What Makes Fuel Go Bad?
Fuel is not shelf-stable indefinitely. It degrades through chemistry, biology, and neglect — and the consequences show up in blocked filters, failed injectors, and generators that won’t start when you need them most.
The assumption that fuel is inert once it’s in a tank is one of the more expensive misunderstandings in storage and fleet management. Petrol begins oxidising within weeks. Diesel with a biodiesel component can support biological growth within months. Generator fuel sitting in a tank between outages may look fine, smell different, and perform catastrophically. Understanding the mechanisms behind fuel degradation is not an academic exercise — it is preventive maintenance for everything downstream of your fuel supply.
Oxidation — The Chemistry of Fuel Going Stale
Fuel reacts with dissolved oxygen from the moment it leaves the refinery. The question is how fast.
Petrol and diesel are not pure compounds — they are complex mixtures of hundreds of hydrocarbon molecules, each with different stability characteristics. Some of these molecules, particularly the olefins (alkenes) present in petrol, are reactive with atmospheric oxygen. The refinery adds antioxidant packages — hindered phenol compounds and phenylenediamine derivatives — to slow this reaction. Once those additives are consumed, oxidation accelerates.
Hydroperoxide Formation
The first stage of oxidation converts unstable olefin molecules into hydroperoxides. These are relatively innocuous in small quantities but act as reaction initiators — each hydroperoxide molecule catalyses further oxidation, creating a self-accelerating cycle. This is why fuel degradation is not linear: it proceeds slowly at first, then rapidly once the antioxidant package is exhausted and the hydroperoxide concentration climbs above a threshold. The antioxidants in fresh fuel are buying time, not stopping oxidation.
Acid and Aldehyde Formation
Hydroperoxides decompose further into alcohols, aldehydes, and carboxylic acids. Aldehydes are responsible for the sharp, sour smell of stale petrol — recognisable immediately to anyone who has opened a container of fuel that has been stored for several months. Carboxylic acids are directly corrosive to metal fuel system components: fuel lines, tank walls, carburettor jets, and injector bodies. Old petrol does not merely lose performance — it becomes chemically aggressive toward the system it runs through.
Gum and Varnish
The final oxidation products are high-molecular-weight polymers — collectively called gum — that are semi-solid to solid at fuel system temperatures. Gum deposits as a brown-to-black varnish on every internal surface the degraded fuel contacts: injector tips, intake valves, carburettor jets, fuel pressure regulators, and filter media. These deposits do not dissolve in fresh fuel. They narrow flow passages, alter spray patterns, cause sticking valves, and progressively reduce engine performance until the affected components are cleaned or replaced. ASTM D381, the standard gum content test, measures this directly: fresh petrol should show washed gum content below 5 mg per 100 ml. Fuel stored for six months without stabiliser can show values many times higher.
South Africa’s Warm Climate Accelerates Everything
Oxidation rates are temperature-dependent — roughly doubling for every 10°C increase, following the Arrhenius relationship in chemistry. Fuel stored in a sun-exposed above-ground tank in Limpopo at 45°C degrades 4–8 times faster than fuel in a cool European warehouse at 15°C. Storage life ratings on fuel stabiliser products are typically calibrated for temperate conditions. In a South African context, halve the stated storage life before applying it to any above-ground or outdoor storage scenario. Underground tanks are significantly more protected from this effect due to soil thermal insulation — but not immune, particularly in summer.
Water Contamination — The Enabler
Water in fuel doesn’t just cause problems on its own. It enables everything else to get worse faster.
Water and fuel are incompatible in every direction. Water accelerates oxidative degradation by providing a reaction medium for the acid products of oxidation. It enables microbial growth. It causes phase separation in ethanol-blended petrol. It damages high-pressure injection components directly. And it enters storage tanks through more routes than most operators account for.
Ethanol Phase Separation — The Hidden Petrol Risk
South African E10 petrol contains up to 10% ethanol. Ethanol is hygroscopic — it absorbs water vapour from any gas-phase environment it contacts, including the vapour space in a partially filled storage tank or the atmosphere drawn through vent lines. As the dissolved water content in E10 petrol rises toward 0.3–0.5% by volume, phase separation triggers: the ethanol abandons the petrol blend and drops out of solution with the water, forming a distinct water-ethanol layer at the bottom of the tank. This layer has very low energy content, is directly corrosive, and — critically — is what gets drawn into the engine if the fuel pickup sits near the tank bottom. A carburettor or injector receiving a water-ethanol slug behaves exactly as you would expect: the engine stumbles, misfires, and stalls.
Free Water in Diesel — Steam Hammer
Water does not dissolve in diesel. It either settles as free water at the tank bottom or remains as emulsified droplets suspended throughout the fuel after agitation. Free water reaching a common-rail diesel injector operating at 1,600–2,500 bar is instantaneously vaporised on injection into the hot combustion chamber. The volumetric expansion of liquid water to steam is approximately 1,700 times. At injection pressures, this expansion is explosive at the microscopic scale — it erodes the injector nozzle tip, destroying the precision-machined spray holes and degrading the atomisation pattern permanently. A single severe water contamination event can write off a set of injectors that each cost R5,000–R15,000 to replace.
Corrosion of Storage Infrastructure
Water at the bottom of a steel storage tank, combined with the organic acids produced by fuel oxidation, creates one of the more corrosive environments in ordinary industrial use. The tank floor and lower walls corrode, releasing rust particles into the fuel, which block filters and ultimately damage injection equipment. The corrosion of the tank itself eventually causes structural failure — a leaking bulk diesel tank on a farm or mine site is both an environmental liability and an unbudgeted capital expense. Plastic tanks eliminate the corrosion problem but introduce their own degradation pathways from UV exposure and chemical attack by certain fuel components over time.
Diesel Bug — The Biology Problem
If there is water in your diesel tank and the temperature is above 10°C, the conditions for microbial growth are met. In South Africa, that is most of the year, in most places.
Diesel bug is not a single organism. It is a collective term for a community of fungi, bacteria, and yeasts that colonise the interface between diesel fuel and free water. The most well-studied member is Hormoconis resinae — a filamentous fungus that metabolises hydrocarbon compounds in diesel as a carbon source and uses water for cellular functions. It does not live in the fuel or in the water; it lives precisely at the boundary between the two, where it has access to both.
South Africa’s SANS 342 diesel specification includes up to 5% FAME (fatty acid methyl esters — biodiesel). FAME is derived from vegetable oils and is significantly more hygroscopic than mineral diesel: it absorbs moisture from the environment more readily, providing a more hospitable water-rich environment for microbial colonisation. FAME also degrades faster than mineral diesel, producing degradation products that some diesel bug organisms metabolise more readily. The introduction of FAME into South African diesel did not create diesel bug, but it created better conditions for it.
Biomass and Filter Blocking
The primary mechanical damage is straightforward: the microbial colony produces a thick, dark, gelatinous biomass — a biofilm — that accumulates at the tank bottom and in sump points. This material is heavier than diesel and denser than water, making it resistant to simple fuel draw-off. When disturbed by tank agitation (a delivery, a pump cycle, or a vehicle moving), fragments break off and migrate through the fuel system. A well-established diesel bug colony can block a primary fuel filter in hours. Operators who see filter blocking that returns rapidly after replacement — rather than progressively over weeks — are usually dealing with a biological contamination event, not a sediment accumulation problem.
Organic Acid Production and Tank Corrosion
Diesel bug organisms produce organic acids as metabolic by-products — primarily short-chain fatty acids and sulphur compounds if sulphate-reducing bacteria are also present. These acids are directly corrosive to ferrous tank walls, pipework, and filter housings. A mature diesel bug colony in a steel tank accelerates corrosion of the tank floor measurably compared to equivalent storage without biological contamination. The corrosion compounds the contamination: rust particles join the biofilm as additional filter-blocking material, and eventually the tank itself becomes a maintenance liability.
Treatment and the Critical Sequence
Diesel bug treatment requires a specific sequence to be effective. Biocide alone — products such as Biobor JF (a 2-ethylhexyl borate compound widely used in commercial diesel applications) or equivalent registered products — kills the active organisms but does not remove the dead biomass or the water that enabled the colony. The correct sequence is: drain free water → dose biocide → circulate → remove dead biomass physically → verify water removal. Treating with biocide while leaving the free water interface in place allows recolonisation from surviving cells within weeks. Many operators treat repeatedly and achieve only temporary improvement because the water removal step is skipped or incomplete.
Generator Fuel — South Africa’s Most Neglected Fuel
Load shedding changed how South Africans use generators. It didn’t change how most of them maintain the fuel in them.
Before sustained load shedding became a feature of South African life, most backup generators sat dormant for months between tests. Fuel loaded at installation or at the last outage sat in a tank, degrading quietly, while the generator waited. When the next outage arrived, the generator started on fumes of habit and luck.
Extended load shedding changed the usage pattern — generators in many commercial and residential applications ran frequently, sometimes daily. But it introduced a different problem: fuel was loaded in large quantities during panic-buying at the start of severe load shedding stages, and then consumed inconsistently. Tanks were topped up at random intervals with fuel of unknown age from forecourts that may have had their own storage issues. The result is often a generator fuel tank containing a mixture of fuel at different ages, different contamination histories, and occasionally different additive packages — none of it tracked or managed.
Petrol Generators — Carburettor Varnish
Small portable generators predominantly use petrol and carburettor-based fuel delivery. Petrol degrades faster than diesel — varnish formation begins noticeably from 30–60 days in an unsealed or partially filled tank. The carburettor is particularly vulnerable: its precision-machined jets and passages are exactly the kind of narrow-tolerance components where varnish deposits cause failure. A generator that ran perfectly at last test will not start six months later if stale petrol has varnished the main jet. The fix — carburettor cleaning or rebuild — is straightforward but requires a mechanic and costs time the operator does not have during an active power outage. The preventive measure is to either run the carburettor dry before storage (for seasonal use) or use a fuel stabiliser at every fill-up.
Diesel Generators — Diesel Bug in a Warm Confined Tank
Commercial diesel generators — the units that power hospitals, data centres, office buildings, and larger residential complexes — typically have integral fuel tanks that are warm, partially filled, and vented to atmosphere. This is close to ideal for diesel bug development: there is always a fuel-water interface from condensation, the tank temperature is elevated by the generator’s own heat output, and the FAME component in SA diesel provides good microbial nutrition. A generator that runs infrequently does not agitate the tank sufficiently to break up establishing biofilm. The worst-case scenario — and it occurs more often than generator maintenance records suggest — is a unit that fails to carry full load during a critical outage because the fuel filter is blocked with biological contamination that developed silently over three months of standby.
The Bottom-of-Tank Problem
Generator fuel tanks, unlike vehicle fuel tanks, often draw fuel from near the bottom of the tank — where sediment, water, and biological material accumulate. Vehicle fuel systems have multi-stage filtration designed for continuous operation. Generator fuel systems are often simpler, with a single primary filter and no secondary fine filter. When the tank bottom is disturbed by a load start — the high fuel demand of a generator coming online under load — concentrated sediment and contamination is drawn through a system not designed to handle it. The result is fuel filter blocking under load, which manifests as the generator starting, running at low load, and then dying as load is applied. This is one of the most common and most avoidable causes of generator failure during an actual power outage.
Fleet and Bulk Storage Fuel — The Commercial Picture
A bulk tank with contaminated fuel does not damage one vehicle. It damages the entire fleet simultaneously.
Farms, mines, construction companies, and transport operators that manage on-site bulk diesel storage face a version of every problem described above, scaled by tank size and multiplied across every vehicle drawing from the same supply. The economics of fuel quality management look different at this scale: a single contaminated 50,000-litre tank does not produce one blocked filter — it produces blocked filters across every vehicle that refuelled in the contamination window, potentially dozens of vehicles, simultaneously.
The cost of treating a contaminated bulk tank — biocide dosing, professional cleaning, filter replacement across the fleet, potential injector inspection on high-hours units — consistently exceeds the cost of a fuel quality management programme that would have prevented it. The problem is that prevention has a diffuse, invisible cost (the programme), while failure has a concentrated, visible cost (the breakdown event). Budget conversations tend to go in predictable directions.
Slow Stock Turnover in Outdoor Tanks
A bulk diesel tank on a farm or mine site that is filled quarterly and drawn from daily has acceptable turnover. One that is filled annually and drawn from irregularly does not. Slow turnover combines with temperature cycling (outdoor tanks experience full diurnal temperature swings, unlike underground commercial forecourt tanks) to accelerate both oxidation and water accumulation. Fuel sitting in an outdoor tank for more than six months in South African conditions without stabiliser treatment is fuel with a meaningful probability of quality degradation. The calendar cost of replacement fuel is visible; the mechanical cost of running on degraded fuel is distributed across engine hours and shows up months later in increased filter consumption, injector wear, and power output decline.
No Baseline Quality Testing
Most fleet operators do not test their bulk fuel on receipt. They accept delivery, check the docket volume, and assume the product meets specification. In a well-managed supply chain from a reputable terminal, this assumption is usually correct. In an extended supply chain that includes smaller distributors, decanted product, or storage at intermediate points, it is an assumption with real risk attached. Basic fuel testing — density, water content by Karl Fischer titration, and visual assessment for clarity and colour — is inexpensive and provides a baseline that makes contamination events traceable. Without a baseline, operators cannot determine whether a problem originated in their own storage or arrived with the delivery.
Water Draw Discipline
Every bulk diesel tank with a properly positioned sump valve can be drawn for water bottom samples. This takes minutes. Done monthly, it provides early warning of water accumulation before it reaches levels that support biological colonisation. Most fleet operations with formal maintenance programmes include this in their service schedule. Operations without formal programmes do not do it until a problem has already manifested — by which point the water has been present for long enough that biological growth is already established. The intervention is cheap. The timing is the variable that determines whether it prevents a problem or merely confirms that one has already occurred.
What to Do with Fuel You Suspect Has Gone Bad
Visually: fresh petrol is nearly clear to pale yellow. Fresh diesel is pale yellow to amber. Darkened colour, cloudiness, visible sediment, or foam on agitation are all indicators of degradation. Stale petrol has a sharp, sour smell quite distinct from fresh fuel. Contaminated diesel may smell mildly of sulphur or have an unusual sweet-rot odour from biological activity.
Do not run suspect fuel through a modern common-rail diesel injection system. The cost of injector damage is not proportionate to the value of the fuel being saved. Drain the tank, dispose of the fuel appropriately, clean the tank if biological contamination is suspected, and refill with fresh product. For petrol equipment with carburettors, drain the carburettor bowl as well — residual varnish in the bowl will continue to cause problems even after fresh fuel is added to the tank.
Fuel Has a Shelf Life. Treat It Like One.
Every mechanism described in this article — oxidation, water contamination, phase separation, biological growth — is predictable, measurable, and largely preventable. None of it is mysterious. All of it is manageable with the right routine and the right timing.
The pattern in fuel quality failures is consistent: they are not discovered during routine inspection. They are discovered at the worst possible moment — during a power outage, at the start of a long haul, on the first cold morning of the season. The fuel was bad before that moment; it just hadn’t been checked.
For commercial operators, the economics of prevention are not complicated. The cost of a fuel management programme — stabilisers, biocide, monthly sampling, annual testing — is a small, predictable budget line. The cost of a contamination event affecting a fleet, a generator, or a bulk storage system is unpredictable, concentrated, and always larger than the prevention would have been.