A boiler that keeps cutting out on a cold morning. A tractor that coughs and loses power halfway through a field. A fuel filter that’s blocked again, only weeks after you changed it. If any of that sounds familiar, the cause could be sitting at the bottom of your tank. So, what is diesel bug, and how worried should you be?
Diesel bug is one of the most common problems we come across with stored fuel. As a family-run supplier delivering heating oil, red diesel and DERV across Staffordshire, Cheshire, Shropshire and Derbyshire, we see it in home heating tanks, farm stores and business backup supplies alike. This guide explains what it is, which fuels are at risk, how to spot it, and what to do about it.
What Is Diesel Bug?
Diesel bug is the everyday name for microbial contamination in stored fuel. It’s not one single organism. It’s a mix of bacteria, yeasts and moulds that live in water at the bottom of a fuel tank and feed on the fuel above it. As they grow, they form a dark slime that blocks filters and damages equipment.
These microbes are in the air, in rainwater and in small amounts in most fuel. The trouble starts when they find water. Water is heavier than fuel, so it sinks to the bottom of a tank, and the colony grows where fuel and water meet.
As they feed, they produce:
- Sludge and slime (also called biomass), which clogs filters, fuel lines and injectors
- Acids, which can corrode steel tanks and fuel system parts
- More water, as a by-product, which helps the colony spread further
Is diesel bug a type of algae?
No. You’ll sometimes hear it called diesel algae or diesel fungus, but algae need sunlight to grow, and there’s no light inside a fuel tank. Diesel bug is mainly bacteria, yeasts and moulds. Fungal moulds are often the worst offenders, as their long strands knit together into mats that block filters quickly.
What Causes Diesel Bug?
Water causes diesel bug. Without water in the tank, microbes can’t grow, however long the fuel sits. Water gets in through condensation, rain and damaged tanks, and modern diesel’s biodiesel content makes the problem worse by attracting moisture. Fuel that’s left standing for months gives the bug time to build up.
The main ways water and microbes get into a tank are:
- Condensation: in a half-empty tank, warm air cools overnight and leaves droplets that run to the bottom, especially in summer
- Rainwater: through loose caps, faulty seals, damaged vents or cracks. Bordering the Peak District, our area gets plenty of rain
- Biodiesel content: UK road diesel contains up to 7% biodiesel, known as FAME (fatty acid methyl esters). FAME is hygroscopic, meaning it draws in moisture, and it’s an easier food source for microbes
- Fuel left standing: generators, seasonal machinery and little-used tanks are at the highest risk
- Old sludge: residue from years of use can reinfect fresh fuel
You’ll see the biodiesel content on forecourt pumps as the “B7” label, which the Department for Transport introduced to show that diesel can contain up to 7% biodiesel.
Which Fuels Can Get Diesel Bug?
Despite the name, diesel bug can affect any stored fuel that has water in it. Road diesel (DERV) and red diesel are most at risk because they can contain biodiesel. Kerosene heating oil is less prone, but it isn’t immune. HVO is the most resistant of all.
DERV (white or road diesel): DERV contains up to 7% FAME and has a higher risk of diesel bug when sat in a tank for months.
Red diesel (gas oil): Also contains up to 7% FAME. The risk of diesel bug whilst in storage is also high, particularly in farm tanks and generators that are left idle between seasons.
Kerosene (heating oil): Standard kerosone doesn’t contain biodiesel, as a result, it has a lower chance of developing diesel bug when in storage. However, water in the tank can lead to sludge and microbial growth.
HVO: HVO is completely FAME-free so has a much lower risk of diesel bug, but water should still be kept out.
If you heat your home with oil, our guide to what kerosene is and how it differs from diesel explains why it stores better. If you run vehicles or machinery, our guide to DERV and road diesel covers storage life in more detail.
What Does Diesel Bug Look Like?
Diesel bug looks like a dark brown or black slime, often sitting as a layer between the fuel and any water at the bottom of the tank. In a fuel filter, it shows up as a brown or black gunk, sometimes like wet coffee grounds. The fuel itself may look hazy, dark or have strands floating in it.
You may also notice a sour or rotten egg smell from the tank or filter. The easiest check is to take a sample from the very bottom of the tank, where water and sludge collect, and pour it into a clear glass jar.
Diesel Bug Symptoms to Look Out For
The most common diesel bug symptom is a fuel filter that keeps blocking. In a home heating system, that usually means the boiler cuts out or locks out. In an engine, you’ll notice power loss, rough running or difficulty starting. If the same problem keeps coming back after a filter change, diesel bug is a likely cause.
Watch out for:
- Fuel filters that block repeatedly, sometimes within weeks
- An oil boiler that keeps cutting out or going to lockout
- Engines losing power, stuttering or stalling under load
- Hard starting, particularly after machinery has stood idle
- Black smoke from the exhaust
- Rust or corrosion inside a steel tank
Is it diesel bug, water or wax?
Not every blocked filter is diesel bug. Wax, plain water and sediment can all cause similar symptoms, and the fix is different for each. Looking at what’s in the filter, and when the problem happens, will usually point you to the right cause.
- Wax: appears in very cold weather as a cloudy, pale gel. Problems ease once the fuel warms up. Winter-grade fuel and anti-wax additives help
- Water alone: shows as a clear layer at the bottom of a sample. Water-finding paste on a dipstick confirms it
- Sediment: gritty rather than slimy, and often appears just after a delivery has stirred up the tank
- Diesel bug: slimy, dark, often smelly, and keeps coming back
One thing worth knowing is that a fresh delivery can stir up water and sludge that was already sitting in the bottom of a tank. It’s easy to blame the new fuel, but the real cause is usually what was there before. We cover this in more detail in our guide to water damage and fuel storage tank maintenance.
How Do You Get Rid of Diesel Bug?
To get rid of diesel bug, you need to remove the water, kill the microbes with a biocide, and clear out the dead sludge. In mild cases, a biocide treatment and new filters will usually do the job. Heavy contamination can require professional cleaning.
- Confirm the problem with a sample from the bottom of the tank or a diesel bug test kit
- Remove the water with a water absorber, or have larger amounts pumped out by a specialist
- Treat with a biocide, following the manufacturer’s curative dose exactly
- Change the filters, as dead sludge gets flushed through the system after treatment
- Polish the fuel or clean the tank if contamination is heavy, so fresh fuel isn’t reinfected
- Fix the cause, such as a damaged cap, seal or vent, or the problem will return
We stock a range of fuel additives, including biocides, suitable for heating oil, red diesel and DERV tanks.
Does petrol kill diesel bug?
No, and you shouldn’t try it. It’s an old bit of advice that still turns up online, but adding petrol to diesel or heating oil lowers the fuel’s flash point. That creates a real fire risk, especially in a home heating tank, and it can damage engines and boilers. Use a proper biocide instead.
How to Prevent Diesel Bug
The best way to prevent diesel bug is to keep water out of your tank. Keep tanks well filled to cut condensation, check regularly for water, keep caps and vents sealed, and use a biocide if fuel will be stored for months. Prevention is far cheaper than treatment.
- Keep your tank topped up in summer: less air means less condensation
- Check for water regularly: particularly after heavy rain and before winter
- Keep caps, seals and vents sound: look for cracks, splits or a cap that won’t close properly
- Use a preventative biocide for fuel that will sit for months. Our guide on whether diesel additives work explains when they’re worth it
- Don’t overstock: aim to use fuel within six to twelve months
- Have your tank inspected yearly: OFTEC, the Oil Firing Technical Association, recommends an annual check by a registered technician, which can include a test for water
Preventing diesel bug in a heating oil tank
For heating oil, the biggest risk is over summer, when the boiler barely runs and warm days create condensation. Keeping the tank reasonably full through the warmer months, and checking for water before the heating season starts, prevents most problems.
It’s also worth looking over the tank itself. Cracks, splits, bulging and rust can all let water in. Our guide to spotting heating oil tank damage shows what to look for.
Preventing diesel bug on the farm
On farms, the highest-risk tanks are those that sit idle for long periods such as a red diesel store between busy seasons, a standby generator, or a bowser parked up after harvest. Dosing with a biocide before fuel is left standing, and draining off any water regularly, keeps machinery running when you need it most.
Frequently Asked Questions About Diesel Bug
Can you get diesel bug in heating oil?
Yes, although it’s less common than in diesel. Standard kerosene doesn’t contain biodiesel, so it’s less attractive to microbes and holds less water. But if water collects in the bottom of a heating oil tank, microbes can still grow and form sludge that blocks the boiler’s filter.
Does HVO get diesel bug?
HVO is far less prone to diesel bug because it contains no FAME and doesn’t absorb water the way biodiesel does. That’s one reason it can be stored for much longer than standard diesel. It isn’t completely immune, though. If water gets into the tank, microbes can still grow at the water layer.
Is diesel bug harmful to humans?
Diesel bug isn’t generally considered a health risk in itself. The bigger concern is handling the fuel and sludge. Wear gloves, avoid skin contact and don’t breathe in fumes. Contaminated fuel and sludge must be disposed of properly by a licensed waste carrier, never poured away.
Worried About Diesel Bug? Talk to Stoddards
If you think diesel bug or water might be in your tank, the sooner you check, the cheaper it is to fix. We offer customers a free tank check, where we’ll dip your tank for water and let you know if it needs further attention.
We’ve supplied heating oil, red diesel and DERV across Staffordshire, Cheshire, Shropshire and Derbyshire for over 50 years, and we can add biocide to the same delivery so your fuel goes into a tank that’s ready for it.
Get in touch with our team to book a free tank check or to talk through the right treatment for your tank.