Does Diesel Go Bad? What RVers Need to Know!

If you’ve got an RV or any vehicle with a diesel engine, you may have heard or read that diesel fuel can go bad. But, does it really? And if so, how long does it remain stable before that happens? 

Our rig is a Class A diesel pusher, so this topic is important to us. We’ll try to clarify some of the information we’ve found on this topic in hopes of better informing anyone who owns an RV or other vehicle with a diesel engine.

Whether you’ve got a diesel pusher like ours, a diesel truck that hauls a travel trailer, 5th wheel, truck camper, a Class B van (like those on the Sprinter chassis), or even a diesel generator, here’s what you need to know.

Does Diesel Fuel Have an Expiration Date?

The short answer is “yes” but it’s not like a gallon of milk with a date printed on the package. While diesel fuel used to last for years, things have changed over time, and the shelf life of diesel fuel has dropped considerably. 

Nowadays, the usable life of diesel fuel is usually measured in months as opposed to years.

Let’s take a look at why this is the case.

Why Does Diesel Expire?

There are several factors that influence the integrity of diesel fuel over time. If left sitting long enough, diesel can turn into sludge that just won’t burn as it’s supposed to.

So, while there isn’t a literal expiration date on diesel, performance is impacted significantly by these factors.

• Water
• Air
• Warm temperature

But why is this the case now if it didn’t use to be?

The answer is a lack of sulfur. 

Microbes hate sulfur, and diesel used to contain a whole lot more of it. That’s the main reason it ages faster than it did in the past. The lack of sulfur provides an environment that allows more microbes to develop.

The carbon chain molecules that diesel is made from break down when they’re exposed to water, air, and warm temperatures. As they break down, they provide a great food source for microbes. 

In addition to microbial growth, the waste products from the microbes themselves further the breakdown of the chemicals in the fuel.

Bottom line? The quality of stored diesel fuel degrades over time.

How Long Does Diesel Fuel Last?

In general, diesel will last for between six and twelve months under ideal conditions. This is true whether it’s held in a storage tank, in your fuel tank, in your generator – anywhere.

Pro Tip: Fueling up at more popular locations increases your chances of buying fresher fuel.

But even under ideal conditions including being kept cool at temperatures under 70℉, if diesel is going to be unused for 12 months or more, it should be treated with a biocide and a fuel stabilizer.

Adding fuel treatment will positively impact long-term diesel storage, considerably increasing its usable lifespan.

There are other important considerations, however. 

The integrity of diesel fuel over time can also be improved by the use of good storage tanks that are well-maintained to prevent water from contributing to fuel contamination. 

Keeping the tank full also makes a difference. That’s because space allows condensation to form and the water from that condensation speeds the breakdown of the fuel. We always leave our fuel tank as full as possible whenever we store our RV before traveling away from it.

Remember, heat, water, and air are the enemies of stored diesel fuel.

How Do I Know If My Fuel is Still Good?

There are a number of ways to tell if your diesel fuel storage life has been exceeded.

If you can look at the fuel (which of course may not be easy, or even possible), you may see that that’s it’s turned a darker color. You may also see sediment or even sludge.

If you’re experiencing poor fuel efficiency or your fuel filter is clogging (or your fuel pump is damaged), those are also indications that your diesel may be outdated.

If the diesel in your tank is very old, you may also find that your motorhome, truck, or generator has difficulty starting. You may also see black smoke, another sign that your fuel is too old.

What Treatments Can I Use to Keep Diesel Usable Longer?

If you have a known problem with microbial growth in your fuel tank (remember, it’s not a gas tank on a diesel-powered vehicle!), you’ll need to use a diesel biocide. Again, microbes are far more common now with low-sulfur diesel.

The purpose of a biocide is to kill microbes in fuel, including bacteria and fungus, ridding your fuel system of microbial contamination. 

You’ll also want to use a fuel and tank cleaner that’s safe for use in all diesel engines and tanks.

These products remove water and grime and keep the fuel fresher during long-term storage. It also prevents the clogging of fuel filters and the damaging of fuel pumps. 

This product can be used in conjunction with the previously noted product.

So, if you’ve got a diesel engine on your RV, generator, or another vehicle, it’s important to remember that diesel fuel has a limited lifespan.

For more information on diesel pushers, see our post on diesel Class A motorhome benefits.

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