**1. Diesel Engine Failure to Start**  

For a diesel engine to start successfully, it must meet several conditions: sufficient cranking speed, adequate cylinder 

compression, timely injection of finely atomized fuel, and a certain temperature to ensure the atomized fuel reaches ignition conditions.

When the engine fails to start, begin by checking the electrical circuit. If pressing the start button produces no response 

from the starter motor, there may be an open circuit or poor contact. If only a gear collision sound is heard between the 

starter pinion and flywheel ring gear, the gears are not meshing properly. This can usually be resolved by manually rotating 

the flywheel one or several teeth. If the starter motor turns weakly or at a low speed, the battery may be insufficiently charged.

If the starter motor operates with normal power and speed but the engine does not fire—no fuel injector sound is heard and no 

smoke comes from the exhaust—the issue likely lies in the fuel system. Alternatively, if the engine starts but stalls after a few 

rotations, there may be air or water in the fuel.

If the starter motor achieves the required cranking speed and fuel injector sounds are audible, but the engine still struggles to 

start, accompanied by air leakage sounds, insufficient compression may be the cause. After stopping the engine, try rotating 

the flywheel with a screwdriver. If it turns too easily or with minimal resistance, and the flywheel shows no rebound, 

compression is likely inadequate.

Low ambient temperatures, prolonged inactivity (resulting in thickened oil), excessive resistance in moving parts, 

excessive piston-cylinder clearance, or insufficient compression can all make starting difficult. In such cases, preheat 

the engine with hot water or steam before attempting to restart.

**2. Insufficient Power Output of the Generator Set**  

The main symptoms include unstable operation at low speeds, black smoke from the exhaust at high speeds, 

and abnormal noises. If the diesel engine has not yet reached its major overhaul interval, insufficient power is usually 

caused by faults in the fuel supply system or inadequate cylinder compression.

Begin by checking under no-load conditions. If increasing the throttle allows the no-load engine to reach maximum speed, 

the fault likely lies in the connected machinery. If the no-load speed cannot increase, the fault is within the diesel engine.

Check the temperature at the base of the exhaust manifold. If one cylinder’s temperature is lower than the others, that 

cylinder is either not operating or operating poorly. At low speeds, touch the base with a finger (caution: do not attempt 

at high speeds to avoid burns). Alternatively, spit on the base of the exhaust manifold; if the saliva does not sizzle, that cylinder is faulty.

Pinch the high-pressure fuel line with your fingers. If the pulsation is strong and the temperature is higher than other cylinders, 

the fuel pump is likely functioning, but the injector may be stuck closed or its pressure spring may be too tight. If the pulsation 

is weak and the temperature is similar to other cylinders, the injector may be stuck open or its pressure spring may be broken. 

If there is no pulsation at high speeds and the temperature is higher than other cylinders, the high-pressure fuel pump may be faulty. 

If smoke rings appear from the exhaust at low speeds, the outlet valve spring of the high-pressure fuel pump may be broken or 

the gasket may have failed. If no abnormalities are found in the fuel system, the issue is likely poor cylinder compression.

During operation, if blow-by gases from the oil filler port increase and the smell of oil is strong, the piston-cylinder clearance 

may be excessive, causing poor sealing. When manually rotating the flywheel with a screwdriver after stopping, if the resistance 

varies and does not correspond to the number of cylinders, the cylinder with poor compression can be identified. If there is a 

leakage sound at the cylinder head-block interface, accompanied by smoke and a burnt smell, the cylinder gasket may be leaking. 

If a metallic knocking sound is heard from the cylinder cover, varying with engine speed, the rocker arm-valve clearance may be 

too large. If a leakage sound is heard at the cylinder head, the intake manifold base is hot at low speeds, and a leakage sound is 

audible in the intake pipe after stopping, the intake valve may be leaking. If black smoke appears from the exhaust at high speeds 

and flames are visible at night, the exhaust valve may be leaking.

**3. Sudden Automatic Shutdown During Operation**  

Automatic shutdown of a diesel engine can result from various causes, primarily falling into two categories: interruption of fuel 

supply or obstruction of moving parts.

If the engine gradually slows down during operation, combustion sounds weaken, and coolant temperature, oil temperature, 

and pressure remain normal, but adjusting the throttle has no effect—eventually leading to shutdown—and the flywheel can be 

manually rotated after stopping, the issue is likely insufficient or interrupted fuel supply.

If during operation, oil pressure is normal but oil temperature rises, coolant temperature increases, a paint-like odor is detected, 

coolant output decreases or stops (with steam emerging), the engine runs laboriously with rough noises, and cylinder scoring sounds 

are heard near shutdown—followed by difficulty in rotating the flywheel forward or backward after stopping—and the flywheel 

becomes movable only after the engine cools, the piston may be seized.

If during operation, coolant temperature is normal but oil pressure drops, oil temperature rises, metal debris is found in the oil, 

a burnt oil smell is present, engine vibration intensifies, abnormal noises occur, speed gradually decreases, inertial rotation time 

shortens before stopping, and the flywheel cannot be rotated even after the engine cools, the issue may be main bearing seizure 

(also known as "throwing a bearing").

**4. Rising Oil Level in the Oil Pan**  

An increase in the oil level indicates coolant leakage into the oil. This is typically caused by cracks in the cylinder head, cylinder liner, 

or engine block, or failure of the cylinder liner seal or oil cooler seal.

During inspection, place your hand over the oil filler port. If condensation forms on your hand, coolant is likely mixed with the oil. 

Additionally, if oil pressure is low or absent at low speeds or after stopping, coolant contamination may be the cause.

If the oil level rises, starting becomes difficult ("water lock" occurs), and white smoke emerges from the exhaust at low speeds, 

the cylinder head may be cracked, allowing coolant into the cylinder. If the oil level rises significantly but starting is normal and 

no white smoke appears at low speeds, the issue may be cracks in the engine block or failure of the cylinder liner seal, allowing 

coolant to directly enter the oil pan.

**5. Increased Oil Consumption**  

Increased oil consumption is mainly caused by oil entering the combustion chamber, leaks at engine joints, or failure of the crankshaft oil seal.

If blue smoke appears from the exhaust at low speeds, oil is entering the combustion chamber. This is often due to excessive 

piston-cylinder clearance or misaligned piston rings. If this occurs after maintenance, the oil ring may have been installed backward. 

Leaks can also result from loose screws or damaged seals at joints.

If oil droplets are thrown from the rotating flywheel, the main shaft oil seal is likely damaged.

**6. Low or No Oil Pressure**  

Causes include a faulty oil pump, clogged or leaking oil passages, a broken pressure-regulating spring, or a malfunctioning pressure gauge.

If the pressure gauge pointer remains at a low position during operation and does not change after stopping, the gauge may be faulty.

If the oil pressure gauge shows no movement upon starting, the oil pan level does not drop, and no sound is heard from the fine filter rotor, 

the oil pump may be malfunctioning.

If oil pressure is low and unstable, gradually decreasing, an oil leak is likely. This occurs when weak points in seals develop 

small gaps under pressure, which gradually expand, allowing oil to short-circuit back to the oil pan.

During operation, if adjusting the oil pressure-regulating screw with a screwdriver produces no change in pressure, the pressure-regulating spring may be broken.

**7. Excessive Clearance Between Piston Pin and Connecting Rod Bushing**  

At low speeds, a clear knocking sound is heard in the middle-upper part of the cylinder, intensifying with increased load. 

The sound disappears at high speeds and is unrelated to oil pressure (the pressure gauge shows no fluctuation). After stopping, 

manually rock the flywheel back and forth with a screwdriver. If a knocking sensation is felt, the clearance between the piston 

pin and connecting rod bushing is likely excessive.

**8. Excessive Clearance or Fracture of Connecting Rod Bearing**  

At low speeds under load, a knocking sound is heard in the lower part of the engine block. The frequency of the oil pressure 

gauge needle fluctuations matches the knocking frequency, and the sound varies with engine speed, often louder than the 

combustion noise. After stopping, manually rock the flywheel back and forth with a screwdriver. If a clear knocking sensation is felt, 

the connecting rod bearing clearance is likely excessive or the bearing is fractured.

**9. Crankshaft Fracture**  

During operation, a sudden, severe knocking sound may occur, followed by a sharp drop in oil pressure and automatic shutdown. 

Grasp the crankshaft front pulley and push-pull it axially. If significant axial movement is detected but the flywheel does not move 

synchronously, the crankshaft is likely fractured.

Foshan Generators | Guangzhou Generators | Guangdong Longhong Electromechanical Equipment Co., Ltd. | www.longhongpower.com