In-depth interpretation of Three Cases of diesel Engine faults

1. It is difficult to start when the clutch pedal is depressed

One Komatsu excavator is equipped with a Mitsubishi S6D115 diesel engine. When the clutch pedal is depressed, the diesel engine will have difficulty starting, and the hot engine cannot be started either. At this point, if you manually rotate the crankshaft, you will feel that it is very difficult for the crankshaft to turn. Only by releasing the clutch pedal will the above phenomenon not occur.

The inspection is as follows: When the diesel engine is running, if the clutch pedal is fully depressed, the clutch will emit a continuous "clanging" sound. When the diesel engine is idling, as long as the clutch pedal is pressed, the engine will immediately shut down. It is initially judged that there is a fault with the clutch.

When the diesel engine is stationary, an on-site inspection is carried out. When the clutch pedal is depressed, the driven disc of the clutch can rotate freely, and there is no damage or abnormality in other components, but the release bearing cannot be turned.

To identify the cause of the malfunction, it was decided to disassemble and inspect the clutch. However, when disassembling the transmission, it was unexpectedly discovered that the inner and outer rings of the release bearing and the cage and other parts were severely burned and seized. After replacing the new separated bearings and testing the machine again, the fault phenomenon disappeared.

2. The diesel engine cannot be started after a major overhaul

A Hitachi excavator was equipped with an Isuzu 6B115 diesel engine. After a major overhaul, the diesel engine could not be started. When it was forcibly started, although the diesel engine was running at a high speed, there was still no sign of fire.

The fuel system was inspected and no blockage in the pipelines or air intrusion was found. Each cylinder of the fuel injection pump pumps fuel, and all the fuel injectors also spray fuel well. The cylinder pressure of the diesel engine is normal, and the fuel supply timing is also fine. Later, it was suspected that the valve clearance was not adjusted properly, and it was decided to readjust the valve clearance. When adjusting the valve clearance, to ensure accuracy, the high-pressure oil pipe was removed and the diesel engine was turned forward. When oil came from the high-pressure oil pipe joint of cylinder I of the fuel injection pump, it was found that cylinder I of the diesel engine was not at the top dead center of compression, while cylinder VI was at the top dead center of compression.

When the diesel engine was turned clockwise, it was found that the working sequence of the fuel injection pump was chaotic. The sequence was IV - II - VI - III - V - I. However, when the diesel engine is rotated counterclockwise, its working sequence is the normal I - V - III - VI - II - IV. This situation is exactly the opposite of the working sequence of the diesel engine and the fuel injection pump.

After disassembling the fuel injection pump for a careful inspection, it was found that the camshaft of the fuel injection pump was installed in reverse. When the camshaft is installed in reverse, the diesel engine will experience a special fault of disordered working sequence and inability to start. After reassembling the camshaft in the correct way and then testing the machine, the fault was eliminated.

3. Idle vibration after major overhaul of the diesel engine

One Komatsu excavator equipped with a Mitsubishi S6D110 diesel engine had severe shaking when idling after a major overhaul. Analysis suggests that for newly overhauled diesel engines, it is highly likely that the fit clearance at a certain part was inappropriate during the assembly process. For this purpose, the diesel engine will be disassembled for a comprehensive inspection.

The clearances of the crankshaft journal and main bearing, connecting rod journal and connecting rod bearing, piston skirt and cylinder wall, piston and piston ring, etc. were inspected, and no problems were found. During the process of disassembling the piston and connecting rod, it was found that the fit clearance between the small end bushing of the connecting rod in cylinder II and the piston pin was too small, only 0.01mm.

After the major overhaul of the diesel engine, the small end bushing of the connecting rod and the piston pin should be able to rotate flexibly. However, due to the too small fit clearance between the small end bushing of the connecting rod and the piston pin in cylinder II, the connecting rod rotates with difficulty, resulting in unbalanced rotation of the crankshaft and causing vibration. This phenomenon is particularly obvious at idle speed. Use a scraper to trim the small end bushing of the connecting rod in cylinder II. While trimming, test fit the piston pin until the gap between the two meets the requirements. Then, the engine was assembled and tested, and the idle vibration fault of the diesel engine was eliminated.