Nightmare Fuel: The Effects of Using Gas in a Diesel Engine

Diesel vs. Gas: What’s the Difference?

To fully grasp the potential issues that can arise from filling your diesel truck with the wrong fuel, you’ll first need to understand the differences between diesel and gasoline on a chemical level.

Production and Refining

Gasoline and diesel are both derived from crude oil, also called petroleum. Petroleum comprises countless goods, including plastics, cosmetics, asphalt, and automotive fuels. But how does that thick, black gunk turn into the gas and diesel we put into our trucks?

Crude oil is heavily refined before it arrives at the gas pump as diesel or gasoline. While both products undergo a similar refining process, gasoline is distilled at a lower temperature range than diesel, which makes for a lighter, more volatile final product. On the other hand, diesel is refined at a higher temperature than gasoline, leading to a heavier, denser, and less volatile product.

Energy Potential

Since diesel fuel is denser and less volatile than gasoline, it burns slower and more comprehensively than the lighter, more volatile fuel. This density, paired with diesel’s chemical composition (longer carbon chains compared to gasoline’s shorter), leads to increased energy density. In other words, diesel fuel can produce more energy than gasoline per unit volume of fuel. 

This characteristic is why diesel-powered vehicles tend to be more fuel efficient than their gasoline counterparts, making them ideal for commuter vehicles and long-haul trucking. There’s a reason most heavy-duty trucks feature diesel engines under the hood!

Engine Operation

The fuels aren’t the only different things—so are the engines that ingest them. 

All engines need three ingredients to run, including air, fuel, and an ignition source. Gasoline-powered engines burn gasoline as fuel, inhale air from the intake, and ignite said mixture via spark plugs. Diesel engines, on the other hand, don’t use spark plugs. Instead, they employ a process called compression ignition. 

Compression ignition works by compressing air to approximately 1/16 of its original volume as its pistons rise in the cylinders. This process produces extreme temperatures within the cylinders—approximately 400 degrees Celsius. Once superheated, fuel injectors introduce diesel fuel into the cylinders, which instantly ignites upon contact with the high-temp air, resulting in combustion.

What Happens When You Put Gas in a Diesel Engine?

With that engineering lesson under our belts, what happens when you dump the wrong fuel type into your diesel engine? 

If you’re driving an older diesel with a mechanical fuel system, the consequences of a fuel mixup may not be as severe. While some knocking from predetonation may occur, it’s likely to be more of an irritant than a problem.

On the other hand, modern diesel fuel systems feature electronically controlled high-pressure pumps and fuel injectors that are incredibly sensitive to contamination. These systems rely on the unique chemical composition of diesel fuel for lubrication, as the fuel itself lubricates all wear components. Since gasoline is far thinner and less lubricating than diesel, excessive gasoline in a diesel fuel system can quickly wipe out expensive injection pumps and fuel injectors, leading to thousands of dollars in repairs.

Did I Damage My Diesel Engine?

Now, accidentally filling your diesel fuel tank with gasoline isn’t an immediate cause for concern, especially if you realized your mistake before starting up your truck. 

However, if you drove for some distance after the error, especially with primarily gasoline in the fuel tank, you could be in for some bad news when the fuel system is finally inspected. However, a quick cruise across town, especially with a mixture of diesel and gasoline in the fuel tank, will not likely cause permanent damage.  

So long as the engine was only run for a short period with gasoline in the fuel system, you’re probably in the clear. Still, there are some precautions you should take in the event of a fueling mixup.

What to Do if You Put Gas in a Diesel Engine

If your truck finds itself at the wrong end of a gas nozzle, you should take a few precautions to prevent damage to the fuel system and engine. 

1. Don’t Start It

While you may be tempted to cruise your pickup to the local mechanic, we’d advise against it. Any driving after a fill-up of gasoline, especially in a newer diesel, can wreak havoc on the fuel system. Instead, play things safe and call a tow truck. If you’ve already started the vehicle with gasoline in the fuel system, turn it off as soon as possible. 

2. Drain the Fuel System

Once the vehicle is in a safe location, such as a mechanic’s shop, the contaminated fuel system must be drained completely. In most cases, this requires removing the fuel tank to extract all fuel, including the couple of gallons that sit below the fuel pickup tube.  

3. Flush the Fuel System

Even with the tank drained, contaminated fuel is likely to remain in the fuel lines, fuel filter, high-pressure pump, and fuel injectors. These components must also be flushed to remove all traces of gasoline. 

4. Change the Oil

Though this step is often overlooked, it’s essential. Because gasoline is thinner, it can bypass seals and enter the crankcase, diluting the engine oil, reducing its lubricity, and increasing engine wear. Performing an oil change rids the engine of any residual gasoline and ensures proper lubricity of hard-wearing components. 

After performing these steps, fill the tank with fresh diesel and keep on trucking! You should be in the clear as long as the engine operates normally—without any worrisome noises, check engine lights, or odd driving characteristics. 

For more, check out our other diesel resources on RealSource—the number one source for information on trucks, Jeeps, and more.