Ⅰ.Judging from performance and operating status (most intuitive)
Fault Phenomenon | Possible Causes | Description |
| 🔻 Difficult Start-up | Insufficient compression, poor fuel injection, piston ring wear | Typical sign of aging |
| 🔻 Power Loss (Significant reduction in output power) | Compression leakage, incomplete combustion, cylinder wear | Generally reaches the critical point for overhaul |
| Significant Increase in Oil Consumption | Piston ring and cylinder liner wear; valve guide wear | Often accompanied by blue smoke from the exhaust |
| ⚫ Abnormal Exhaust Color | Blue smoke: Oil burning; Black smoke: Poor combustion; White smoke: Coolant leakage | Persistent abnormality indicates issues with sealing or fuel injection system |
| ⚙️ Increased Fuel Consumption | Insufficient fuel injection pressure or fuel injector wear | Decreased fuel economy |
| 🔊 Increased Noise or Knocking Sound | Excessive clearance in connecting rod bearings or main bearings, piston slap | Machine shutdown for inspection is required |
| 🌡️ Abnormal Fluctuations in Temperature and Oil Pressure | Aging of lubrication or cooling system | If abnormalities persist, maintenance should be scheduled |
Ⅱ.Judging from the state of mechanical wear (inspection required)
1.Cylinder Pressure or Compression Ratio Test:
- Test the pressure difference between each cylinder using a compression gauge.
- If the compression pressure of any cylinder drops by more than 15%–20% of the standard value, or the difference between cylinders exceeds 10%, it indicates severe wear, and a major overhaul is recommended.
2.Crankcase Venting (Blow-in) Measurement:
- If the amount of gas (combustion gas leaking into) the crankcase is large, it indicates poor piston ring sealing.
- Blow-in exceeding the manufacturer’s standard by 1.5–2 times → Major overhaul signal.
3.Oil Consumption Rate:
- Normal diesel engines consume approximately 0.2–0.5 g/kWh.
- If oil consumption suddenly increases (e.g., doubles or continues to increase), it indicates severe internal leakage.
4.Oil Analysis (Content of Abrasive Particles such as Iron, Aluminum, and Lead):
- Laboratory oil sample analysis is a routine method for professional marine engines or large equipment.
- Excessive abrasive particle content indicates accelerated wear of bearings, pistons, cylinder liners, etc.
III. Judging from disassembly or maintenance records
Item | Standard Reference |
| Cumulative Operating Time | A major overhaul is generally required after 8,000-15,000 hours of operation (varies by model). |
| Number of Minor Repairs | If performance cannot be restored after multiple consecutive minor repairs, a major overhaul is necessary. |
| Wear of Cylinder Liner & Piston Ring | Cylinder bore wear > 0.15mm, or piston ring end gap > 1.5 times the standard value. |
| Crankshaft Journal Wear | Exceeding the allowable limit (generally > 0.05mm). |
| Cylinder Head & Valve Seat Cracks or Deformation | Unrepairable or beyond the specified tolerance. |
| Low Compression Ratio & High Fuel Consumption | No improvement observed even after adjustments. |
Ⅳ.Comprehensive Judgment Process (Practical Version)
- First, check performance → Reduced power, high fuel consumption, increased oil consumption → Initial suspicion.
- Measure data → Compression pressure, oil sample analysis, air volume → Quantitative judgment.
- Check cumulative working hours & maintenance records → Exceeded design life or multiple repairs.
- Determine whether a major overhaul or assembly replacement is necessary.
Ⅴ.Major Items During Major Overhaul
Module | Overhaul Content |
| Cylinder Assembly | Replace cylinder liners, piston rings, and piston pins |
| Cylinder Head | Grind valve seats, replace valve guides, and repair cracks |
| Crankshaft System | Inspect journal wear, replace bearing shells, and align the crankshaft |
| Fuel System | Calibrate fuel injection pumps and replace fuel injectors |
| Lubrication & Cooling System | Clean oil passages and water channels, and replace seals |
| Commissioning | Re-calibrate parameters such as fuel injection timing, idle speed, and oil pressure |
Ⅵ.Wear and Overhaul Limits
Item | Standard Value | Wear Allowance Limit (Repair or Replacement Required) | Inspection Method |
| Cylinder Bore | Ø165.0 mm | Cylinder liner replacement required when wear reaches +0.15 mm | Measured with an inside micrometer |
| Piston Ring End Gap | 0.25–0.50 mm | Replacement required if ≥ 1.0 mm | Measured by placing the ring in the lower part of the cylinder liner |
| Piston Ring Side Clearance | 0.05–0.10 mm | Replacement required if ≥ 0.20 mm | Inspected with a feeler gauge |
| Piston Pin Outer Diameter | Ø55.000 mm | Replacement required if wear ≥ 0.03 mm | Measured with a micrometer |
| Piston Pin and Bushing Clearance | 0.018–0.040 mm | Replacement required if ≥ 0.08 mm | Measured with an inside gauge |
| Connecting Rod Small End Bushing Bore Diameter | Ø55.04–55.06 mm | Replacement required if ≥ 55.10 mm | – |
| Connecting Rod Big End Bearing Clearance | 0.06–0.12 mm | Replacement required if ≥ 0.20 mm | Feeler gauge / Plastic gauge |
| Main Bearing Clearance | 0.06–0.14 mm | Replacement required if ≥ 0.25 mm | Feeler gauge / Plastic gauge |
| Crankshaft Journal Roundness | ≤ 0.01 mm | Unusable if ≥ 0.05 mm | Measured with a dial indicator |
| Crankshaft Axial Clearance | 0.15–0.35 mm | Thrust washer replacement required if ≥ 0.50 mm | – |
| Valve Guide Clearance (Inlet / Exhaust) | 0.05 / 0.07 mm | Replacement required if ≥ 0.15 / 0.20 mm | Inspected with a dial indicator |
| Valve Spring Free Length | 68.5 mm | Replacement required if ≤ 67.0 mm | Measured with a caliper |
| Valve Seat Seal Width | 1.5–2.0 mm | Repair required if ≥ 2.5 mm | Blue oil inspection |
| Fuel Injection Pressure | 260–280 bar | Fuel injector calibration required if ≤ 250 bar | Test bench inspection |
| Compression Pressure (Warm Engine) | 3.2–3.8 MPa | Repair required if ≤ 2.8 MPa | Measured with a compression gauge |
