08/27/25
Table of Contents
Imagine this: You’ve planned a dream summer road trip in your diesel RV: the open road, stunning campgrounds, and adventure ahead. But halfway up a mountain pass, your engine sputters and dies. You’re left stranded, vacation on hold, waiting for a tow truck. Unfortunately, this nightmare happens to diesel RV owners more often than you’d think, and the culprit is usually a simple fuel mistake. In this article, we’ll explain the diesel mistake too many RV owners make and how to avoid it. We’ll also cover how to improve diesel MPG (miles per gallon) on your RV this summer while keeping your engine safe. By the end, you’ll know exactly how a few easy diesel RV maintenance steps, like using the right additive for diesel fuel, can save you from costly breakdowns and boost your rig’s fuel economy.
Diesel RV owners often take great care of their motorhome’s oil changes, coolant, and tires. But there’s one maintenance item that is frequently overlooked: fuel quality. The mistake too many make is running on old or untreated diesel fuel. If your RV sits for long periods (like over the winter or between trips) and you don’t treat the fuel, you’re asking for trouble. Diesel fuel can degrade, collect water, grow “algae,” and lose the lubricity your engine needs. This isn’t just a theory; RV forums and mechanics are full of stories of trips cut short by fuel problems that could have been prevented.
Why is unused diesel fuel such a ticking time bomb? Here are a few hidden dangers when diesel sits in your tank for weeks or months:
In summary, the big mistake is not using a diesel fuel additive for storage and trips. Old, untreated diesel can contain water, lack lubricity, grow algae, and even gel in surprise cold conditions. Any one of these issues can cause fuel starvation or injector failure that leaves you stranded.
If you’re thinking “fuel is fuel, I buy the good stuff from the pump,” it’s important to realize that diesel fuel quality directly affects your engine’s reliability and performance. A diesel engine is robust, but it’s also precision-engineered, especially the fuel system. Modern diesel RV engines use high-pressure common rail (HPCR) systems with extremely tight tolerances in the injectors. Tiny particles or lack of lubrication can wreak havoc in these systems.
Let’s break down the key fuel-related factors that can make or break your RV trip:
A small amount of water in gasoline might not stop a gas engine (it can cause a stumble, but gas engines are more forgiving). In a diesel engine, however, water is a menacing intruder. Diesel fuel injectors pressurize fuel to incredibly high levels (several thousand PSI) to atomize it. If a drop of water gets in there, it instantly flashes into steam and expands ~40 times in volume. That can literally blow the tip off an injector. Even if it doesn’t reach the injector, water contributes to corrosion and can freeze into ice crystals that block fuel flow in cold weather.
Most diesel RVs have a water separator filter, a component that traps water and keeps it out of the engine. But these separators only work if you maintain them (drain them regularly) and if the water accumulation isn’t overwhelming. An untreated tank with a lot of condensation can produce more water than the separator can handle. Many RV owners are shocked to find out their water separator is full of water because they never thought to check it, especially after storage. Water can also come from bad fuel at a station (perhaps a fuel stop with an old storage tank or fuel stations in remote locations). It’s wise to drain your water separator before a long trip and periodically during the trip. But an even better strategy is to use a diesel additive that handles water for you (more on that soon).
What people call diesel algae is actually a mix of bacteria and fungi that thrive in diesel fuel when water is present. You won’t see them until it’s too late – when they’ve grown into a black or green slimy crud clogging your fuel filter or coating your tank walls. If your engine suddenly loses power or won’t rev, and you find a dark jelly-like substance in the fuel filter, you’ve got a microbial infestation. This usually happens in diesel RVs that have been left idle with some water in the tank. Microbial growth accelerates fuel breakdown into acids and sludge, so not only do you have “bugs” in your fuel, you also now have poor-quality fuel. The only cure once this happens is to treat with a biocide (diesel algae treatment) and potentially go through the hassle of pumping out and filtering the fuel, and replacing filters multiple times. It’s a messy, time-consuming problem, and not what you want during your vacation.
Preventing diesel algae is much easier than fixing it. The key is keeping water out (so the bugs have nowhere to live) and using a fuel stabilizer additive if the fuel will sit for a long time. Many top rated diesel fuel additive products include biocide or stabilizer features, or you can add a dedicated biocide periodically. If you know your RV will be stored for a season, it’s smart to fill the tank full (to minimize air space for condensation) and add a quality diesel conditioner that prevents microbial growth. This falls under basic diesel motorhome maintenance: think of it as “feeding” your RV clean fuel so it stays healthy.
Lubricity might sound like a technical term, but it’s easy to understand: it’s the fuel’s ability to lubricate moving parts. Diesel fuel itself (especially older high-sulfur diesel) is oily and provides lubrication for the fuel pump and injectors. However, today’s ULSD fuel is “dry” and severely lacking in lubricity. That’s why many diesel owners started adding two-stroke oil or other additives to fuel when ULSD was introduced, they were trying to put the “oiliness” back in. Nowadays, this is an outdated tactic, because modern diesel additives help restore that lubricity! Without sufficient lubricity, the metal parts in your fuel pump and injectors grind against each other and wear out faster. Think of it like running an engine with poor oil – not good at all.
For an RV owner, injector or pump wear is a big concern because these rigs are expensive to repair. A set of diesel injectors can easily cost upwards of thousands of dollars to replace (parts and labor). In fact, depending on your vehicle, replacing diesel injectors could run anywhere from a few hundred up to more than ten thousand dollars in total. It’s not a bill you want to have to pay. Unfortunately, if you never use any additive for diesel lubricity, you might face those bills earlier than expected. The diesel mistake of ignoring fuel lubricity doesn’t usually cause an immediate breakdown; rather, it’s a stealthy long-term threat. Your engine loses power or starts smoking, and when you take it into a diesel shop, a mechanic informs you that the injectors are shot due to scoring and wear.
The solution is simple: use a diesel fuel additive with a lubricity improver. The difference can be dramatic. For example, Howes Diesel Defender additive packs 2.5 times more lubricity than most competing products to protect and extend the life of your fuel system. With such an additive, the wear on injectors and pumps is greatly reduced. In fact, lab tests show treated fuel can beat stringent U.S. and European lubricity standards for wear scar, meaning much less wear on metal parts. By maintaining lubricity, you keep the fuel system running smoothly, which also helps maintain proper fuel pressure and spray patterns.
Even if your diesel fuel is fresh and dry, over time your fuel injectors can get dirty. Diesel injectors have tiny orifices that spray fuel into the engine in a fine mist. But deposits (carbon and varnish from fuel) can build up on the injectors, especially if you idle a lot or use lower-quality fuel. Deposits cause the spray to become irregular (droplets instead of mist) leading to incomplete combustion. The symptoms? Rough idle, loss of power, more smoke, and lower fuel mileage because not all fuel is burning efficiently.
Deposits accumulate gradually, so many RV owners don’t realize their performance is down until they clean the injectors and suddenly feel the difference. One common method to deal with this is adding a fuel injector cleaner additive. A good additive will have strong detergents to clear out deposits in the fuel system. In fact, Howes Diesel Defender uses a state-of-the-art IDX4 detergent that cleans your entire fuel system within about three treatments. It removes stubborn carbon build-up and even tackles internal diesel injector deposits (IDID) that can form in modern HPCR injectors. By keeping injectors clean, you restore the precise fuel spray needed for optimal combustion.
For you as the driver, the benefits of clean injectors are immediately noticeable: smoother running, more responsive power, and better fuel economy. Many RV owners report that after using a fuel system cleaner, their rig “feels like new” with more pep and sometimes improved MPG. It’s an easy form of diesel RV maintenance that can save you from bigger problems down the road.
By now, you might be thinking, “Okay, I see the problems: water, algae, low lubricity, gelling, deposits, but what’s the solution?” The answer is straightforward: use a quality diesel fuel additive regularly. It’s truly the secret weapon for both RV reliability and fuel mileage. Let’s see how one small bottle can address all the issues we discussed:
Now, a skeptic might ask: “Do I really need to use additive at every fill-up?” The answer from many experts and experienced RVers is yes, if you want consistent protection and benefits. Quality additives are designed to be used continuously, and they even say using them regularly is key to keeping the improvements going. Each tank of fuel you buy can vary in quality and adding your conditioner ensures a consistent, high standard for what’s going into your engine. It’s a small cost (often just a few cents per gallon treated) that can prevent huge costs later and give you peace of mind.
It’s worth highlighting that preventive maintenance with fuel additives isn’t just theory, it has saved many RV owners from trouble. On the flip side, those who skipped it often regret it.
In fact, testimonials for additives can be quite eye-opening. One Diesel Defender user reported “initial improvements in fuel economy and overall engine performance” after just a month of use. They noticed the engine ran smoother and quieter, too.
Another driver using a diesel fuel additive saw nearly a 2 MPG increase in fuel mileage under normal driving, and about 1 MPG improvement when towing heavy; a significant boost for a diesel truck that might normally get ~10 MPG towing. And these aren’t isolated cases. While results vary, it’s common to hear about 3-10% fuel economy improvements from keeping the fuel system clean and optimized. For instance, Howes Diesel Defender is formulated to improve fuel economy by 5% or more.
Let’s put that into perspective: How many MPG does a diesel RV get? On average, a diesel motorhome will get between 8 and 15 miles per gallon (larger Class A rigs on the lower end, smaller Class C or B on the higher end).
Suppose your RV normally gets 10 MPG and you drive 10,000 miles a year. That’s 1,000 gallons of diesel. A 5% improvement would save you 50 gallons of fuel, which, at say $4.00 per gallon, is $200 saved. If you get even better results, say 10% (1 MPG gain to 11 MPG), you’d save ~91 gallons, or about $364. Meanwhile, a bottle of additive treating that 1,000 gallons might cost $50-$100 total. That’s a pretty good return on investment, not even counting the intangible savings of avoiding breakdowns and repairs. And if you drive more or fuel costs rise, the savings grow.
Fuel economy is a hot topic for all RV owners, especially with diesel prices often higher than gas. While the focus of this article is fuel additives, it’s worth mentioning a few other tips to improve diesel MPG in your RV, because additives are just one piece of the puzzle. You want every edge you can get for better mileage:
By combining these practices, you might be surprised how much you can improve your diesel RV MPG. Some drivers who do “everything right” report getting at or above the upper range of expected MPG for their rig, while others who neglect these might fall to the lower end or worse. And when you consider the cost of diesel, every extra mile per gallon is money in your pocket (or miles further you can go between fill-ups).
We’ve zeroed in on fuel additives and their benefits, but let’s step back and look at the bigger picture of diesel RV maintenance: specifically fuel system maintenance. Here are some practical tips to ensure your diesel motorhome stays reliable on those long hauls:
By following these maintenance tips, you’re setting yourself up for worry-free travels. Worry-free is what we want: RVing is supposed to be fun, not a source of anxiety about whether the engine will make it up the next hill! As the saying goes, “An ounce of prevention is worth a pound of cure.” A $20 bottle of diesel additive and a $10 fuel filter are cheap compared to a $500 tow and a $1,500 repair job. And beyond money, it’s about keeping your vacation on track.
Taking care of your diesel RV’s fuel might not be as visible as shining the exterior or greasing the chassis, but it’s arguably one of the most important things you can do to ensure a smooth trip. When you improve diesel MPG through good maintenance and additives, you save money on fuel, which could be the difference between taking that extra excursion or staying on budget. When you prevent breakdowns, you save not only money but the priceless experiences that you would have missed if you were stuck in a repair shop for two days.
Let’s recap the key points and how they benefit you:
Confidence and peace of mind: Perhaps the best payoff is simply peace of mind. Instead of nervously listening for engine hiccups after a bad batch of fuel, you can cruise down the highway confident that you’ve treated your fuel and your RV is in top shape. RVing should be about freedom, and knowing your fuel system is taken care of frees you from one of the big worries. You can focus on the scenery, not the next truck stop repair center.
In conclusion, don’t let your summer RV trip end in a tow truck ride. The diesel mistake of neglecting fuel quality is so easily avoidable with the right knowledge and products. Now you have that knowledge: you know what can go wrong and exactly how to prevent it. By using a quality diesel additive like Howes Diesel Defender as part of your routine, you address all the major threats in one go: poor lubricity, water, sludge, gelling, deposits, and more. It’s a small step that delivers big results in both protection and performance.
Your diesel RV has given you great adventures; give it the care it needs to keep the adventure going. A well-fed (and well-defended) diesel will reward you with many worry-free miles. So before you set out on that next journey, make sure you’ve got a bottle of your trusted diesel fuel additive handy and top off your tank with some extra peace of mind. Here’s to better MPG, robust engine health, and epic road trips: see you on the open road, and not on the back of a tow truck!
