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Putting Unleaded Gas in Diesel Engines is a Bad Idea

Putting Unleaded Gas in Diesel Engines is a Bad Idea

If you have ever thought about putting gas in a diesel engine, or vice-versa, you might be curious about would happen. The bottom line: using the wrong gas type is a very bad idea. They aren’t mix and match.

Auto Repairs Are EXPENSIVE


Many modern drivers play a little game when driving. We keep a running score of gas prices on our commutes. If we travel to the city, we gripe about higher gas prices. When we journey to the far suburbs or countryside, we marvel at the lower prices.

Sometimes, we get a little mixed up. In our game of price checking, we might confuse the two prices – one always seems to be a little cheaper than the other.

Worse, if we’re distracted when we pull up to the pump, we might almost grab that nozzle that pumps diesel into a regular engine (or the other way around). What would occur?

The short answer is disaster. Your car could be totally fried by the error. Don’t turn it on if you mix the fuels and realize it after the fact.

The fuel types cannot be mixed or swapped for one another. The chemical composition of the two fuels, gasoline and diesel, are different. The engines into which they go are different, too. You will ruin your car if you try to put gas in the diesel engine and start it up.

Putting Gas in a Diesel Engine is a Big No-No

If you pull up to the gas tank with your cellphone to your ear, not paying attention, you might accidentally put gasoline in a diesel engine. It wouldn’t be your finest hour, and here’s why.

The flashpoint will be all wrong in the engine. In summary, a car engine works by igniting fuel in a very controlled away. The fuel ignites at a specific temperature, and if the fuel doesn’t, the engine won’t operate as designed. 

Even a small amount, say about one percent, could change the fuel so that the flashpoint falls. In other words, the fuel will ignite a temperature far below the norm.

How Regular Gas Affects the Diesel Engine

What happens if you find yourself putting regular gas in a diesel engine? Nothing good.

A basic review of your high school science: diesel is an oil with long hydrocarbon chains; gasoline is a solvent. 

This means that that lubrication is reduced when adding gasoline to the diesel engine. Not only would the combination cause a lower flashpoint, but the engine will be operating without the proper amount of lubrication. 

That’s definitely not good for the engine!

Where there’s smoke, there’s fire. When you top of your diesel with a little gasoline, you’re going to see black smoke billowing from your vehicle. 

Today’s modern engines have many computer pieces in them. Many people don’t realize how many sensors are involved in their car’s operations. If the sensors fry or get covered in grime from the smoke, they’re not going to work, shutting down the engine entirely.

What to do if You Put Diesel in a Gas Engine

Imagine you put diesel fuel in a gas-powered engine. It wouldn’t be a smart idea, but for the sake of blog science, we should know what would happen.

It’s straightforward science. The gasoline in the car’s tank has a lower flashpoint. The diesel fuel is heavier, with a higher flashpoint. It’s just as bad as putting gas in the diesel tank. 

Adding diesel to gasoline reduces the octane factor. Octane measures the pace of the ignition of the gasoline, not too late or too early, right on time. 

When the timing is off, you’re going to hear terrible knocking in the engine. This problem could hurt the engine. 

Gasoline, if you pay attention at the pump you know this, comes in octane ratings, like 87, 89, and 91. Usually 87 is the cheapest. 

Now, compare the octane ratings of diesel fuel. It’s between 25 and 40.

If you put just 10% of diesel into the gas tank, that’s a deduction of five points in octane.

Problems ahead if that occurs!

The burning diesel will leave behind soot on engine parts like valves and pistons. It will even end up on the spark plugs!

A car in this condition is good for driving.

You might see other problems down the line from the snafu: diesel fuel in the oil crankcase, locked cylinders, blown head gasket, the catalytic converter, etc.

The damage will run thousands of dollars.

If you accidentally add diesel fuel to your gas tank, and it’s a car that takes unleaded fuel, you have a big problem on your hands. Do not drive it. Call a mechanic. Call a tow truck. Car a car rental agency and rent something else until the repairs are made.

If you don’t run the car, the problem can be more easily resolved with a thorough fuel system flush.

Anecdotal evidence of friends and coworkers who “mix the fuels” is hearsay and rumor. Even if they went on with their lives without incident, their experience doesn’t compete with the science. 

Mechanics know how to deal with this problem better than your random neighbor, unless they are also in the industry. 

Putting Diesel Fuel in Storage

Perhaps you are wondering about putting extra diesel fuel or gasoline you want to put into storage or into a container for transport. Be careful and follow all safety regulations.

Storage of diesel fuel is possible. Today, many types of storage tanks are available. Fuel cans are also on the market.

Fuel can be stored for a few months is safely protected and rightfully maintained. 

Diesel fuel, like gasoline, can be put into five-gallon gas cans – the very same ones that people use to store gas for a boat or a lawn mower.

Large options exist for specialty purposes. It might look like a large free-standing tank or a drum.

These large tanks of are usually metallic or polyethylene, specifically designed for the purpose of storing diesel fuel. Just choose the right size tank for your needs.

How Long can Diesel Fuel be Kept if Putting in Storage

If you will be putting your diesel fuel or gas in storage, you need to know about the basics about how long it can be safely kept.

In general, diesel fuel has a shelf-life of six months to a year. However, the fuel must be kept in warm temperatures to avoid oxidation within the tank.

If you check out store fuel, and it seems kind of like glue, do not use it. It will clog up your engine.

There are other problems that can happen, too. Fungus sometimes grows, breaking down the fuel. Water or other fluids can come in contact with the diesel fuel, running it along the way. If rust or insects enter the tank, the diesel fuel is bad.

Beware of additives! Whereas fuel stabilizer might work for gasoline, additives aren’t the answer for diesel fuel.

WARNING: Diesel is flammable. It should be stored based on all OSHA safety recommendations. The average Joe shouldn’t be storing diesel fuel in his garage if he doesn’t have a handle on the situation. Doing so is dangerous. 

In other words, don’t store the fuel near fire hazards like electrical circuits or incinerators. If you have diesel fuel tank on your property, you shouldn’t be hosting bonfires near it.

All of this information may seem like it goes without saying, but some people forget the basics when it comes to not mixing fuel and fire, so it is worth mentioning.

Caution is key.

What is the Difference Between Putting Gas and Putting Diesel in an Engine?

When researching the difference between putting gas or diesel fuel into an engine, you may start to wonder about what makes these fuel types so different.

The first gas engines weren’t effective, but they did pave the way for modern gas engine vehicles. 

Scientists were inspired to come up with a more efficient engine – and that’s when diesel fuel entered the picture.

In some ways, an engine is an engine. Internal combustion causes bursts that translate into the energy that makes a car go.

The little bursts, however, are not the same in nature in a diesel and fuel engine.

In a regular gas-powered car, the gasoline encounters compressed air, thanks to the pistons. The spark plugs set the fuel ablaze. 

Diesel engines operate with compressed air to generate heat instead. The process works thanks to how fuel is injected into the cylinder.  For this reason, fuel injectors are a major player in diesel engines. They hold their own against heat and pressure.

It isn’t a perfect set-up. There isn’t 100% efficiency. It remains a challenge for modern science. 

Gasoline offers horsepower; diesel, on the other hand, offers torque. This is why diesel is such a hit with big rigs, busses, and trucks.

Some people prefer diesel engines because they offer more engine efficiency. They go farther on less fuel. 

Both fuels come from the oil refineries we see so often on the news – they are a hot topic. Without fuel, we can’t run our nation. We can’t move product across the country. We can’t even take the kids to school. We are dependent, as a society, on both gasoline and diesel taken from the ground.

For this reason, new types of diesel are emerging! These fuels are not petroleum-based. Instead, they are made from vegetable byproducts. It might just be the next big thing!

Diesel fuels are popular for logistics because the engines have fewer problems thanks to a lack of spark plugs and electrical to operate it.

Final Comments on Putting Diesel in Gas Engines (or the Other Way Around)

It’s clear by this point that putting diesel in gas engines is a no-go. It’s also clear that the other way around, putting gas in a diesel engine, is also not a good idea.

At the end of the day, they are just two fuels that do not mix. The chemistry involved is complicated for the everyday driver, but the rule of thumb should be to watch out that you don’t mix the fuels.

If you have an old diesel car, you may love the increased MPG, but it could also be a real nightmare. Older cars were more likely to use diesel fuel back in the day, especially heavy pick-up trucks.

With environmentalists on your case, and the increasing cost of repairs for an old vehicle, it may be better to send that diesel fuel engine car to a junkyard near you. 

If you accidentally put the wrong fuel in a very old car, and turn it on, you might save money by junking it.

Those who care about the planet should like the idea; it gets polluting cars off the road. It also allows for the parts to be recycled. 

When you call for a tow to the junkyard, you might get cash money on the spot for the old vehicle. This is great news because you can put the money toward an environmentally friendly car or even a public transit pass.

For the rest of us, we just need to take caution when fueling our cars. Nobody should be smoking, texting, or talking on a cellphone when adding fuel to a vehicle as it is. This could ignite a fire or explosion.

Additionally, even if the explosion doesn’t happen, the distraction might cause you to mix up the fuels. If that does happen, remember the rule: don’t turn on the vehicle. Call a tow truck so that the fuel system in the car be properly flushed. 

It’s the only safe way to get the car back on the road.

Keep checking those prices at the pump, but mind which is which so you don’t find yourself searching the Internet for “putting gas in a diesel engine.”