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.
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 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.
- 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.
- 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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Vince S.
Saturday 12th of August 2023
Wow, there’s a lot to unpack here for a career diesel mechanic….LOL
Diesel fuel doesn’t “go bad”, it becomes contaminated. The myth that it’s only good for a few months has been disproven too many times to count yet still persists.
Listed in order of prevalence are the “Top Three” contaminants that will ruin a travel day if not a diesel fuel system:
DIRT This occurs from improper handling. Watch the delivery truck fill the underground tanks at your favorite fuel stop. Rarely do you ever see the driver clean the tank cap/connection or his discharge hose. Nope, he doesn’t even knock the road dust off it when he connects. When the driver opens the dump valve, all that “stuff” goes into the service station’s tank and churns all the previous contaminants for you to pump into your tank.
Look at the pump fill nozzle. Do you clean it inside and out prior to sending its contents into your tank? Nah. Most folks just jam it into the filler neck and let ‘er rip.
HINT: You have a 2 micron fuel filter trying to protect your fuel system. A human hair is roughly 70 microns. If you can see any dirt/dust/organics, you’re only seeing a fraction of what’s going into your tank. Change your primary and secondary fuel filters often (avoid exceeding every 500 hours of engine operation) to reduce wear and potential downtime. Absent pre-filtering, you just don’t know *how* dirty the fuel is that you’re putting into your tank.
WATER Water will get into any tank simply by condensation alone (even with desiccant breathers). It only gets worse if it seeps in as rainwater, flood water or whatever. Like all living creatures, aerobic and anaerobic bacteria (Diesel bugs) exist in the presence of water. Get rid of the water and you eliminate the fungi and bacteria from procreating in your tank.
HINT: Drain the water from your fuel tanks frequently. Monitor and drain your water separator frequently. Avoid allowing any form of water into your tank even if you need to put up an umbrella while fueling in the rain and wipe the fuel nozzle. Water will not only allow bugs to thrive in your tank, it will degrade the high pressure components of your fuel system.
THERMAL Temperature can not only affect condensation but it also affects how “lustful” aerobic and anaerobic bacteria can be. Diesel bugs can’t live a good life below 50F or above 104F but are quite comfy within that range IF there’s water. Extremely cold fuel becomes cloudy fuel (gels) as the conditioners and natural paraffins separate from the liquids. This makes it all but impossible to pump through a 2 micron fuel filter. Extremely hot diesel on the other hand increases condensation.
HINT Keeping your fuel tank full (storage or in use) reduces temperature variability within your tank. Reduced variability reduces condensation.
SUMMARY Additives do not make water, dirt, bugs or temperature variation disappear. They merely “spread out” or disperse the water in your fuel but the water is still there. Additives might kill the diesel bugs that live off that dispersed water but the bodies of them dead critters don’t disappear either. They slime up your fuel filters. Every time you add fuel you’re likely to add microbes, dirt and water. It just takes time for those contaminants to overwhelm your fuel system.
Treating a tank every few months is akin to buying a mop to compensate for bad window seals. Sure it works to pick up the dust, bugs and moisture and it beats doing nothing but prevention is better. Eliminate contaminants and in most cases, your fuel pump and injectors will last until overhaul.
Happy RV’ing!
Gay Travel Enthusiast (Jason)
Saturday 12th of August 2023
Interesting article. Call me naïve, but I never thought it was possible for Diesel fuel to go bad.
Gay Travel Enthusiast (Jason)
Monday 14th of August 2023
@TheRVgeeks, I could be wrong of course, but I would think that if you don't run the engine often enough that'll happen.
TheRVgeeks
Saturday 12th of August 2023
We don't think you're alone in that assumption, Jason!
Dr. Mike
Saturday 12th of August 2023
I am ashamed to admit it, but due to a very low amount of staff, our coach has rested peacefully in the garage connected to power for the batteries for the last 15 months. I check on her from time to time and the only thing I see are more cobwebs.
OK, enough boo-hoo about me. I do know that the fuel tank is full and so is the DEF.
Questions: How much of the above products will be required to treat 200 gallons of diesel?
Should I drain the DEF tank, rinse with fresh DEF, and refill with new DEF?
TheRVgeeks
Saturday 12th of August 2023
Hi Dr. Mike. Sorry to hear that things have gotten in the way of your ability to use your motorhome. We hope things turn around again so you can use it.
For the DEF, you'll probably need to drain it out and replace it with new DEF. If there's not an easy way to drain the tank, you could get a small transfer pump (like this one on Amazon: https://amzn.to/3KCx9is) and some clear plastic tubing (like this: https://amzn.to/3QFclKX) online or at a big box hardware store and use that to get as much of the old DEF out as you can. Then, if you're still not planning to drive the RV or run the engine much for the foreseeable future, you could just leave the DEF tank empty and only get new DEF once you know you have plans to travel again. That way, you won't run into the same issue with it aging out before you can drive again.
On the diesel fuel treatments, one bottle of each is more than enough to treat all 200 gallons.
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.
TJL
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.