TL;DR

Black smoke means unburned fuel: too much fuel or not enough air. Check the air filter first (quickest fix), then inspect the injectors. Blue or white smoke has a different cause and needs separate investigation.

What you might see

black or dark grey exhaust smoke
smoke visible at startup and under heavy load
air filter restriction indicator tripped
excess fuel smell in the exhaust area

Likely causes

Tap to mark as ruled out

Clogged air filter restricting combustion air causing a rich fuel-air mixture
Fouled or worn injectors spraying poorly atomized fuel droplets
Generator running at overload, demanding more fuel than the engine can efficiently burn
Fuel injection timing advanced, causing incomplete combustion

Required tools

Replacement air filter element (correct part number)
True-RMS power meter (kW, not just ammeter)
Exhaust opacity meter (optional for documentation)

Safety first

Diesel exhaust is toxic. Ensure adequate ventilation before running the generator. Do not work in a pit or low-lying area where exhaust gases can accumulate.
Allow the exhaust system to cool before inspecting exhaust components. Exhaust temperatures can exceed 600 degrees Celsius.

Procedure

7 steps · ~30 min

1
Check the air filter restriction indicator. If it shows red or restricted, replace the primary air filter element.[1]
2
Start the generator and observe whether the black smoke clears after the air filter is replaced. If it clears, the air filter was the cause.
3
If smoke continues after a clean air filter, check the generator load against nameplate kW rating with a true-RMS power meter. Sustained overload drives rich combustion.
4
Inspect the exhaust connections for any restriction downstream of the engine that could cause backpressure.
5
Check the fuel filter for restriction. A clogged filter can cause pressure fluctuations that affect injector spray quality.
6
If air filter, load, and fuel filter are all normal and smoke persists, the injectors need inspection by a CAT-trained technician. Injector flow testing and calibration require specialized equipment.[1]
7
Document the smoke color, conditions when it occurs, and all corrective actions for the service record.

Sources

Caterpillar C-Series Generator Set Operation and Maintenance Manual, Caterpillar Inc.
Caterpillar C-Series generator set operation and maintenance manual, air filter replacement and fuel system inspection (general)
View source

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