(1) Static Compression Method

The static compression method refers to troubleshooting faults while the machine is in a stationary state. Generally, 

the procedure begins by checking the low-pressure fuel circuit, followed by the high-pressure fuel circuit, and finally 

inspecting the fuel injection quality. If necessary, the injection timing should also be examined.

Check the low-pressure fuel circuit. 

First, inspect the fuel circuit for blockages or airlocks. This is done by loosening the drain bolt at the end of the 

low-pressure fuel line and observing the fuel flow. If the fuel flows unevenly or contains air bubbles, 

it indicates a partial blockage or air ingress in the low-pressure fuel circuit. Gradually troubleshoot along the fuel circuit until clean, 

bubble-free diesel flows out, then tighten the bolt. If the fuel flows smoothly without bubbles, 

the low-pressure fuel circuit is functioning properly, and the issue likely lies in the high-pressure fuel circuit.

Check the fuel delivery of the high-pressure fuel pump.  

First, loosen the air vent screw on the high-pressure fuel pump. If no air bubbles emerge, immediately tighten the screw. 

Next, inspect the fuel delivery by positioning the high-pressure fuel pump’s rack at a medium load setting, 

pressing your thumb firmly against the outlet port, and quickly lifting the hand pump lever for the corresponding cylinder 

(for pumps without a hand lever, use a screwdriver to pry the plunger for fuel delivery). If no fuel sprays or insufficient pressure is felt, 

the plunger pair may be worn, or the outlet valve may have poor contact. In this case, remove the high-pressure fuel pipe connector bolt, 

press your thumb against the outlet valve, and check for diesel around the outlet valve. If diesel is present, 

the outlet valve gasket may be damaged or poorly sealed. If this is normal, disassemble the high-pressure fuel pump for further inspection. 

If fuel sprays properly, the plunger pair is functioning correctly. Then, check the outlet valve for integrity. 

If the outlet valve’s decompression band and conical contact surfaces are in good condition, 

the high-pressure fuel pump is not the issue, and the fault likely lies with the injector.

Check the injection quality of the injector. 

Generally, the injector must be removed from the machine and tested using a pressure tester to inspect and 

adjust the injection pressure and quality. Only after confirming a fault should the injector be disassembled for further examination.

“Incorrect injection timing” can also prevent the machine from operating properly. 

Therefore, it is essential to verify whether the injection timing is accurate.

(2) Dynamic Compression Method

The dynamic compression method refers to troubleshooting faults while the machine is running. 

The inspection procedure involves sequentially stopping the operation of each cylinder and observing changes in fault symptoms, 

focusing primarily on exhaust smoke, voltage, and frequency. This helps determine the performance of each cylinder. For example, 

if a fault disappears after cutting off the fuel supply to a specific cylinder, the fault likely lies in that cylinder. 

After cutting off the fuel to a cylinder, manually push the fuel rack and observe changes in engine speed. 

If the cylinder has a significant impact on the engine’s operation, it indicates that the cylinder is functioning properly.

It must be emphasized that the static and dynamic compression methods must be used in combination, 

as a single fault can often manifest in several ways. For example, delayed injection timing may cause insufficient power, 

poor acceleration, and excessive engine temperature. Therefore, based on the symptoms, 

multiple inspection methods should be employed to achieve quick and accurate troubleshooting.

(3) General Steps for Troubleshooting

Common faults in diesel engines include difficulty starting, abnormal exhaust color, power loss, unstable speed, 

low-speed "knocking," and "engine runaway." Here, we take difficulty starting as an example to outline the general troubleshooting steps. 

Causes of difficulty starting in diesel engines include:

- No fuel in the fuel tank.

- Fuel supply failure due to clogged fuel lines.

- Water in the diesel fuel, making ignition difficult.

- Air ingress in the fuel supply system, where the high-pressure fuel pump’s plunger compresses only air, 

  reducing fuel pressure and decreasing or stopping fuel delivery.

- Excessive fuel in the combustion chamber or excessively low temperature, making starting difficult. 

  Low temperatures in the combustion chamber and cylinders hinder fuel atomization, while excessive fuel creates a rich mixture that is difficult to ignite.

- Poor fuel atomization due to suboptimal injector spray quality, making it difficult to form a combustible mixture.

- Insufficient fuel delivery from the high-pressure fuel pump or excessively high or low fuel pressure.

- Injection timing that is too early or too late.

Given the many potential causes of difficulty starting, the following steps can be taken for analysis and inspection:

First, check the fuel level in the tank and whether the fuel valve is open. 

If the fuel level is sufficient and fuel flows when the valve is opened, the fault likely does not lie here.

Crank the engine and listen for a crisp fuel injection sound in the cylinders. If absent, it indicates that fuel is not entering the cylinders. 

In this case, loosen the air vent screw near the last cylinder’s high-pressure fuel pump and observe the following:  

- If air bubbles emerge from the fuel, air has entered the fuel system and must be purged.  

- If water droplets are present in the fuel, it indicates water contamination, and the fuel should be replaced.  

- If fuel flows unevenly, the fuel filter or part of the fuel line is clogged. Clean the fuel passages or the fuel filter.  

- If fuel flows smoothly, the fault may lie with the high-pressure fuel pump or injector. At this point, 

disconnect the high-pressure fuel pipe near the injector and crank the engine (or operate the fuel pump lever) to check 

the high-pressure fuel pump’s fuel delivery. If no fuel or insufficient fuel is delivered, the fault lies with the high-pressure fuel pump, 

which should be disassembled for inspection. If fuel delivery is normal, the fault lies with the injector, which should be disassembled for inspection.

If a crisp injection sound is heard while cranking the engine, the fault does not lie in the fuel supply system. 

Instead, it may be due to excessive fuel in the cylinders, low starting speed, low cylinder temperature, or faults in other components. 

If the cylinder temperature is too low, preheat the engine. If excessive fuel is present, stop fuel delivery from the high-pressure fuel pump, 

open the decompression valve, and crank the engine to expel the excess fuel from the cylinders.

Guangdong Longhong Power Machinery Equipment Co., Ltd. 

specializes in: generators, diesel generators, Cummins generator sets, and Weichai generator sets. For details, visit [https://www.longhongpower.com]