Accidentally mixing gasoline and diesel fuel - What happens then?

Here are a couple of scenarios we've come across. Someone calls us at the Bell office and starts to describe a situation where they had a diesel tank, and they accidentally put some gasoline into it. What should they do?

We’ve also seen the converse – one of our customers accidentally put gasoline directly into one of his tractor’s diesel fuel tanks. He wanted to know if it was going to be a problem.

If you deal with fuel long enough, you’re going to come across this kind of situation at least once. Mixing gas and diesel is never advisable, but it isn’t necessarily a disaster. The biggest factor is how much of each you accidentally dropped in. Here’s what you might expect if it happens to you.

Big Differences between gasoline and diesel fuel

When we talk about diesel fuel here, we’re talking about #2 diesel fuel – on-road or off-road, it doesn’t matter.

When you’re trying to predict what kind of issues might arise from accidentally adding one to the other, you have to take into account the biggest differences between the two fuels.

Diesel fuel is heavier than gasoline (because it’s made up of large molecules). It atomizes differently due to a different density and viscosity. And its flash point and autoignition temperatures are significantly higher. Same is true of the differences with gasoline, albeit in the opposite direction. These differences in physical properties are what cause problems in engines and fuel systems when you put in fuel that isn't supposed to be there.

Putting Gasoline in Diesel Fuel

Let’s say you accidentally introduce a small amount of gasoline into diesel fuel. This creates a chain of problems that can escalate quickly — especially in modern diesel engines.

One of the first impacts is flash point depression. Diesel fuel is designed to be relatively stable and resistant to vapor ignition during storage and handling. Gasoline is highly volatile. When gasoline is blended into diesel, the flash point of the entire fuel supply drops sharply. It doesn’t take much. Around 1% gasoline contamination can lower diesel flash point by roughly 15 to 20°C. That creates safety concerns for fuel storage, transfer operations, and enclosed equipment environments.

Inside the engine, the bigger issue isn’t early ignition — it’s combustion stability and system protection. Diesel engines rely on controlled compression ignition combined with precise injection timing and proper fuel lubricity. Gasoline disrupts this balance. It alters ignition delay behavior and reduces combustion stability, which can lead to rough operation, power loss, and increased thermal stress on engine components.

Fuel system damage is often the most expensive consequence.

Modern diesel injection systems — especially high-pressure common rail systems — depend on diesel fuel for lubrication. Diesel contains natural lubricating compounds that protect fuel pumps, injectors, and precision internal components. Gasoline contains virtually none. Even small amounts of gasoline can significantly reduce lubricity. The result can be accelerated wear, internal pump scoring, injector damage, and metal debris circulating throughout the fuel system.

As combustion quality deteriorates, excessive smoke and incomplete burning may develop. Over time, this can foul exhaust aftertreatment components, raise exhaust temperatures, and trigger engine derate or limp-mode conditions. Sensors exposed to abnormal soot loading and temperature stress can also suffer long-term reliability issues.

In short, gasoline contamination doesn’t just change how a diesel engine runs — it directly threatens the durability of the fuel system and emissions hardware. That’s why even low-percentage misfueling events are treated as serious maintenance events rather than minor inconveniences.

Putting Diesel into Gasoline

Now let’s look at the reverse scenario — introducing a heavier, high-flash-point fuel into gasoline, which is designed to be light, volatile, and fast-burning. Some people assume this is less serious than putting gasoline into diesel. That assumption is wrong.

One of the first problems is octane loss.

Gasoline engines rely on controlled ignition timing. Octane rating measures gasoline’s resistance to premature ignition under compression. When octane drops, the fuel-air mixture can ignite too early in the compression stroke. Instead of a smooth pressure rise that pushes the piston down, you get pressure spikes colliding with the upward-moving piston. The result is detonation — commonly heard as knocking — and over time this can damage pistons, rods, bearings, and cylinder walls.

Diesel fuel doesn’t have a gasoline-style octane rating, but when blended into gasoline it behaves like an extremely low-octane component. Even small amounts matter. As a rule of thumb, roughly 2% diesel contamination can drop gasoline octane by about one point. At 10% contamination, octane can fall four to six points — enough to push many modern engines well outside their safe operating range. The more diesel present, the worse the knock resistance becomes.

And octane loss is only the beginning.

Diesel fuel is far less volatile than gasoline. When this heavier fuel is pulled through gasoline injectors and intake systems, it does not vaporize properly. The result is uneven combustion and partially burned fuel. Over time this creates heavy carbon deposits on pistons, intake valves, combustion chambers, and spark plugs. Engines may begin running rough, misfiring, losing power, and fouling plugs.

With more severe contamination, liquid fuel can accumulate in the cylinders during misfire events. While hydrolock is not common at low contamination levels, large misfueling events can create conditions that stress head gaskets, valves, and connecting components.

Another downstream issue is oil dilution. Unburned diesel fuel can wash past the piston rings and enter the crankcase. This thins the engine oil, reduces lubrication film strength, and accelerates wear on bearings, cam surfaces, and other critical engine parts.

Finally, unburned diesel hydrocarbons that reach the exhaust system can overload the catalytic converter. The catalyst attempts to oxidize this excess fuel, driving internal temperatures far beyond normal operating limits. The result can be catalyst meltdown, internal plugging, and permanent converter failure — often turning a simple misfueling mistake into a four-figure repair bill.

The Bottom Line - Don't Drive It

Because it's impossible to know exactly how much of the wrong kind of fuel is in your tank and fuel system, the bottom line advice is that if you have good reason to believe you (or someone else) put the wrong kind of fuel in your gasoline or diesel engine, you need to have it towed to a mechanic's garage where they can remedy the problem.

Once at the garage, they will remove all of the fuel from the filter and flush the system to remove the problem fuel.

Some might respond with well, my ________ (fill in with friend, coworker, relative, general practitioner) accidently got some in his tank, and he drove it and it was just fine.

In those situations, there's no way to know how your situation compares to theirs (and human nature is such that we always want to minimize our description of potential problems if it stems from a mistake that we're responsible for). If you drive the vehicle after you think the wrong fuel has been dispensed, you've been warned. We recommend that you not take that chance in any case.

You may be interested in these other posts:

• Oops...someone put diesel in President Obama's gasoline limo
• Does premium gas have ethanol in it?
• Using Premium Fuel In Your Vehicle
• What Does Octane Do In Gasoline? Octane Ratings