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Does Diesel Go Bad? What RVers Need to Know!

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. 

Low-sulfur diesel notice

The use of low-sulfur diesel has decreased the time it takes for the fuel to age. (Photo credit: U.S. Department of Energy)

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?

Black smoke from the tailpipe of a truck

Black smoke may indicate that the diesel fuel is outdated.

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. 

Power Service 09016-09 Bio Kleen Diesel Fuel Biocide - 16 oz.
  • Dual-phase effective in both diesel fuel and water as recommended by major diesel engine manufacturers

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.

Power Service 09280-06 Clear-Diesel Fuel & Tank Cleaner - 80 oz.
  • Safe for use in all diesel engines: Disperses diesel fuel contaminants, removes water and slime
  • . +Petro fresh provides maximum long-term storage stability – keeps fuel fresh

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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Not So Free

Saturday 13th of August 2022

Thankfully I don't have to worry about DEF. My truck ,2000 Cummins Ram, doesn't require it. As to ultra low sukfur, I was unsure if I was going to have issues when they switched to it. At 235K I haven't had any problems Last year I only put 328 miles total on it. I don't use it much since I sold the last trailer and bought a small gas motorhome. I will be adding some treatments next time I fill up for the winter. I still take it out and drive it enough to get to normal operating temp from time to time.


Saturday 13th of August 2022

As if worrying about DEF expiration weren't enough - now diesel! I store my 9 year old rig for 6-7 moths every year and have never had a problem starting in the Spring. What I may do in the future is fill the tank before storage to minimize space for air and moisture. Simle enough.

John S.

Saturday 13th of August 2022

Thanks for writing this post. As someone who is NOT a full timer our coach is often idle for several months at a time so this article is very informative.

As you suggested when not running for a month or two I always make sure the tank is nearly full - less room for air moisture. But this year was only the second time I've decided to use an additive.

The reason? I was advised such precautions are not necessary for short-term periods but this year our coach may be idle for twice (or 3X) as long as normal.

One thing I did right was add the treatment, fill the tank and then drive for a couple of hours thus ensuring that the tank, the filters, the pumps the ejectors and all the fuel lines have diesel that has been 'stabilized'.

Russel Hawkins

Saturday 13th of August 2022

Good post. I might add that another product of the microbes is water. Years ago my brother’s Mercedes 300 shut down for no reason. When serviced at a the dealership he was informed that there was water in the tank caused by microbes. The Technician rubbed his finger around the filler and showed him the dark creatures. Microbes.

Gay RV Enthusiast (Jason)

Saturday 13th of August 2022

I don't get why Diesel fuel has changed at all. I don't think Diesel is as bad as people make it out to be. I don't think the problem is with the fuel itself, but with how its burned. I'd like to breathe clean air as much as most people, but I fail to see how changing the fuel is supposed to help clean up the air and make it less polluting to the environment.

Gay RV Enthusiast (Jason)

Sunday 14th of August 2022

@TJL, For decades? How long is that? 40 yrs? 50 yrs?

Gay RV Enthusiast (Jason)

Sunday 14th of August 2022

@Larry Lee, I'm all for low polluting, or no polluting fuel. But how does Ultra-Low Sulfur fuel help?


Saturday 13th of August 2022

@Gay RV Enthusiast (Jason), Simple. The sulfur in the fuel burns to sulfur dioxide which is a pollutant that affects health and contributes to acid rain. Efforts to reduce sulfur in fuel (coal, oil, gasoline, diesel) have been ongoing for decades.

Larry Lee

Saturday 13th of August 2022

@Gay RV Enthusiast (Jason), I Googled this and quickly found out how vitally important it is to all of us to use Ultra Low Sulfur Diesel (ULSD) in all diesel engines built since 2006. The reason for lowering the sulfur content so drastically is that it allows the catalytic converter to work ultimately reducing air pollutants by about 95%. This is a serious lung health benefit as well as reducing acid rain. Just read up on this subject on the internet and I believe you will be convinced that is is money well spent.

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