I. Problem Overview
Scavenging is a crucial process in the operation of two-stroke (and four-stroke) engines. It is responsible for expelling the combusted exhaust gas from the cylinders and charging fresh air for the next combustion cycle. Scavenging efficiency directly affects engine power, fuel efficiency, and emissions. Therefore, both excessively high scavenging air temperature and excessively low pressure are issues that require immediate attention and resolution. In the following sections, we will provide a more in-depth analysis and explanation of the causes of failures and countermeasures you provided.

II. Analysis and Countermeasures
Methods Scavenging air is typically cooled by an air cooler before entering the cylinders to increase air density and intake volume, thereby improving engine efficiency. If the scavenging air temperature remains too high after the air cooler, it usually indicates a problem with the cooling system or the air cooler itself.

1.Excessive Scavenging Air Temperature (After Air Cooler)
(1)Causes of the Issue：
1-1. Excessive Cooling Water Temperature or Insufficient Flow in the Air Cooler (Divided into Seawater and Freshwater Cooling; Taking Seawater Cooling as an Example): Air coolers typically use cooling (sea) water as a cooling medium. If the temperature of the cooling water is too high or the flow rate through the air cooler is insufficient, the heat exchange efficiency will decrease, and the scavenging air cannot be effectively cooled. Excessive cooling (sea) water temperature may be caused by overall overheating of the engine cooling system (increase in seawater temperature), cooling (sea) water pump failure, blockage of the cooling water circulation line, etc. Insufficient cooling water flow may be caused by (sea) water pump failure, line blockage, incomplete valve opening, or air in the system, etc.

1-2. Fouling or Scaling of the Air Cooler (Air Side or Water Side): During operation, the air side of the air cooler may accumulate dust, oil, or other particulate matter on the cooling fins. This fouling forms an insulating layer that hinders heat exchange between the air and the cooling surface. The water side may also have scale or rust adhering to the inner walls of the cooling tubes, which similarly reduces heat transfer efficiency and prevents the scavenging air from being adequately cooled. In addition, structural damage to the air cooler itself, such as deformed or corroded cooling fins, affects its cooling performance.

1-3. Excessive Engine Room Temperature: If the overall temperature of the engine room is too high, the scavenging air entering the air cooler will already have an elevated temperature. Even if the air cooler is operating normally, it will be difficult to reduce the scavenging air temperature to the ideal level. Excessive engine room temperature is usually caused by poor engine room ventilation, accumulated heat from the engine, high ambient temperature, or ineffective insulation measures.

(2)Solutions：
2-1. Inspect the Cooling Water System: Verify that the engine seawater cooling system is operating normally, including the seawater pump operation and the condition of the air cooler.
2-2. Disassemble and Clean: Periodically clean the inside of the cooler to prevent the accumulation and blockage of salt scale and marine particles.
2-3. Inspect Engine Room Ventilation Equipment: Check whether the ventilation fans are operating normally, whether the ducts are unobstructed, and whether the air inlets and outlets are blocked. Ensure sufficient air circulation in the engine room to remove heat promptly.
2-4. Contact YANMAR Technical Representatives: If the problem persists, contact professional technical personnel for system adjustment or modification.

2. Insufficient Scavenging Air Pressure
(1)Causes of the Issue：
1-1. Clogged Air Filter or Air Side Obstruction: The main function of the air filter is to filter the air entering the engine, preventing dust and other impurities from damaging engine components. If the air filter is not cleaned or replaced for a long time, it will be clogged with dust, oil, etc., increasing intake resistance and reducing the amount of air entering the turbocharger, thereby reducing the turbocharger's boost effect and resulting in insufficient scavenging air pressure. Similarly, blockage of the turbocharger intake side or the scavenging air lines themselves by foreign objects will also cause insufficient scavenging air pressure.

1-2. Carbon Deposits, Scaling, or Damage on the Turbine and Compressor Sides: The turbocharger consists of a turbine side (exhaust turbine) and a compressor side (scavenging air turbine). The turbine side is in long-term contact with high-temperature exhaust gas, which may produce carbon deposits and scaling, affecting the turbine's rotational efficiency. If the air drawn in by the compressor side is not sufficiently filtered, it may also cause blade fouling, reducing the boost effect. In addition, mechanical failures such as turbocharger bearing wear, blade damage, or seal failure will also lead to insufficient boost pressure, further causing insufficient scavenging air pressure.

1-3. Increased Exhaust Back Pressure (Blockage): Blockage of the exhaust pipe will lead to increased exhaust back pressure, making it difficult for exhaust gas to be smoothly discharged from the cylinders, thereby reducing scavenging efficiency and resulting in insufficient scavenging air pressure, foreign objects blocking the exhaust pipe, or damage to the internal structure of the muffler, etc.

1-4. Engine Room in a Negative Pressure State (Insufficient Ventilation or Improper Operation of Ventilation Equipment): If the engine room has insufficient ventilation, or if the exhaust volume of the ventilation equipment is greater than the intake volume, a negative pressure may form in the engine room. Negative pressure will impede air from entering the engine's intake system, reduce the amount of air entering the turbocharger, and thus cause insufficient scavenging air pressure.

(2)Solutions：
2-1. Clean or Replace: Based on the inspection results, clean or replace the clogged air filter or remove foreign objects from the intake lines.
2-2. Disassemble and Clean: Periodically clean the inside of the cooler to prevent the accumulation and blockage of water (salt) scale, and inspect the cooling fins for deformation or corrosion.
2-3. Inspect Exhaust Manifold and Main Exhaust Pipe: Inspect the exhaust manifold and main exhaust pipe for deformation, damage, or leaks that reduce scavenging efficiency or blockages from foreign objects.
2-4. Inspect Engine Room Ventilation Equipment: Check whether the ventilation fans are operating normally, whether the ducts are unobstructed, and whether the air inlets and outlets are blocked. Ensure sufficient air circulation in the engine room to remove heat promptly.
2-5. Contact YANMAR Technical Representatives: If the problem persists, contact professional technical personnel for system adjustment or modification.

III. Conclusion and Recommendations
Scavenging air temperature and pressure are important indicators for measuring the operating efficiency of two-stroke (and four-stroke) engines. Both excessively high scavenging air temperature and excessively low pressure can have a negative impact on engine performance and reliability. Therefore, it is crucial to regularly monitor the operating status of the scavenging system and take appropriate handling measures promptly based on the problems that arise.