Guide
Contaminated Diesel & Diesel Bug in Generators
A standby generator can sit for months between outages, and the fuel in the tank does not stay as it went in. Water, sludge and microbial growth are a common reason a set that was fine last year now blocks filters or will not fire.
- Fuel checked before injection equipment is condemned
- Filters and water separator inspected on site
- Findings explained before work is quoted
- Onsite across Gauteng
Why it happens
Standby fuel sits still, and still fuel changes
A standby set is built to wait. That is also the problem for the fuel in it. Diesel absorbs water over time, tanks breathe as temperatures swing between day and night, and condensation collects at the bottom where the pickup is. Modern diesel with a biodiesel component holds water more readily than older fuel did.
Where water and diesel sit together there is an interface, and that interface is where microbial growth lives. This is what the trade calls diesel bug. It forms a dark slime that blocks filters, coats tank walls and breaks down into acidic by-products that attack the tank and the fuel system.
ProService Generator Repairs checks the fuel and the filtration before anyone blames injectors or a pump. Replacing injection equipment on a set that is still being fed bad fuel does not fix the fault, it repeats it.
Symptoms
How contaminated fuel usually presents
These are the complaints that most often turn out to be a fuel-condition problem rather than a mechanical failure.
Filters blocking early
A fuel filter that restricts again soon after it was changed is the classic sign. Clean fuel does not need a new filter every few runs.
Cranks but will not fire
The engine turns over willingly and never catches because delivery is restricted or the fuel reaching the pump is not combustible. This overlaps with the cranks-but-will-not-start page.
Starts, then dies under load
Enough fuel passes at idle and not enough when the set is asked to work. The engine starts, runs, then shuts down when load is applied.
Dark slime or sludge
Visible growth on a filter element, in a water separator bowl or on a dipstick sample is direct evidence rather than a theory.
Water in the separator
A separator bowl that keeps filling with water points at the tank, not at the separator. The source matters more than the drain.
Smoke and rough running
Poor-quality fuel burns badly, which can show as smoke or uneven running. Smoke has other causes too, covered on the black smoke and white smoke pages.
The visit
What is checked and what is done
The order matters. The fuel condition is established before parts are recommended.
Sample the tank
Fuel is drawn from low in the tank, where water and sludge collect, and inspected. A clear sample and a cloudy, dark or layered sample lead to different work.
Inspect the filters
The primary filter, the secondary filter and the water separator are examined. What is on the element describes the state of the tank.
Check for water
Water is drained from the separator and the tank low point where a drain is fitted. Finding water is a result, not an inconvenience.
Replace filtration
Blocked elements are replaced as part of the work. On a badly contaminated system more than one filter change can be needed as the system clears.
Address the fuel itself
Where the fuel is beyond use, the options are cleaning or removal and replacement. Tank cleaning and fuel disposal are quoted for what the tank needs, and some tanks require a specialist.
Confirm the engine side
Once clean fuel is reaching the engine, delivery and running are rechecked. If injection equipment is genuinely damaged, that is diagnosed on the fuel system and injector pages.
Prevention
Keeping stored diesel usable
Keep the tank full
A fuller tank has less air space for the humid air that condenses into water. Half-empty tanks between outages are the worst case.
Drain water regularly
Where a water separator or tank drain is fitted, draining it on a schedule removes the layer that microbial growth needs.
Turn the fuel over
Fuel that is used and replaced does not sit long enough to degrade badly. A set that only test-runs briefly barely consumes anything.
Service on a schedule
Fuel filtration is part of a generator service, which is when early contamination tends to be caught rather than discovered during an outage.
Keep records
Noting filter changes and water drained makes a worsening trend visible. Maintenance plans exist to hold that record.
Mind how long fuel has sat
Fuel delivered years ago is not the same product today. How long it has been in the tank is a fair question to ask before an outage, not during one.
Common Questions
Contaminated diesel — questions
What is diesel bug?
It is microbial growth that lives at the boundary between water and diesel in a tank. It appears as a dark slime or sludge, blocks fuel filters and produces acidic by-products that attack the tank and fuel system.
How does water get into a generator fuel tank?
Mostly by condensation. Tanks breathe as temperatures rise and fall, and the moisture in that air collects in the tank. Diesel with a biodiesel component also holds water more readily than older fuel.
Can contaminated diesel stop a generator starting?
Yes. Restricted filters and poor-quality fuel reaching the pump are a common reason a set cranks and will not fire. The fuel is checked before injection equipment is blamed.
Do you clean generator fuel tanks?
Fuel condition is assessed on the visit, and filtration is replaced as part of the work. Where a tank needs cleaning or the fuel needs removing, that is quoted for what the tank actually requires, and some tanks need a specialist contractor.
Will a fuel additive fix it?
An additive does not remove water or sludge that is already in the tank. The water and the contamination are dealt with first. Anything beyond that is a decision for the fuel supplier, not a repair promise.
How often should standby fuel be checked?
Practically, whenever the set is serviced, and before a period when it is likely to be needed. Fuel filtration and water draining are part of a generator service rather than a separate errand.
Still unsure what the fault is? Call or WhatsApp 082 057 4605 for onsite diagnosis.
Related Pages
Fuel, filters and the faults this causes
- Fuel System RepairsFilters, pumps and delivery
- Injector ServiceWhen injection is genuinely damaged
- Fuel Pump RepairsLift and injection pump faults
- Cranks But Will Not StartTurns over and never fires
- Starts Then StopsRuns briefly, then shuts down
- Black SmokeBurning fuel badly
- Generator ServicingFuel filtration on schedule
- Maintenance PlansHolding the service record
- Diesel Generator RepairsThe wider diesel work
Service Areas
Onsite visits across Gauteng
Technicians travel to the generator. These are the main Gauteng areas. Suburb pages sit under each region.
What to send before we attend
ProService Generator Repairs confirms an onsite visit from these details. Send what you have. A missing model number does not stop the booking.
- Location
- The suburb or site, so the visit can be booked in the right area.
- Make and model
- The generator brand and model, if they are known.
- kVA
- The size printed on the data plate.
- Fuel type
- Diesel, petrol, LPG, natural gas, or dual-fuel.
- Controller model
- The module name on the panel, when it is visible.
- Fault or alarm
- What the set does, and any text on the screen.
- Photos
- The generator, the data plate, and the controller or alarm. Send these on WhatsApp.
Book a Visit
Tell us what the filters look like
The brand, the hours and what came out on the last filter change are useful. A photo of the filter element or a fuel sample helps more than a description.
- Call
- 082 057 4605
- Message us on WhatsApp