Does Diesel Fuel Go Bad? How Long Does It Last? (Facts vs. Myths)

It’s important to be selective with the fuel we put into our vehicles and equipment. After all, these fluids are the life-blood equivalent for these machines.

There are several aspects to consider when choosing fuel for your car: compatibility, quality, type, and cost being the most important. Choosing the wrong one can impact how a diesel engine runs, and can also cause damage that’ll cost you some good money down the line.

Another essential factor to know about and take into consideration is whether or not this liquid power source is still viable. If you’ve left your automobile parked with a full tank for a few months or you have one or two cans of fuel that have been stored for quite a while now, how do you know if they’re still safe to use?

If you’ve found yourself asking the question “does diesel fuel go bad?” we’re here to help! In this article, you’ll learn everything about why and how diesel fuel can turn bad. We’ll detail all the factors that influence its lifespan and also include useful tips on how you can preserve diesel fuel in your gas tank or container.

You’ll also get familiar with some of the most common signs that show it’s time for disposal, and how you should go about that.

Can Diesel Fuel Expire? How Does This Happen?

You might be shocked to learn that diesel doesn’t last forever. It’s sourced from fossil fuels that have survived for centuries, so how is it possible that it can ever turn bad?

Diesel fuel goes through processing changes that have some beneficial effects. However, such changes can also shorten its shelf life. Chemical reactions can also take place, compromising stability.

These may be influenced by external causes, such as climate, or can simply occur with time. It’s possible for breakdown to occur both in fuel storage containers and inside gas tanks.

Like most petroleum products on the market, diesel goes through a refining process.

Crude oil is chemically and physically altered (refined) to form different substances. This includes distillates (such as the one we’re discussing), lubricating oils, waxes, and more.

The majority of this distillate sold in the United States is ultra-low sulfur diesel (ULSD). The composition has been altered to contain up to 97 percent less sulfur, which is a harmful pollutant.

Biodiesel is another variation available on the market. This mixture includes vegetable oils or animal fats. As with ULSD, it was developed to help lower toxic emissions.

Additionally, almost all forms of diesel include additives. These are compounds that are added during the refinement process. They can range from detergents to fight corrosion to ethanol, which is a common ingredient in biofuels.

What Makes Diesel Fuel Go Bad?

Now that we’ve gone over some of the important basics above, let’s talk about what can cause diesel fuel to expire. 

Water and Oxygen

Remember that most modern diesel fuels are ultra-low sulfur. The lack of sulfur is great for the environment, but not so good for protection against bacteria. The same is true of additives such as animal fats.

So-called “diesel bug” is contamination by bacteria or other microbes, such as fungi. This occurs due to condensation or water penetration—e.g. rainwater.

In highly humid environments, absorption of water from the air can take place. Oxygen will speed up the multiplication of these organisms.

Extreme Heat

Extreme heat can speed up the natural deterioration process, resulting in sediment or gummy sludge buildup. A hot environment can also promote the growth of certain species of bacteria.

Certain Ingredients

There are some compounds (e.g. specific additives) that will start to degrade within a certain period of time.

In these cases, there’s not much you can do about it aside from avoiding long-term storage.

Stagnation

The gas tanks of vehicles, equipment, and power tools are far from an ideal place to store diesel fuel for the long term.

They’re not 100 percent airtight, which means that water and oxygen can get in. Since it’s impossible to shield diesel from these elements indefinitely, it has to be used quickly. So, moral of the story: don’t fill up your tank and let it sit for months on end!

Improper Storage Container

This one is pretty obvious but needs to be mentioned anyway. It’s recommended to keep any fuel in a proper storage container that’s well-sealed.

Not only is this a priority for safety reasons, but also for preservation. Oxygen, dust, and other contaminants such as trace metals can get in and speed up the breakdown process.

Long-Term Storage Without Precautions

If you must let this fluid sit in storage, it’s necessary to take preventative measures. The fact of the matter is this: you can’t expect it to stay fresh for months on end!

There are substances you can use that work to keep your fuel fresh and stable, such as the best fuel stabilizers and biocides. They’re also practical for seasonal equipment such as snow blowers, and you can also try them on vehicles that aren’t driven all that often.

These products could save you the chore of having to drain your tank, or being faced with bad diesel fuel.

How Does Diesel Fuel Go Bad?

In the following section, we’ll be talking about the science behind the deterioration process and explaining exactly how diesel fuel goes bad.

There’s more than one way in which this fluid can go bad, so let’s have a look at all of them:

Phase Separation

Phase separation is what happens to biodiesel that contains ethanol. Under certain circumstances (e.g. oxidation), the alcohol can separate from the distillate.

As you might expect, this will make the fluid useless, since it won’t be able to ignite normally. The separated alcohol will be corrosive to your tank and metal engine components.

This phenomenon can also happen with water. Rather than remaining mixed with the diesel, the water will begin to separate.

The water tends to accumulate towards the bottom of the storage tank or container. In turn, this can provide a hospitable environment for fungi and bacteria to grow.

Destabilization

Dirt and dust from the outside world can contain various compounds, including copper and zinc. These metals can destabilize the distillate, making it unusable.

Deposits Forming

Sediment and gummy deposits can form in the fluid. If the consistency becomes thick enough, it will affect performance.

The average engine isn’t built to run on a viscous gel. Moving mechanisms won’t be able to function properly if they’re gummed up.

Biological Contamination

Fungi, bacteria, and other microbial growth will transform stored diesel fuel for the worse. They can produce organic acids, which are corrosive.

Plus, these organisms can become big enough to the extent that they alter the consistency. You don’t want a clump of fungi circulating through your pipes, that’s for sure!

Bacteria can also destroy protective additives, which can reduce lifespan and alter performance.

How Long Does Diesel Fuel Last?

Under optimal conditions and assuming the quality of the fuel is high, you can expect it to last for up to one year or more.

At higher temperatures (86 degrees Fahrenheit and above), this estimate decreases. In this instance, storage life expectancy is six months to a year.

Don’t forget that we’re talking about the best-case scenario here. Other variables (humidity, oxygen, fuel quality, etc.) can also come into play, and have to be taken into account as well.

What Are the Signs of Bad Diesel Fuel?

The good news in all of this chemistry lesson is that you don’t have to get out a chemistry set to identify bad diesel fuel!

As a matter of fact, all you’ll need to perform an inspection is your eyes, nose, and some common sense.

For now, let’s go over the different methods through which you can tell whether the fuel you currently have is good or bad:

Nasty Smell

Some people appreciate the smell of petroleum fuels, while others hate it.

Either way, you’re likely familiar with the standard smell of diesel. If you notice a foul odor, it may be contaminated with microorganisms, and may no longer be good to use.

Fuel Discoloration

Depending on the type of diesel fuel you buy, the fluid can range from a clear to yellowish-brown color.

If it’s begun to darken, this could indicate that it’s starting to break down.

Separation or Visible Sediment

When you can see that the consistency has started to thicken, this more often than not means that it’s gone bad.

You may spot sediment, solid deposits, or even biomass (e.g. fungus).

Poor Engine Performance

A sudden drop in engine performance can be caused by hundreds of potential issues. Still, look for easy fixes before you jump to more serious conclusions.

Rough idling and higher fuel consumption could signal bad fuel. You may also notice black smoke or white exhaust smoke with a nasty smell to it.

The Dangers of Bad Diesel Fuel

Ignoring any unusual symptoms your vehicle shows you isn’t wise, and the same holds true for ignoring the symptoms of bad diesel.

Deliberately circulating this fluid if it’s gone bad comes at a high price.

For one, it’s more likely to cause damage to your injectors, fuel pump, and engine. As you’re probably aware, trips to the mechanic aren’t exactly cheap!

Sediment and deposits can clog up moving mechanisms and fuel filters as well. Acids, the byproducts of bacteria and fungus, will corrode metal parts.

How to Make Diesel Fuel Last Longer

There are a few simple strategies you can take advantage of to keep diesel fresher for longer.

Consider these tips when you next buy a can or fill up your tank:

Appropriate Storage

Keep your cans in a temperature-regulated environment. Don’t leave them outdoors to endure the sweltering heat or freezing cold!

Take care to keep them stable and protected. For example, don’t balance cans on tightly packed shelves.

Don’t Forget It

Keep track of the fuel you have, both in your fuel tanks and any cans you may have. Try to use it within a reasonable period.

Try Additives and Biocides

As we briefly touched on, fuel treatment with additives and biocides can work to keep your diesel fresher for longer.

You can add them every time you fill up your tank. Or, you can make use of them for unavoidable storage. (For instance, if you’re not willing to drain your boat’s tank every winter).

Stabilizers

These products are formulated to slow the breakdown of fuels. They contain ingredients that work against the chemical changes that happen with time.

Most are also packed with additives to boost performance, detergents to fight the buildup of sediment, and lubricating agents to keep things running smoothly.

They can also enhance existing properties. Typical ingredients are substances like methanol, which encourages cleaner burning and fights corrosion.

Biocides

Diesel biocide is the equivalent of pesticide for diesel fuels and diesel engines. In other words, they’ll kill or suppress any organisms that may be growing.

Not all biocides are created equally, though. Some brands require repeated doses, which may not be convenient for longer storage periods.

Make sure you check the guidelines before buying. If you’re not able to follow up with fuel treatments, get a biocide that will kill rather than suppress.

Where and How to Dispose of Bad Diesel Fuel

By now, you know enough to have confirmed that your stored fuel or the stuff in your tank has gone bad. The question remains: What do you do with it now?

Responsible disposal is the only solution. Pouring it down a storm drain, flushing it or similar strategies are bad for the environment and should be entirely avoided – not to mention that they could be illegal.

So, save yourself the hassle by following good disposal practices!

For starters, you can check online for local regulations regarding diesel fuel disposal. If you have bad fuel in your tanks, drain them into appropriate containers.

Keep in mind that you’ll have to make sure you’re using containers the disposal center approves of. A plastic bucket or soda bottle probably won’t make the cut.

Conclusion

The next time anyone around you asks “Does diesel fuel go bad?”, you’ll now have all the answers and will be able to inform them about what’s facts and what’s myths.

Understanding how and why our automotive products expire is always a good thing.

If we don’t know about this stuff, we might end up incurring way more costs on repairs down the line, while all we were trying to do was save a few bucks on not having to buy another bottle of a product we already have laying around the garage.

You’re not entirely helpless in this situation, though. You can certainly take action to avoid speeding up the degradation process.

For example, using biocides and additives is crucial if you’re leaving the fluid unused for lengthy periods.

You’re also now all aware of the reasons why rancid fuel is better disposed of immediately. If you don’t do that, you’ll end up with an engine that acts up, or even worse, paying for expensive repairs!