Why You Should Avoid Mixing Gasoline with Diesel Fuel

Gasoline and diesel both made from crude oil and come out of pumps at the gas station — but that’s about it in terms of similarity. Gasoline and diesel engines are built differently, and they’re only going to work with the appropriate engine. Mixing the two fuels, or putting the wrong kind of fuel in your tank, can be a costly mistake.

The Difference between Diesel and Gasoline

Both diesel and gasoline are fuels derived from crude oil. Diesel is both more stable and more energy-dense than gasoline (which is why diesel has such great fuel economy). As a result, diesel needs more compression than gasoline to ignite. Most often, diesel engines are compression engines, and gasoline engines are spark-ignition engines. Beyond that difference, both kinds of engines work on about the same principles, despite the chemical differences between diesel and gasoline.

What Gasoline Does to a Diesel Engine

The short answer is nothing good.

The long answer is the gasoline will either not combust, or, because gasoline is different in composition compared to diesel (with gasoline being generally more volatile and explosive), the gasoline will combust at the wrong stages in the diesel engine’s combustion process. While a diesel engine is built to withstand the force of combustion, combustion at the wrong time can lead to serious damage to almost every component of the engine — especially the pistons and connecting rods.

Diesel is also a lubricant, while gasoline is not. If gasoline makes its way into a diesel engine, it can jam the fuel pumps and the injectors. So gasoline will gum up the works in a few different ways, all of which can be expensive to repair.

Diesel engines will run on a lot of different types of fuel (including biodiesel and leftover frying oil), but they will not run on gasoline. Mixing in a little amount of gasoline with diesel is a bad idea, but might not be catastrophic. A large amount of gasoline will cause significant damage to a diesel engine.

The good news is it that it is relatively difficult to mis-fuel a modern diesel engine. Starting around 2009, manufacturers began adding mis-fuel protection devices to diesel vehicles. Vehicles with these protection devices will only accept diesel fuel (or fuel from diesel pumps). If a diesel is older than that, however, it’s probably still at risk for mis-fueling.

If you’re wondering what diesel does to a gasoline engine, the answer is a lot less. Gas engines can’t combust diesel fuel — there’s not enough pressure in the engine. If you pump diesel into a gas engine, you’ll drive until you run out of gasoline and then stop. The diesel will then need to be flushed out of the fuel tank, and the engine itself before the engine will run again, however. The process of getting all of the diesel out can be difficult. Sometimes, the engine will need to be completely taken apart, cleaned and then put back together.

What to do if you Fuel a Diesel Engine with Gasoline (or Vice Versa)

First of all, don’t start the engine. You might be tempted to be polite and move your vehicle out of the way of the pump. Starting the engine, however, puts it at risk of damage right away. Instead, let the gas station attendant know you won’t be able to move the vehicle right away and pay for the fuel that you’ve dispensed.

Call your roadside service provider or a towing company and have your car brought to a dealership or mechanic. Once there, let them know what’s happened, and they’ll flush your fuel tank. This can be a little expensive — anywhere from $50 to $100 — but it absolutely beats damaging the engine, or having to pay to have your entire engine flushed of fuel.

Mixing Gasoline and Diesel Fuel

Mixing gasoline with diesel (or vice versa) will at best cost you a good chunk of money. Stick with the fuel for your engine, and if you do accidentally fuel up with wrong kind, don’t try to drive on it — it won’t end well.

Scott Huntington is an automotive writer from central Pennsylvania. Check out his work at Off The Throttle or follow him on Twitter @SMHuntington.

Editor’s Note: The opinions expressed in this article are those of the author.

Photo attribution: Paul Brennan licensed under CC0 Public Domain.

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2 Responses to “Why You Should Avoid Mixing Gasoline with Diesel Fuel”

  1. Levi Armstrong says:

    My wife’s car runs on a diesel engine, and this morning I caught her almost putting my spare gasoline on her car because she ran out of gas. It’s a good thing I was able to stop because, as you said in your article, the gasoline would’ve jammed the fuel pumps and injectors, which would’ve damaged her car engine. I’ll start looking for a diesel fuel delivery service in town to solve my wife’s dilemma so she won’t try to put gasoline in her car ever again.

  2. Howie says:

    Use to drive military vehicles, and over-the-road trucks, we often would put about 5 Gallons of gasoline in our 100+ diesel tanks during the winter to prevent the diesel from jelling up in the cold. So, one can use up to 20% gas to diesel ratio, and it will be ok for winter weather, we never ran this mixture above 20 degrees Fahrenheit.