The SEM919 grader is a new product from Caterpillar SEM, retaining the design characteristics of Caterpillar graders. Compared to similar graders from other domestic brands, its appearance is similar, but it stands out in several areas with unique advantages and advanced features.
1. Key Configurations with Proven Technology from Reputable Brands

• Engine: The SEM919 is powered by the Shandong D9 turbocharged engine. This engine belongs to the D114 series, which has been under development since the mid-1980s. After more than 20 years of continuous improvement and innovation, it has become a mature and widely used engine in the construction machinery industry. It is compact, lightweight, durable, fuel-efficient, low noise, low emissions, and highly reliable, making it an excellent engine choice.
  
• Torque Converter and Transmission: The grader features the Hangzhou ZF WG180 torque converter and transmission system. It includes a torque converter and a multi-speed gearbox with an integrated transmission. This setup ensures smooth power delivery, minimal vibration, and stable operation. The transmission system is commonly used in domestic 180-horsepower graders, known for its low failure rate, high transmission efficiency, and ease of maintenance.
  
• Drive Axle: The drive axle is supplied by Xuzhou Meichi, which is known for its strong product development and innovation capabilities. The drive axles are durable, have a long lifespan, and can handle various working conditions. Meichi has established itself as an industry leader.
  
• Hydraulic System: The hydraulic system uses Rexroth load-sensing pumps, CAT directional control valves, and high-quality domestic components. This system is robust, long-lasting, and capable of handling various construction tasks.
  

SEM is set to launch two new motor graders, the SEM919 and SEM921, in the domestic market. In anticipation of the product release, SEM invited operators to experience these machines firsthand and provide valuable feedback to develop more targeted marketing strategies. To gain a deeper understanding of user impressions, SEM invited four experienced motor grader operators to participate in a test drive at their factory in Qingzhou, Shandong.
The test drive took place on July 22 in a spacious dirt field near the SEM factory, which provided an ideal terrain for various grader operations. After an in-depth introduction to the new products by SEM staff, operators began working with the SEM919 and another competing product currently available on the market.
According to SEM, both the SEM919 and SEM921 integrate Caterpillar’s proprietary technology, such as proportional priority and pressure-compensating control valves (PPPC). This technology makes the machines more responsive, improving overall performance and efficiency. The graders also feature world-class blade control systems, and the use of a control linkage pin and seven-hole connecting rods allows for fast and precise blade positioning. The optimized blade curvature and larger throat clearance improve material flow and passability, enhancing production efficiency. Furthermore, the new cooling system design ensures the machines can operate in extreme temperatures ranging from -40°C to 50°C.
The SEM919 and SEM921 are designed to offer three key features: fuel savings, ease of operation, and broad adaptability.
Test Drive Performance
During the test drive, operators put the SEM919 through various tasks including heavy load, light load, trenching, turning (to test maneuverability, turning radius, and efficiency), fly-cutting, and road grading. These tests evaluated the machine's handling capabilities. The machines' configuration, built with mature technology from well-known brands, earned high scores for control performance. Key configurations include:

• Engine: The SEM919 features a Shangchai D9 turbocharged engine, known for its compact size, lightweight design, long service life, good fuel economy, low noise, and high reliability.
  
• Hydrostatic Transmission: The SEM919 uses the Hangzhou ZF WG180 transmission, which combines a torque converter and a multi-gear power shift transmission. This system provides smooth transmission, low vibration, and minimal impact. Additionally, the rear-mounted hydrostatic transmission design saves space in the front and makes maintenance easier.
  
• Hydraulic System: The hydraulic system incorporates Rexroth load-sensing piston pumps, Caterpillar's directional control valves, and domestically recognized components. This robust system ensures long service life and supports various construction conditions.
  

As small-scale engineering projects continue to increase, there is a growing demand for compact and flexible machinery. Skid steer loaders, in particular, have gained popularity due to their small size and versatility. These machines are widely used in road maintenance, snow removal, landscaping, as well as in forestry, farming, and construction sites. BobCat, the global leader in skid steer loaders, recently launched its latest M Series model, the S770, in China. Compared to its predecessor, the S300, the S770 offers increased lifting capacity, greater lift height, and significantly improved hydraulic flow, delivering impressive power. The S770 belongs to BobCat's M Series, which was introduced in 2011 as the company's flagship lineup, representing the highest standard in skid steer loader technology today.
Exterior Design
The S300 belongs to BobCat's K Series, with "300" indicating the rated load capacity. In contrast, the S770 is part of BobCat's M Series, and its "770" refers to the model number within the series, marking a shift in naming conventions where the model numbers no longer reflect the load capacity.
The M Series has a more robust and commanding appearance. Notably, the S770 features a wider rear door panel and introduces a rear door locking mechanism, a standard feature across all M Series machines. The M Series also provides 30% greater visibility compared to the K Series, thanks to improved door design. Additionally, the M Series eliminates the front footstep found in the K Series by lowering the bottom edge of the door, making it easier to access the machine.
The "cat ears," the lighting system on BobCat skid steer loaders, also received a design update. The M Series offers a rounder shape compared to the K Series, featuring robust steel plate protection around the headlights. These new vacuum-sealed headlights provide 50% greater illumination, ensuring excellent visibility during nighttime operations.
Cab and Operator Comfort
The M Series represents a significant step forward in operator comfort and ergonomics. The S770’s cabin is far more luxurious compared to the S300, featuring standard adjustable suspension seats for comfort, 10 air conditioning vents to prevent temperature variations, and a 60% reduction in cabin noise levels compared to the K Series. The external noise is also reduced by 50%, ensuring a quieter working environment.
The S770 is equipped with a pressurized cabin, which prevents dust from entering the interior, a crucial feature when working in harsh environments such as cement plants or mining sites. All M Series machines, including the S770, come standard with this pressurized cabin.
Additionally, the M Series offers more storage compartments and a cup holder, addressing long-standing convenience issues. Legroom is more spacious than in the K Series, providing greater comfort for operators. As more younger workers enter the construction machinery industry, the demand for in-cabin entertainment has risen. In response, BobCat has designed the M Series to accommodate an optional audio system, allowing operators to enjoy music during long hours of work.
Internal Components and Hydraulic System
The M Series introduces several updates to key internal components, including the hydraulic quick-connect system. The hydraulic lines are now routed inside the lift arms, offering better protection for the quick-change mechanism. During the launch event, many attendees expressed interest in the air filter location, which has been relocated in the M Series to a more accessible position on top of the engine, replacing the previous round air filter with a square one. The M Series also adds a centrifugal filter, improving filtration efficiency by 30%.
The M Series also uses a three-point engine mounting system, compared to the K Series’ four-point system. This change results in reduced engine noise and improved stability. The radiator cleaning process has also been simplified; the M Series features a support rod fixation method for easier maintenance compared to the K Series' snap-on system.
Work Performance
Since its debut in 2011, the M Series has been synonymous with high-performance machinery. The S770 is a prime example, with a 15% increase in traction capacity and an 18% faster cycle time for dumping and turning. The S770 achieves a remarkable vertical lift height of 3.35 meters, supported by a hydraulic system with a flow rate of 23gpm and a high-flow rate of 36.5gpm, with a hydraulic pressure of 3500psi, providing substantial power for various tasks.
For example, the S850, another powerful model in the M Series, has an operating weight of 4540kg and a maximum lifting capacity of 1792kg. It also boasts a maximum lift height of 3.66 meters, making it ideal for a variety of tasks.
Each BobCat skid steer loader comes with numerous configuration options, including four control modes (Standard, Power Assist, Advanced Control System, and optional Joystick Control). Additional optional features include a deluxe dashboard, climate control, travel control, Bob-Tach attachment system, high-flow hydraulics, and a power management system.
Standard and Optional Features
Each BobCat skid steer loader comes with the following standard configurations:

• Bob-Tach attachment mounting system
  
• High-strength rear steel plates
  
• Reinforced lift arms
  
• Dual-channel cooling system
  
• Maintenance-free chain systems
  

• Model: S770
  
• Series: M Series
  
• Rated Load Capacity: 1520kg
  
• Tipping Load: 3039kg
  
• Max Lift Height: 3353mm
  
• Operating Weight: 4162kg
  
• Engine Power: 92hp
  
• Engine Type: Turbocharged Diesel
  
• Fuel Tank Capacity (dual fuel tanks): 124.3L (103.1L + 21.2L)
  
• Hydraulic System Pressure: 3500psi
  
• Hydraulic Flow Rate: 87.1L/min
  
• High Flow Rate (optional): 138.2L/min
  
• Travel Speed (Low): 11.4km/h
  
• Travel Speed (High): 19.8km/h
  

Kobelco excavators are renowned in the Japanese market for their fuel efficiency, earning them a reputation as the "Earthwork King" and "Shift King" among many users. These excavators have become the preferred choice for numerous construction professionals, especially those concerned with fuel consumption. However, while some enthusiasts view Kobelco machines as almost "legendary," their performance may not meet the high expectations of all users. The actual value of these excavators depends on the specific needs of the operator, and setting realistic expectations is crucial.
Advantages of Kobelco Excavators

1. Fuel Efficiency
   Compared to other brands like Caterpillar and Komatsu, Kobelco excavators stand out for their low fuel consumption. With rising fuel costs, Kobelco’s fuel efficiency becomes even more appreciated. For example, in earthmoving tasks, a 210-liter tank of diesel can last over 15 hours, and the fuel consumption rate on a six-speed setting is around 14-15 liters per hour. The fuel efficiency of Kobelco’s 200 and 210 models is impressive, particularly in smaller machines.
   
2. Speed and Productivity
   When it comes to earthmoving and loading tasks, Kobelco’s speed and fuel efficiency are hard to match. The 200 and 210 models, especially, offer an exceptional level of productivity. These machines provide a noticeable increase in speed and can perform significantly faster than their competitors, maximizing work output and profitability for the operator.
   
3. Reliability and Comfort
   Kobelco machines are known for their durable build. Despite their relatively low engine power, they are well-suited for earthmoving tasks. The operator cabins in Kobelco machines are spacious and comfortable, with ample room for additional personnel, making long hours more manageable. However, the seats tend to be harder, resembling those in older excavator models, which can be uncomfortable for some users.
   

1. Delicate Engine Performance
   Kobelco engines are somewhat delicate, meaning they are best suited for earthmoving tasks. The quality of the diesel used is crucial; poor-quality fuel can lead to significant engine wear. If the fuel filter is not regularly replaced with genuine parts, the machine's performance may degrade after about three years, leading to serious problems.
   
2. Complex Electrical System
   One common complaint among users is the complexity of the electrical system. As the machine ages, electrical components tend to malfunction, and repairs can be expensive and time-consuming. This issue is a notable weakness for Kobelco excavators, which is something prospective buyers should consider.
   
3. Lower Resale Value
   While Kobelco machines perform well, their resale value tends to be lower than that of American brands. The steel used in the construction of these machines is thinner compared to some competitors, affecting the machine's overall strength. Consequently, the resale market for Kobelco machines is not as strong, and they may depreciate faster than other brands.
   
4. Frequent Alarm System Alerts
   Kobelco's alarm system is designed to warn users of potential issues before they become serious problems. However, users often find that the alarms can be triggered frequently, sometimes even when there is only a minor issue. While this feature helps prevent major damage, it can be annoying and distracting for operators, especially when the alarms are triggered unnecessarily.
   

• Air Filters: The Super 8 includes an additional air filter.
  
• Water Separator: The Super 8 is equipped with an extra oil-water separator.
  
• Improved SCV Valve: The Super 8 features a revised SCV valve.
  
• Updated ECU System: The Super 8’s ECU system has been upgraded to enhance load handling.
  
• Engine Adjustments: In the Super 8, the engine operates more efficiently, with 100 fewer revolutions at idle and a higher top RPM compared to the SK-8.
  
• Side Air Inlets: The number of side air inlets has increased, improving ventilation and efficiency.
  
• Improved Excavation Bucket: The Super 8 models, such as the SK250 and SK260, have upgraded excavation buckets designed for deeper digging.
  

1. Kobelco SK75-8, 1300-Hour Usage Report
   A user shared their experience after 1300 hours of operation. The machine performed well in various tasks, including foundation leveling, trenching, and slope repair. The cabin is spacious, providing comfort even for two people. However, the seat was noted to be somewhat hard, and the inability to see the tracks from the cabin was a disadvantage during loading tasks.
   
2. Kobelco SK75-8, 1000-Hour Usage Report
   After 1000 hours of usage, this user was satisfied with the machine's performance. The cabin is large, with good noise insulation, and the machine is equipped with a powerful engine, delivering fast and efficient work. However, issues with the swing motor and muddy conditions that affected the machine's movement were mentioned. The cost of maintenance was also highlighted, with a total of 4935 yuan spent on 1000 hours of use.
   
3. Kobelco 210, 1600-Hour Usage Report
   After 1600 hours, the user reported that while the machine was effective and offered good value for money, the undercarriage quality could be improved. Despite its strengths, the user noted that the machine wasn’t as robust as Korean models, and its performance in cold weather could be better.
   

I purchased three Komatsu PC400-7 excavators, and after two months of use, the working hours have now reached around 2500 hours for each machine, with the initial hours being about 2000. The machine numbers are 5, 8, and 9, and I will provide an overview of the condition of these three excavators, offering insights for those interested in purchasing a 40-ton class excavator.
Excavator No. 5
This machine is mainly used for soil loosening operations, equipped with a ripper. The ripper was purchased for ¥9800. Initially, we used the bucket teeth from a LiuGong 225 excavator, but it was prone to breaking 5-6 teeth every day. It was considered a miracle if the teeth didn’t break within two days.
After multiple adjustments, we finally switched to Komatsu 400 (Diamond-branded) bucket teeth, which improved the situation. Since the PC400-7 is equipped with an electronic fuel injection engine, it had some issues when it arrived here, due to poor fuel quality at the previous job site. As a result, I had to replace the fuel injectors, which cost ¥13,500.
Excavator No. 8
This machine experienced an issue where the main belt broke within 100 hours of use. We then replaced the belts with a large one, incorporating two smaller belts, which cost ¥1300.
Excavator No. 9
This is the most stable of the three machines and the fastest in terms of operation. It is equipped with a 2.1 cubic meter bucket and has no trouble loading material. It can easily outpace the Doosan DH500 with a 3.2 cubic meter bucket.
Performance Comparison
In general, the Komatsu PC400 is significantly more powerful than the PC360. It is also nearly 10 tons heavier than the PC360, but the price is about ¥400,000 higher. Due to its long arm and deep bucket design, the PC400 is better suited for excavating weathered rock. The Doosan DH500, on the other hand, struggles when trying to cut into weathered rock with its oversized bucket, while the PC360’s wider bucket also has difficulty penetrating the material.
In terms of loading speed, the PC400 outperforms the PC360 by a wide margin, making it a much more efficient machine in this regard.
Fuel Consumption
The fuel consumption of the PC400 has been somewhat surprising. Currently, we operate the machine at full throttle:

• In Mode A (high power mode), the fuel consumption is around 50 liters.
  
• In Mode E (eco mode), the fuel consumption drops to about 40 liters.
  

1. No designated grease points
   Surprisingly, the PC400 does not have dedicated grease points, except for the open area in the rotating section.
   
2. Overweight transportation issues
   The weight of the domestic Komatsu PC400 is 38,500 kg, while the domestic PC400CSE weighs 42,000 kg, and the imported PC400 weighs 42,300 kg. This causes problems with overweight transportation, and even without overloading, we still face penalties. For example, when transporting across provinces, each fine can amount to ¥5000, which makes it risky to use highways.
   

I purchased the Sumitomo 200-3 excavator in early 2002, and it has now accumulated over 17,200 working hours. I’ve been the sole operator since it was new, so I believe I’m qualified to provide a detailed review of this machine.
Advantages of the Sumitomo 200-3
1. Speed
When this machine was new, its speed was exceptional—at least compared to any excavator I had operated before. Although I haven’t used the famous Kato model frequently mentioned in forums, I’ve seen it work, and I can confidently say that my Sumitomo 200-3 outperforms the Hyundai 215-7 in terms of speed (no offense to Hyundai 215 users, just my personal observation).
2. Reliability
This excavator has been quite reliable, with no unusual or obscure issues. Maintenance has been relatively straightforward:

• The small arm oil seals were replaced at around 9,000 hours.
  
• The bucket arm oil seals were replaced at over 10,000 hours.
  
• At 12,000+ hours, I had to replace the large arm seals. Two seals leaked within a week, which was likely due to natural wear.
  
• At over 13,000 hours, I replaced the hydraulic pump O-rings and the valve distribution repair kit.
  
• At over 15,000 hours, I replaced the track chain link segments.
  
• At 16,800+ hours, I had to overhaul the engine due to issues caused by the fuel pump. It had frequent stalling problems, which led to excessive oil burning. However, the engine didn’t have any visible leaks, and the overhaul was mostly a result of the fuel pump issues.
  

This Caterpillar 330D excavator arrived at the site on January 2, 2008, and has since accumulated 7928 hours of operation (I have mentioned this time before, and many people may not believe it, but I am reaffirming today that the hours are entirely accurate. As an operator, I don’t need to boast about my hard work; I’m here to have a sincere conversation with everyone). As the operator of this machine, I will share my experience regarding its faults and advantages.
Advantages Overview
Caterpillar, as a globally recognized brand in the construction machinery industry, has earned a stellar reputation, and its excavators are considered some of the best in the world. The Caterpillar 330D is no exception. It ranks among the top in global sales and also boasts an excellent resale value. Despite some minor issues, the overall performance is commendable.
Common Faults and Maintenance History

1. Oil Consumption:
   • From the time of delivery, this excavator has consumed oil at a relatively high rate, with approximately one bucket of oil consumed every 100 hours. It’s important to note that this is not a fault with the engine but rather a design feature by the manufacturer. All 330D models exhibit this behavior.
     
   • My recommendation is to change the oil every 1000 hours or even longer intervals. At this rate, 1000 hours would consume approximately 10 buckets of oil, which is equivalent to replacing the oil five times.
     
2. Oil Filter Replacement:
   • The oil filter should be replaced every 500 hours to ensure the engine remains in optimal condition.
     
3. Boom Cylinder Maintenance:
   • At 3000 hours, the oil seals of the boom cylinder were replaced, which was done free of charge by the manufacturer.
     
   • At 6000 hours, the boom cylinder cracked, and the cylinder body was replaced, costing approximately 4500 yuan.
     
4. Hydraulic Pump Replacement:
   • At 6500 hours, all six hydraulic pumps were replaced together, with each pump costing approximately 8000 yuan. This damage is primarily due to the poor quality of local oil.
     
5. Water Pump Replacement:
   • At 7000 hours, the water pump was replaced at a cost of around 10,000 yuan.
     
6. Other Issues:
   • Currently, there is slight oil leakage at the fan pump, but it has not yet been addressed.
     
   • The boom cylinder needs a new repair kit.
     
   • The fuel consumption is slightly high, but considering the amount of work performed, the overall cost remains reasonable.
     

1. Excavator Models:
   • There are two models of the Caterpillar 330D excavator in China, with the main difference being the length of the boom. The long boom is 50 cm longer than the short boom, with a bucket capacity of 1.67 cubic meters and 1.87 cubic meters, respectively. I operate the short-boom version.
     
2. Performance:
   • On the construction site, my Caterpillar 330D easily outperforms other 400-series excavators. Its control is extremely precise, and it responds quickly with ample power.
     
3. Cab Issues:
   • The cabin’s sealing isn’t perfect, and dust tends to get in. Additionally, there’s no radio installed in the cabin, which I had to install myself. These are minor drawbacks.
     

The Kubota KX161 is a 6-ton compact excavator introduced by Kubota in the Chinese market. The machine is fully imported and comes with a 47-horsepower engine, weighing 5.7 tons, and equipped with a 0.22 cubic meter bucket. It also features standard attachments such as a breaker pipe, bulldozer blade, and anti-theft key (note: the STD models from 2008 onward do not come with the breaker pipe). The KX161 has achieved solid market performance in China due to the strength of Kubota’s brand. Here's a detailed review from a user who owns two KX161 excavators, sharing their experience with the machine’s pros and cons.
1. Purchase Details

• Purchase Time: March 2007, with approximately 5000 hours of use so far.
  
• Price: ¥200,000 upfront payment + ¥200,000 financing lease + ¥5,000 transportation fee (from Shanghai to Changsha) + ¥5,000 GPS fee, totaling ¥410,000.
  
• Warranty: 2000 hours or one year, whichever comes first, excluding the fuel pump and fuel injectors.
  

• Engine Noise: The Kubota engine is relatively quiet during operation. The engine also has an external fuel cooler, which is an unusual design.
  
• Fuel Consumption: At 1,900 RPM, the fuel consumption is around 4.7 liters per hour for loading tasks, which is considered average.
  

• Hydraulic Pump and Cylinders: The hydraulic system has been stable, though there have been occasional oil leaks. The main cause of these issues is high temperatures during operation, causing some components to leak.
  
• Fuel System: There was an issue with the water separator not being emptied on time, leading to the fuel pump running dry, which resulted in the engine stalling and emitting black smoke. The issue was resolved by replacing the fuel pump at a cost of ¥4,300.
  

• Boom and Arm: The arm broke within 3,000 hours of use due to improper operation with a breaker hammer. The dealer replaced the arm for free, but the user noted that the arms on Kubota’s 6-ton excavators appeared too thin compared to competitors like Komatsu’s PC60 series.
  
• Tracks: The standard 550mm tracks work well in mountainous regions, providing good traction. However, the track roller sagged before 700 hours, requiring multiple replacements. Eventually, the user modified the machine to add extra support to fix the issue.
  

• Cabin Design: The cabin lacks adequate sealing, and the space is cramped. This results in operator fatigue during long hours of operation. In comparison to other machines like Doosan DH55, the Kubota KX161 offers superior digging power and speed, but the cabin lacks luxury and comfort, which affects operator productivity.
  

• Parts Cost: Parts are expensive, with one instance where the cost to replace a broken glass panel reached ¥500. The user found this pricing unreasonable, especially for small items like the glass.
  
• Grease Gun Design: The design of the grease gun storage is inconvenient, as it requires opening both the rear and side covers to access the grease gun. This is a cumbersome process that most operators are not willing to perform regularly.
  

• Flexibility: The Kubota KX161 excels in tight spaces and rough terrain, making it perfect for use in mountain areas or narrow job sites.
  
• Engine Performance: The Kubota engine provides smooth operation with low noise, enhancing user comfort.
  
• Tracks: The 550mm tracks are ideal for working in hilly or uneven terrain, offering excellent traction.
  

• Boom Design: The arms are relatively thin and can break under heavy-duty use, especially when using attachments like a breaker hammer.
  
• High Parts Costs: Replacement parts are significantly expensive, which can lead to high maintenance costs over time.
  
• Cabin Comfort: The cabin’s poor sealing and cramped space lead to operator discomfort, especially on long working days.
  
• GPS System Issues: The GPS system provided with the machine was ineffective, with poor support from the service provider and no useful data for machine management.
  

The SANY SY65 and SY75C mini excavators are commonly used on construction sites, and operators and service personnel often encounter certain issues. These problems often arise from the differences between mini excavators and larger ones, and typically include concerns such as swing bearing clearance, boom drift, minor black smoke, and oil pressure warnings. Below are explanations and solutions for the most frequently asked questions regarding these models:
1. Large Swing Bearing Clearance

• Phenomenon: When the swing brake is released, the bucket still exhibits some swing, usually for about three times. The customer perceives this as an issue with the swing bearing clearance. Additionally, when quickly returning the swing lever to the neutral position, a noise is heard, which is mistakenly believed to be abnormal swing noise.
  
• Cause Analysis: Unlike larger excavators, mini excavators do not have an anti-swing mechanism in the swing motor. The swing is controlled by the hydraulic system, and due to inertia during the braking process, the working arm may swing slightly, which is normal. If a mechanical brake were used, it could damage the swing motor. The noise heard when quickly returning the swing lever is due to the gaps in all gears in the swing reducer. When the lever is returned to neutral, the hydraulic motor brakes, shifting the gear contact surface, causing the metallic collision sound, which is also normal.
  
• Solution: Assure the customer that this behavior is within normal operating parameters and does not require any immediate action.
  

• Phenomenon: Boom drift occurs when the boom, arm, or bucket cylinders move while the excavator is stationary.
  
• Cause Analysis: This phenomenon is caused by the gravity acting on the work equipment when the main valve is in the neutral position. This pressure is transmitted to the main valve, and due to the necessary gap in the valve spool, leakage occurs, leading to the boom drift. The operator’s manual specifies that the bucket must be placed on the ground when not in operation to avoid this issue. For more severe drift (e.g., if the bucket drops more than 95mm within 5 minutes when the oil temperature is around 45°C), this could indicate a fault in the main valve, which may need cleaning.
  
• Solution: Clean the main valve if necessary and ensure the bucket is grounded when the machine is idle.
  

• Phenomenon: A small amount of black smoke appears from the exhaust pipe during engine startup or under heavy load.
  
• Cause Analysis and Solutions:
  
  1. The black smoke may be caused by sudden changes in load, which lead to a temporary decrease in fuel combustion efficiency.
     
  2. The diesel fuel may contain heavy molecules that do not burn completely, causing black smoke. Using high-quality light diesel fuel is recommended.
     
  3. If the engine has previously used poor-quality diesel, it may have caused carbon build-up on the injectors, leading to black smoke. Using better quality fuel for a period of time will often reduce the smoke, or a fuel additive may be used to speed up the process. If the problem persists, cleaning the injectors or adjusting the hydraulic pump may be necessary.
     
• Solution: Educate the customer on the importance of using quality diesel fuel and regular maintenance to prevent such issues.
  

• Phenomenon: The oil pressure warning light occasionally appears on the display when the engine starts, but disappears after a few seconds.
  
• Cause Analysis and Solutions: The excavator has a two-stage oil pressure alarm system. If the warning light comes on for only a few seconds, it is usually not a cause for concern, and the machine can continue to operate. However, it is important to check the oil system, including the oil level, filter, and cleanliness of the oil. If the warning persists for a longer time, the machine should be stopped immediately, and further inspection should be conducted.
  

• Phenomenon: When the pilot lock is lifted, there is a delay of a few seconds before the excavator starts operating.
  
• Cause Analysis: This delay is a design feature of mini excavators to prevent accidental operation when the pilot lock is lifted. It ensures that if the operator touches the control lever while the lock is lifted, the machine will not perform unintended actions, thereby preventing accidents.
  
• Solution: This is a normal safety feature and does not require adjustment.
  

• Phenomenon: The arm cylinder may occasionally stop or move slowly when performing compound movements, especially when the arm is near the ground.
  
• Cause Analysis: To minimize fuel consumption, the return oil back pressure is set low in the design. When the engine operates at a low RPM, the system flow decreases, and the return oil back pressure becomes even lower. This can lead to air suction in the arm cylinder, causing it to stop or move slowly. The recommended solution is to ensure that the engine speed does not drop below 1600 RPM during operation.
  
• Solution: If this does not resolve the issue, removing and adjusting the return oil one-way valve screws may help, but this will increase fuel consumption.
  

• Phenomenon: When using the swing function to level the ground, the excavator feels weak and unable to move the earth effectively.
  
• Cause Analysis: The manual specifically states that swing leveling should not be used on mini excavators. This is because the swing reducer and swing bearing are gear-driven and hardened, which makes them vulnerable to impact during swing leveling. Using the swing function in this way could damage the gears, which is why the mini excavators are designed with reduced swing torque, leading to a lack of power for this operation.
  
• Solution: Avoid using the swing function to level ground to prevent damage to the gears.
  

• Phenomenon: During the replacement of the initial fuel filter, significant diesel waste occurs as the filter is installed with a poor fit.
  
• Cause Analysis and Solutions: In the design of mini excavators, the initial fuel filter is located at the bottom shell. This design causes some difficulty during installation, leading to diesel waste. A more efficient design has been implemented to prevent this issue in newer models.
  

• Phenomenon: The mini excavator struggles to climb steep slopes in high gear, often failing to make it up.
  
• Cause Analysis: Mini excavators do not have an automatic high-low gear switching system. Therefore, when climbing steep slopes, the machine cannot automatically switch to low gear for better traction, resulting in insufficient power. To resolve this, it is recommended to manually switch to low gear when tackling steeper inclines.
  
• Solution: Ensure to switch to low gear when climbing steep slopes to prevent damage to the travel motor.
  

• Phenomenon: While the excavator is moving, there is noticeable jumping and noise from the tracks.
  
• Cause Analysis: The excavator uses tracks, and due to their segmented nature, when the drive wheel, which is gear-shaped, interacts with the track, it causes vibration and noise. This is known as the "polygon effect," where the speed of the machine fluctuates depending on the contact point of the track with the ground.
  
• Solution: This is a normal occurrence. Adjusting the track tension can help reduce vibration and noise, but it cannot be completely eliminated.
  

It has been 3 months since I bought the Doosan DH70 mini excavator. During this time, I’ve been operating both the DH70 and my Yuchai 35-6, so the DH70 hasn’t been used extensively. However, I’m now summarizing my thoughts after using it for 500 hours, highlighting its pros and cons for your reference.
Pros:

1. Compact Design: The machine has a simple, compact appearance, and is designed as a short-tail excavator, making it ideal for working in confined spaces.
   
2. Strong Digging Force: The DH70 has impressive digging power, capable of handling most tasks without struggle.
   
3. Fast Rotation Speed: The machine’s rotation speed is excellent, reaching 11 rotations per minute, improving work efficiency.
   
4. Quick Movements: Each action is executed quickly, and the machine is responsive, making it suitable for fast-paced tasks.
   

1. Slow Compound Movements: The response time for compound actions is slower than expected, which may reduce efficiency in some tasks.
   
2. Air Conditioning and Window Design: The air conditioning lacks an external air circulation function, and the only window that opens is by the door. Unfortunately, it doesn’t allow the operator to stretch their head out for better ventilation.
   
3. Strong Plastic Smell: The interior of the cabin has a strong plastic smell, which is uncomfortable, especially during long working hours.
   
4. Poor Cabin Design: The cabin design lacks adequate storage space, with only one small compartment. Parts often fall to the cabin floor, and it’s unclear whether this is due to vibrations or excess parts from assembly.
   
5. Loud Noise When Digging Hard Soil: There’s a noticeable "grinding" noise when digging into hard soil, even though the grease has been adequately applied. This remains a concern for both the operator and the machine’s longevity.
   
6. Rotation Noise: After 300 hours of use, there was an unusual noise during rotation. However, the issue disappeared by the 310th hour.
   
7. Engine Oil Consumption: While the engine is from Kubota, it does experience some oil consumption. Regular monitoring and topping up are required.
   
8. Poor Fuel Fill Design: The fuel fill cap is poorly designed, making refueling a hassle. I later installed an electric pump to make the process faster and more efficient.
   
9. No Designated Space for Grease Gun: The machine does not have a dedicated storage place for the grease gun, and it is often left lying on the cabin floor, which is not ideal.