Updated
Guide
Contaminated Diesel & Diesel Bug in Generators
Diesel bug is microbial growth where water meets diesel in a tank, and it typically shows up as blocked filters, hard starting and power loss on standby sets that sit for months. Your engine’s manual sets the water-drain interval; Perkins lists every 50 service hours or weekly. We check the fuel and filters before blaming injectors.
Callout fee waived when you accept our written quote. 1-year workmanship guarantee. See prices
- Fuel checked before injection equipment
- Filters and water separator inspected on site
- Findings explained before we quote
- Onsite across Gauteng
Why it happens
Standby fuel sits still, and still fuel changes
A standby set is built to wait, and that is the problem for the fuel inside 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. Diesel with a biodiesel component holds water more readily than older fuel did.
Where water and diesel sit together, microbial growth lives at the boundary between them. The trade calls it 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.
We check the fuel and the filtration before anyone blames the injectors or the pump. New injection parts on a set that still draws bad fuel only repeat the fault, so we fix the fuel first.
Prevention
Keeping stored diesel usable
Keep the tank full
A fuller tank leaves less air space for humid air to condense into water. A half-empty tank between outages is the worst case.
Drain water on schedule
Where a water separator or tank drain is fitted, drain it with the engine stopped and cool. Perkins lists the fuel-tank drain every 50 service hours or weekly.
Turn the fuel over
Fuel that is used and replaced does not sit long enough to degrade badly. A set that only test-runs for a few minutes barely uses any.
Service on a schedule
We check fuel condition and change the fuel filters at each service, which is when we usually catch early contamination, long before an outage.
Keep records
Note each filter change and how much water you drained. A rising trend tells you the tank needs attention.
Know how old the fuel is
Fuel delivered years ago is not the product it was. Ask how long it has been in the tank before an outage, not during one.
Typical published fuel-water checks
These checks come from the published Perkins maintenance schedule, a typical example for diesel engines. Your engine’s manual gives the exact intervals for your set.
| Interval | Check | Who does it |
|---|---|---|
| Daily | Fuel system primary filter or water separator | You or a site operator, with the engine stopped and cool |
| Every 50 service hours or weekly | Drain water and sediment from the fuel tank | Site operator or technician |
| At each service | Fuel filters changed when due, fuel condition checked | Technician |
Source: Perkins: How do you maintain a Perkins engine?. Bad fuel that reaches the injection system can lead to diesel injector replacement, at R3,000 – R9,000 on our cost guide, or fuel pump work at R1,800 – R4,500.
Fault guide
Contaminated diesel in a generator: what it can mean and what to do
Water, sediment or microbial growth in stored diesel blocks filters and starves the engine. We check the tank, the filters and the fuel together. This guide separates what you can safely notice from what a qualified technician tests. One symptom never proves that a particular part has failed.
Symptom, likely cause and repair
Find the line closest to what you see. The cause is the most likely one, not a diagnosis: testing confirms it before any part is replaced.
| What you see | Likely cause | Usual repair |
|---|---|---|
| The engine loses power or dies under load | Filters blocked by dirt or sludge from the tank | Fuel system repairs |
| Water in the fuel filter bowl | Condensation or water in the storage tank | Fuel system repairs |
| Black slime on the filters | Microbial growth in fuel that has stood for months | Fuel system repairs |
| Rough running after a fuel delivery | Poor-quality or contaminated fuel | Injector service |
01
What you may notice
- Filters blocking soon after being changed
- The engine losing power, hunting or stopping under load
- Hard starting after the set has stood for months
- Dark, cloudy or slimy fuel, or water at the bottom of a sample
02
Possible causes
Any of these can produce the same symptom. Testing decides which one applies.
- Water from condensation in the tank or a poorly sealed filler
- Microbial growth ("diesel bug") at the fuel-water boundary
- Old fuel that has degraded in storage
- Dirt introduced during delivery or from the tank
- The wrong fuel delivered
03
What you can safely check
Look and note only, from outside the canopy. Do not open panels, covers, fuel lines or the cooling system.
- When was fuel last delivered, and how long has it been standing?
- Have the filters needed changing more often than normal?
- From the outside, is the filler cap sealed and the tank vent intact?
- Do not open fuel lines or drain fuel yourself.
04
Stop using the generator and call if you notice
- Fuel leaking from the tank, lines or filters
- A strong fuel smell in an enclosed area
- The engine stopping repeatedly under load
If it is safe to reach, stop the generator with its normal stop control or emergency stop, keep people away, and call a technician.
05
What a qualified technician typically tests
These are technician checks, not steps to try yourself. Several involve live electrical measurement, fuel or pressurised systems.
- Fuel sample from the tank bottom for water and sediment
- Filter and water-separator condition
- Tank condition, vents and fill point
- Fuel delivery to the engine under load
06
Common repair categories
The repair follows the test result. These are the services this symptom most often leads to.
07
What to send when you book
- Your suburb and how we reach the generator
- Make, model and kVA from the data plate (a photo is ideal)
- Fuel type: diesel, petrol or gas
- Tank size and the date of the last fuel delivery
- How often the filters have been changed recently
08
Related faults and services
Not sure which symptom fits? Try the generator symptom finder.
Safety before you check anything
- Exhaust contains carbon monoxide, which you cannot see or smell. Run a portable generator outdoors only, away from doors, windows and vents, and never run any generator in a house, garage or other enclosed space.
- A standby generator set to automatic can start at any moment, when the mains fails or on its exercise timer. Keep the canopy closed and stay clear of fans, belts and the exhaust.
- Generator output, the changeover and the control panel can be live, and some parts hold stored electrical energy after shutdown. Leave panels, covers and wiring closed for us to open.
- The engine, exhaust and coolant stay hot after running. Never open a radiator or coolant cap on a hot engine.
- Do not refuel a running or hot generator. If you smell gas at a gas-fuelled set, do not operate switches; leave the area and contact your gas supplier.
Carbon monoxide guidance: US Consumer Product Safety Commission: Generators and engine-driven tools (carbon monoxide).
Send the details on WhatsApp
This opens WhatsApp with a short message already typed: brand, model, capacity, symptoms and suburb. Fill in what you know and send it. Blank lines are fine.
- The data-plate photo is added inside WhatsApp after the chat opens: tap the attachment or camera icon and choose the photo. A link cannot attach the photo for you.
- Take the photo only when the generator is switched off and has cooled down. Do not reach over or into a running or hot generator, and keep away from the exhaust.
- A standby generator set to automatic can start by itself when the mains supply fails. If you are not sure how it is set, photograph the plate from a distance or send a photo of the whole set instead.
Typical repair costs for this fault
Published ranges, last reviewed October 2026. The callout is R400 – R700 in business hours and R700 – R1,400 after hours, and it is waived if you accept the quote. Your written quote after diagnosis is the price that applies.
- Fuel system cleaning and polishing
- R1,500 – R3,500
- Fuel filter replacement
- R300 – R800
- Injector replacement (diesel)
- R3,000 – R9,000
Step 9: book a diagnosis visit.
Common Questions
Contaminated diesel: questions
What is diesel bug?
Microbial growth that lives at the boundary between water and diesel in a tank. It appears as a dark slime or sludge, typically blocks fuel filters and causes hard starting, and produces acidic by-products that attack the tank and fuel system.
How does water get into a generator fuel tank?
Mostly through 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. Blocked filters and poor fuel reaching the pump are a common reason a set cranks but will not fire. We check the fuel before we look at the injection equipment.
Do you clean generator fuel tanks?
We assess fuel condition on the visit and replace the filtration as part of the work. If the tank needs cleaning or the fuel needs removing, we quote for what your tank requires, and some tanks need a specialist contractor.
Will a fuel additive fix it?
An additive does not remove water or sludge already in the tank. We deal with the water and the contamination first; any treatment after that is a matter for your fuel supplier.
How often should standby fuel be checked?
Drain water on your engine’s schedule; Perkins lists a daily water-separator check and a tank drain every 50 service hours or weekly. We check the fuel and change the filters at every service.
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 Repairs Filters, pumps and delivery
- Injector Service When injection is damaged
- Fuel Pump Repairs Lift and injection pump faults
- Cranks But Will Not Start Turns over and never fires
- Starts Then Stops Runs briefly, then shuts down
- Diesel Maintenance Checklist Daily, weekly and 500-hour checks
- Generator Servicing Fuel filtration on schedule
- Diesel Servicing & Maintenance Separator drained and filters changed on each service
- Not Used for Months Readiness after a long standstill
- Generator Maintenance Keeping the service record
- Diesel Generator Repairs The wider diesel work
Service Areas
Onsite visits across Gauteng
We come to your generator anywhere in Gauteng, 24/7. Choose your region for local suburbs, call-out details and the faults we see most there.
What to send before we attend
These details help us plan the visit and bring the right parts. Send what you can on WhatsApp; if you can’t find the model number, we still book the visit.
- 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 at the last filter change all help. A photo of the filter element or a fuel sample tells us more than a description.
- Call
- 082 057 4605
- Message us on WhatsApp
What happens next
- We reply with questions about the generator, or to book a visit.
- A technician comes to you. For a breakdown we arrive within 1 hour.
- The fault is diagnosed on site and you get a written quote. No work starts until you accept it.
Charlie, lead technician: Degree in Mechanical Engineering, Degree in Electrical Engineering, Diesel Plant Fitter. Generator specialists since 2000.