How to determine if an engine needs a major overhaul？

The daily operating conditions of an engine often reveal impending issues. Timely maintenance can extend the engine's service life and ensure equipment efficiency. Beyond regular scheduled maintenance, it is essential to regularly monitor the engine's operating status to determine if a major overhaul is necessary. Proactive assessment significantly improves the quality of major engine repairs.

1. Significant drop in engine power and noticeable increase in fuel consumption.

Check whether engine power has noticeably decreased. Where possible, use a dynamometer to measure engine power and torque. Without a dynamometer, rely on experience and feel to assess. When the throttle is fully open, the engine's maximum output power is only about 60% of its rated power. Additionally, repairs or adjustments to the fuel system, valve train, or transmission mechanism yield poor results. If fuel consumption increases significantly, investigate the cause. If severe engine wear is the reason, consider whether an overhaul is necessary.  

2. Difficult starting or not starting at all in the presence of coolant temperature at approximate 60°C.

General indicators of degradation of the compression system. For instance, cylinder pressure gauge readings indicate that the compression pressure is less than 60% of original reference value.

3. Engine running with a very loud knocking noise.

Severe internal knocking (“knocking” as it is now commonly referred to) generally means that there are abnormal, violent impacts of metal parts against each other, this is a very severe problem. Possible reasons for this are:

1.Worn or damaged connecting rod bearings (main bearings) or crankshaft bearings (journal bearings).

2.Worn or loose piston pins.

3.（Piston seizure) Pistons and cylinder walls are excessively spaced apart.

4.Increased rate of oil consumption and low oil pressure with little response to adjustment.

There are no external leaks of oil, yet consumption is 3.5% to 4.5% of fuel consumption, or double the recommended rate or more. Thick black smoke coming out of the exhaust pipe with a large amount of oil smoke coming out of the crankcase breather. Crankshaft journal wear is beyond limit including roundness and cylindricity are out of tolerance are exceeded.

5. Cylinder and piston clearance exceeds limit values.

Manufacturers specify limit values for cylinder wear roundness tolerance. If any cylinder reaches this limit, the engine requires overhaul. If cylinder wear is below the limit but the cylinder-piston clearance reaches the limit, replace the piston and piston rings. If the cylinder-piston clearance is acceptable but oil leakage occurs, replace the piston rings.

6. For catastrophic failures, such as bearing seizure, especially when the engine is nearing overhaul, a complete engine overhaul is required to restore full performance.

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