The diesel generator set operates normally under light or no load, but stalls once subjected to heavy load. A typical fault analysis for new operators is to check the issues one by one in order of high occurrence.

The diesel generator set operates normally under light or no load, but stalls once subjected to heavy load. A typical fault analysis for new operators is to check the issues one by one in order of high occurrence.

Today, let's talk about the most common problem with diesel generator sets. The unit was running fine before, but now as soon as you put a load on it, it loses speed and power, and in severe cases, it stalls completely. However, when running without load or under a light load, the speed and sound are stable, and it looks completely normal. Many people don't know where to start when they encounter this issue, but it's actually not that complicated.

Below is an article I've put together based on years of experience. I hope it's helpful for everyone, and if there's anything missing, feel free to leave comments and share your thoughts.

1. Insufficient fuel supply in the fuel line (most common)

- Clogged diesel filter, air leak in fuel pipes, pressure loss in manual fuel pump, poor fuel flow from fuel tank
- Low load consumes little fuel and is sufficient, but under high load the instantaneous fuel supply cannot keep up, causing engine to stall due to suppressed pressure

- Symptoms: RPM drops instantly under load → vibration → engine stalls

2. Intake blockage, insufficient air intake

- Severely dirty air filter, turbocharger malfunction, air leak in intake pipe
- Diesel engines require a large amount of air under high load; lack of oxygen causes weak combustion, directly leading to engine stall

3. Poor atomization of fuel injectors, insufficient fuel pump supply

- Worn high-pressure fuel pump, idle fuel amount set too low
- Sufficient under light load, insufficient fuel supply under high load, resulting in weak power and stalling

4. Governor malfunction (freewheel, governor failure)

- Weak or stuck governor spring
- After loading, engine speed cannot be maintained, instantaneous speed drop causes stalling

5. Alternator overload protection or undervoltage protection shutdown
- Not an engine stall; the controller shuts down automatically when voltage is too low
- Upon loading, voltage drops sharply → controller detects fault and shuts down

6. Low engine oil pressure
- Oil pressure drops under heavy load, and oil protection immediately shuts down the engine

Quick self-check guidelines:

1. Black smoke before stalling → insufficient fuel supply in fuel line/air filter/fuel pump
2. White smoke before stalling → poor intake, low cylinder pressure, bad fuel injection
3. Engine stops immediately under any load without shaking → ECU protection shutdown
4. Severe problem when cold, slightly better when hot → fuel line air leak or clogged filter

Simplest troubleshooting steps (without disassembling the machine):

1. Replace/check diesel filter
2. Check intake pipe for leaks, clean air filter
3. Bleed air from fuel line, check if manual fuel pump leaks pressure
4. Check controller fault codes to see if it is under-voltage or low-speed protection