Diesel bug is a build-up of microbes, mainly bacteria, fungi and yeasts, that live in water at the bottom of a diesel tank and feed on the fuel. The first sign is usually a blocked fuel filter, and getting rid of it properly means removing the water and sludge, renewing the filters and checking the fuel system, not just pouring in an additive.
What diesel bug actually is
The name makes it sound like one creature, but diesel bug is a mix of microbes that are naturally present in the air and soil. They get into fuel during storage and handling, and they stay dormant as long as the fuel is dry. Give them water and they start to multiply.
That is the key point. The microbes live in the water, not in the diesel itself, and they grow at the line where the fuel sits on top of the water. Over time they form slime, sludge and sometimes a fibrous mat at that layer. When the fuel is drawn from the tank, some of that material comes with it, and it ends up in your fuel filter.
It is worth knowing that real diesel bug is less common than its reputation suggests. Dirty fuel, rust from an old tank or plain water in the fuel can cause very similar symptoms. That is why it is worth having the fuel and filter looked at before assuming the worst.
Where the water comes from
Most water gets into a fuel tank as condensation. A tank that is part empty has air above the fuel, and as the temperature drops overnight, moisture in that air condenses on the inside of the tank and runs down into the fuel. Water is heavier than diesel, so it sinks to the bottom and stays there.
Water can also get in through a leaky filler cap or seal, from a contaminated batch of fuel, or from a storage tank that has not been drained in a long time. Once it is there, the microbes have what they need.
Who is most at risk
Diesel bug is mostly a problem for diesel that sits still for a long time. It shows up most in:
- Motorhomes and campervans that are parked up between trips, often with a part-full tank
- Boats, where tanks are exposed to damp air and temperature swings
- Farm and stored fuel, including bulk tanks and drums that are not drained regularly
- Machinery and generators that only run a few times a year
- Seasonal vehicles such as a tow vehicle or a work ute that sits through winter
A daily-driven diesel ute that is filled up often and used every day is at much lower risk, because the fuel is used before water has much chance to build up. That said, a bad tank of fuel can still happen to anyone. If you have a motorhome on a Hiace or Transit chassis, our motorhome and small bus servicing page covers the extra care stored vehicles need.
Signs your diesel has the bug
Because the slime and sludge collect in the fuel filter, most of the warning signs come from a filter that is blocking up:
- Loss of power, especially under load or going up hills
- Rough running, hesitation or surging
- Hard starting, or the engine stalling and refusing to restart
- A water-in-fuel warning light, on vehicles that have one
- A fuel filter that blocks again soon after it was replaced
- A filter that is black and slimy when it comes off
If you draw a sample from the bottom of the tank or the water separator into a clear jar, contaminated fuel can look hazy, dark or have a layer of water and sludge at the bottom. Clean fuel should look clear and bright. Even so, a fuel that looks clean can still carry microbes, and a dirty-looking fuel is not always the bug, so these signs point you in the right direction rather than giving a final answer.
Why it matters on a modern diesel
On an older diesel with coarse filtration, a bit of contamination might just mean a few blocked filters. A modern common-rail diesel is far less forgiving. The high-pressure pump and injectors are made to very fine tolerances and rely on clean fuel to lubricate them.
Water breaks down that lubricating film, and dirt and debris score the precision surfaces inside the pump and injectors. That is why contaminated fuel that is left alone can turn a filter change into a much bigger diesel fuel injection repair. The earlier it is dealt with, the more of the system you protect.
How to get rid of diesel bug
The fix depends on how bad the contamination is, but the principle is always the same: get the water and the growth out of the system, then make sure the fuel system is clean and working.
Why an additive alone is not enough. Biocide additives are sold to kill diesel bug, and they do kill the microbes. What they do not do is remove them. The dead growth and sludge are still in the tank and can still block filters, and the water the microbes were living in is still sitting at the bottom. Fuel suppliers recommend cleaning the tank out before any biocide treatment, rather than relying on the additive to do the job.
What a proper fix looks like. When a vehicle comes to us with contaminated fuel, we can drain the tank, renew the fuel filters, and assess the high-pressure pump and injectors on our test bench to check whether any damage has been done. Catching it early is what limits the damage, so if you know you have had a bad tank of fuel, it is worth getting it in before the engine has run on it for long.
If the bug keeps coming back, that usually means water is still getting in somewhere, or that fuel is being stored for too long. Fixing the source is just as important as cleaning out the tank.
How to prevent diesel bug
Prevention comes down to keeping water out of the fuel and not letting diesel sit for too long:
- Keep the tank full when a vehicle, boat or motorhome is going to sit, so there is less air in the tank for condensation to form
- Drain the water separator if your vehicle has one, and keep an eye on any water-in-fuel warning light
- Change fuel filters on time, at the interval in your logbook, and sooner if the vehicle works hard or sits a lot
- Fill up at busy stations where fuel turns over quickly
- Look after stored fuel, by draining water from the bottom of farm tanks and drums, fixing leaky seals and caps, and using stored fuel within a sensible time
- Use the vehicle, because diesel that is regularly used and replaced has less chance to collect water
Regular servicing helps here too. A fuel filter change is part of a diesel service, and a filter that comes off dirty or wet is often the first warning that something is going on in the tank. See our diesel mechanic page for what a diesel service covers.
Think your diesel has the bug?
If your diesel is losing power, blocking filters or showing a water-in-fuel light, get it looked at before the contamination reaches the injection system. Turbo & Diesel has been working on diesel fuel systems in Hamilton for over twenty years, with an in-house injector and pump test bench at our Frankton workshop.
Call us on (07) 847 3339 or use the contact form, and we can talk through what is going on and give you a free quote. You will find us at 281 Kahikatea Drive, Frankton, Hamilton, open Monday to Friday, 7:30 AM to 4:30 PM.