Black smoke
Generator Blowing Black Smoke
Black smoke means your diesel generator is getting more fuel than its air supply can burn cleanly. The usual causes are a blocked air filter, an overload, faulty injectors, a turbo or intake problem, or a governor that over-fuels. We check the air, the load and the fuel delivery together, so you never pay for injectors the engine did not need.
Callout fee waived when you accept our written quote. 1-year workmanship guarantee. See prices
- Air, load and fuel checked together
- Injectors tested before we quote them
- Written quote before any work
- Onsite across Gauteng
Incomplete diesel combustion
What does black smoke from a generator mean?
Black smoke is soot: diesel that did not burn completely. Either the engine is getting too much fuel or it is not getting enough air, and a heavy load can bring on both at once. We work out which before we name a part.
When it smokes matters as much as the colour. A puff on a cold start that clears quickly is normal for many diesels. A plume that carries on under load is a fault, so tell us what was switched on when it happened.
White or grey smoke is a different problem, usually unburnt fuel on a cold engine or, less often, coolant in a cylinder. Our white smoke guide covers it.
Measured, not guessed
Load against the rating
We compare the load your generator actually carries with its kVA rating. Too much load makes it smoke, and so can too little: Caterpillar warns that a diesel set run for long periods below about 30% of its rated load can wet stack, leaving unburnt fuel and soot in the exhaust. Knowing where your set sits tells us whether to look at the load, the air supply or the injectors first.
Fault guide
Generator blowing black smoke: what it can mean and what to do
Black smoke means the engine is getting more fuel than it can burn cleanly. We check the air supply, the load and the injection, because black smoke alone does not prove an injector has failed. 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 |
|---|---|---|
| Black smoke only under heavy load | Overload or a restricted air filter | Load and sizing check |
| Black smoke at every load | Worn or leaking injectors | Injector service |
| Black smoke with poor power | A turbo, intake or exhaust restriction | Generator fault finding |
| Black smoke with rough running | Fuel pump delivery or injection timing | Fuel pump repairs |
01
What you may notice
- Black or dark grey exhaust smoke
- Smoke that gets worse when load comes on
- Soot around the exhaust outlet
- The engine labours or loses speed with the smoke
02
Possible causes
Any of these can produce the same symptom. Testing decides which one applies.
- A blocked air filter or restricted intake
- Overload, or the engine labouring under a heavy load
- Injector faults or incorrect fuel delivery
- A turbocharger or intake problem, on engines that have one
- Long periods of light load leaving unburnt fuel and carbon in the exhaust (wet stacking)
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.
- Is the smoke only at start-up, constant, or only under load?
- What is connected when it smokes?
- Does the set run for long periods at very light load?
- When was the air filter last changed, according to the service record?
04
Stop using the generator and call if you notice
- Smoke drifting towards doors, windows or occupied areas: exhaust contains carbon monoxide
- Heavy smoke with the engine labouring or losing speed
- Flames or sparks from the exhaust
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.
- Air filter and intake restriction
- Load against the set rating
- Injector condition and fuel delivery
- Turbocharger and intake system, where fitted
- Exhaust condition and service history
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
- When the smoke appears: start-up, constant or on load
- What is usually connected to the generator
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.
- Air filter replacement
- R250 – R700
- Injector replacement (diesel)
- R3,000 – R9,000
Step 9: book a diagnosis visit.
Related Pages
Injectors, fuel and load
- Diesel Injector Service Spray and combustion
- Fuel Pump Repairs How much diesel reaches the engine
- Fuel System Repairs Filters and contamination
- Engine Overhauls When the engine itself is worn
- Trips Under Load Fails when the building connects
- Blue Smoke Oil burning, not too much fuel
- Diesel Generator Repairs The wider diesel set
Common Questions
Questions about black smoke from a generator
What causes black smoke from a diesel generator?
Fuel that is not burning completely. The usual causes are too much fuel, too little air, a faulty injector or an overload, and we test for each one.
Is black smoke always the injectors?
No. A blocked air filter or a load that is too big for the set will make an engine with healthy injectors smoke. We check the air and the load before we test the injectors.
It only smokes when the load comes on. Why?
The engine is being asked for more power than it can make cleanly at that moment. We check the air supply, the fuel delivery and the size of the load together.
A puff of smoke on start, then it clears. Is that a fault?
Usually not. A brief puff on a cold start is normal for many diesels. Smoke that continues once the engine is warm needs a diagnosis.
Can an overloaded generator blow black smoke?
Yes. Several motors starting together, or a building load bigger than the set, can push it into black smoke and then a trip.
Is black smoke the same as white smoke?
No. Black smoke is soot from burning too much fuel for the air available. White smoke is more often unburnt fuel from cold running, and sometimes coolant. We diagnose them differently.
What should I tell you about the smoke?
Tell us whether it smokes at start-up, at idle or only under load, and what was switched on when it happened. A short video taken from a safe distance helps too.
Still unsure what the fault is? Call or WhatsApp 082 057 4605 for onsite diagnosis.
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 when the smoke appears
At start-up, at idle or only under load? A one-line description is enough for us to book the diagnosis.
- 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.