and how to avoid costly errors at the pump. Read on to get the full scoop on gasoline and diesel mixing.

Gas and Diesel: An Overview

First, let’s review the key properties and functions of gasoline versus diesel fuel:


  • Made from crude oil refined to lighter fractions
  • Volatile liquid with high vapor pressure
  • Ignites via spark plugs
  • Burns more rapidly than diesel
  • Powers gasoline engines in most cars


  • Made from crude oil’s heavier distillates
  • Less volatile with lower vapor pressure
  • Auto-ignites under compression
  • Burns slower than gasoline
  • Powers diesel engines in trucks, buses, etc.

So gasoline and diesel have quite different physical characteristics and roles in engine combustion, based on their respective distillation fractions. Now let’s look at what happens if they end up mixed together.

Gasoline + Diesel = Big Trouble

Unfortunately, gasoline and diesel are completely miscible. If mixed, they will combine to form a single solution that cannot easily be separated back to the individual components. The resulting blend takes on properties intermediate between pure gasoline and diesel.

More specifically, mixing diesel with gasoline raises the viscosity and density while reducing volatility compared to neat gasoline. This alters the fuel’s ignition and combustion properties significantly. Meanwhile, adding gasoline to diesel reduces viscosity, density, and ignition quality compared to pure diesel.

Either way, the altered fuel mixture will wreak havoc in your engine if introduced into the fuel tank and combustion system. Let’s explore those consequences further.

What Mixing Gas and Diesel Does to Your Engine

Using a blend of gasoline and diesel in your engine is a recipe for disastrous performance and extensive damage. Here are some of the likely outcomes:

Gasoline Engine Problems

  • Failure to start from reduced fuel volatility
  • Vapor lock and stalled fuel pumping
  • Poor acceleration from misfiring
  • Power loss under load due to incomplete combustion
  • Engine knocking or pinging from untimely ignition
  • Potential runaway pre-ignition and cylinder damage

Diesel Engine Issues

  • Hard starting from poor compression ignition
  • Rough unstable idle and misfiring
  • Insufficient power and acceleration
  • Excessive diesel knock and cylinder stress
  • Runaway pre-ignition in extreme cases
  • Fuel system corrosion and leakage

The mechanisms may differ between gasoline and diesel engines, but the end result is the same – unacceptable driveability, immediate performance problems, and probable mechanical damage over time.

Can Mixed Gas and Diesel Be Separated?

Once gasoline and diesel have been inadvertently mixed together in a fuel tank, is there any way to salvage the situation and separate the two fuels? Unfortunately, the options are very limited.

Some approaches that may work include:

  • Manually draining and filtering small amounts of mixed fuel
  • Employing specialty molecular filters to preferentially isolate gasoline
  • Using centrifuge equipment to split fuels by density
  • Boiling off gasoline fractions and condensing for recovery

However, these tactics require specialty equipment not readily available to the average vehicle owner. The complexity, limited fuel recovery, and costs generally make it impractical to attempt separation yourself.

Realistically, mixed gas and diesel in a tank should be considered contaminated and unsuitable for use in any engine. The only prudent options are to safely dispose of the blended fuel and thoroughly clean the fuel system to avoid lingering issues.

How Gas and Diesel Mixes Occur

Before discussing how to prevent gasoline/diesel misfueling, it helps to understand common scenarios that lead to mixing:

Incorrect Fuel Pumps – Driver error filling from the wrong pump at a blended station.

Misfueling Vehicles – Accidentally adding gasoline to a diesel vehicle or vice versa.

Shared Storage Tanks – Gas and diesel stored in the same home heating oil or generator tank.

Portable Containers – Mixing up gasoline and diesel portable cans and equipment.

Fuel Transfers – Pumping fuel into the wrong vehicle or equipment tank.

Contaminated Fuel – Trace amounts of the wrong fuel being introduced and gradually mixing.

Intentional Blending – Attempting to blend fuels intentionally for cost savings or performance reasons.

Situational awareness and clear labeling are the best defenses against accidentally mixing incompatible fuels. Never attempt to intentionally blend fuels with distinct combustion properties.

Avoiding Gas and Diesel Mix-Ups

Here are smart strategies to avoid inadvertently mixing gasoline and diesel:

  • Choose filling locations with dedicated diesel pumps.
  • Read fuel pump labels carefully before refueling.
  • Have gasoline and diesel vehicles refueled separately if possible.
  • Never permit pumping into an unmarked container.
  • Label all gas and diesel portable containers unambiguously.
  • Store gasoline and diesel in separate tanks.
  • Discuss fueling procedures if others may be filling your vehicles.
  • Supervise refueling of rental equipment or vehicles.
  • Report any suspected mixed fuel immediately.
  • Never attempt to blend fuels for economic or performance reasons.

What to Do If Gas and Diesel Mix

If you’ve ended up with mixed gasoline and diesel, follow these steps to mitigate problems:

  • Stop using the contaminated fuel immediately.
  • Drain or pump the mixed fuel into approved containers.
  • Add some fresh diesel to dilute residual gasoline.
  • Fully flush the fuel system, lines, and tank with pure diesel.
  • Replace fuel filters after flushing.
  • Consult mechanics if engine issues persist afterwards.
  • Properly dispose of the mixed fuel at approved hazardous waste facilities.

Never attempt to use blended gasoline/diesel to avoid potentially extensive engine repairs. With proper fuel handling and driving vigilance at the pump, these dangerous mix-ups can easily be avoided altogether.

The Takeaway on Gasoline and Diesel

  • Gasoline and diesel fully mix together rather than separating
  • Using blended gas and diesel causes poor performance and engine damage
  • Once mixed, gasoline and diesel are very difficult to separate
  • Avoid fuel mix-ups through labeling, situational awareness, and proper procedures
  • Immediately address accidental gasoline/diesel mixing to limit consequences

Your engine’s health depends on supplying consistent pure fuel per the manufacturer’s specifications. Drivers have the ultimate responsibility to prevent misfueling situations that endanger their vehicles. Following these best practices at the pump and in stored fuel keeps both gas and diesel engines running smoothly.

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