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Determination Of Overhaul Timing
Generally, the engine needs an overhaul when the compression pressure of the engine becomes low, and the amounts of engine oil consumption and blow-by gas increase.Reduced power output, increased fuel consumption, low oil pressure, difficult in starting, and increased operating noise are also signs that suggest the need for an overhaul; however, since these problems can be caused by various factors, they do not serve as reliable criteria for determining the need for an overhaul.Reduced compression pressure manifests a variety of symptoms, thus making it difficult to accurately determine when the engine needs an overhaul. The following shows typical problems caused by reduced compression pressure.(1) Decreased output power(2) Increased fuel consumption(3) Increased engine oil consumption(4) Increased blow-by gas from breather due to leakage of combustion gas through worn cylinder liners and piston rings(5) Increased gas leakage due to poor seating of inlet and exhaust valves(6) Difficulty in starting(7) Increased noise from engine parts(8) Abnormal exhaust color after warm-up operationThe engine can exhibit these conditions in various combinations.Some of these problems are directly caused by worn engine parts, while others are not.Phenomena described in (2) and (6) can also result from improper injection volume, incorrect fuel injection timing, worn plungers, defective nozzles, and faulty conditions of electrical devices such as battery, starter and alternator.The most valid reason to overhaul an engine is a decrease in the compression pressure due to worn cylinder liners and pistons, as described in (4), and once this is determined, other symptoms should be taken into consideration in order to make the final judgement of whether the engine needs an overhaul.Measurement of Compression Pressure
Preparation For Inspection
Check the following before inspection.(1) Make sure that the engine oil, air cleaner, starter, battery, etc. are in normal operating condition.
Measurement of compression pressureInspection
(1) Move the control lever to the Stop position.(2) Remove the glow plugs from all cylinders, and attach the gage adapter and compression gage to the cylinder to be tested. (3) Crank the engine with the starter, and read the compression gage indication when the indication stabilizes.(4) If the measured compression pressure is lower than the limit, consider overhauling the engine.
(a) Measure the compression pressure in all cylinders.(b) As compression pressure varies with the engine speed, measure the engine speed at the same time.
Measure the compression pressure while the engine is running at 150 to 200 min-1. The oil and coolant temperatures should be between 20 and 30 °C [68 and 86°F].
(a) Measure the compression pressure at regular intervals to keep the record of changes in compression pressure.(b) Compression pressure will be slightly higher when the engine is new or immediately after an overhaul due to tight clearances of piston rings and valve seats, but it decreases to the standard level after the parts break in.