Troubleshooting By Measuring Compression Pressure - Yanmar 3TNV Series Service Manual

3.3 Troubleshooting by measuring Compression Pressure

Compression pressure drop is one of major causes of increasing blowby gas (lubricating oil
contamination or increased lubricating oil consumption as a resultant phenomenon) or starting failure.
The compression pressure is affected by the following factors:
1) Degree of clearance between piston and cylinder
2) Degree of clearance at intake/exhaust valve seat
3) Gas leak from nozzle gasket or cylinder head gasket
In other words, the pressure drops due to increased parts wear and reduced durability resulting from
long use of the engine.
A pressure drop may also be caused by scratched cylinder or piston by dust entrance from the dirty air
cleaner element or worn or broken piston ring. Measure the compression pressure to diagnose
presence of any abnormality in the engine.
(1) Compression pressure measurement method
1) After warming up the engine, remove the fuel
injection pipe and valves from the cylinder to be
measured.
2) Crank the engine before installing the
compression gage adapter.
*1) Perform cranking with the stop handle at the
stop position (no injection state).
*2) See 12.2-18 in Chapter 12 for the compression
gage and compression gage adapter.
3) Install the compression gage and compression
gage adapter at the cylinder to be measured.
*1)Never forget to install a gasket at the tip end of
the adapter.
4) With the engine set to the same state as in 2)*1),
crank the engine by the starter motor until the
compression gage reading is stabilized.
(2) Standard compression pressure
Engine compression pressure list (reference value)
Engine mode
3TNV82A 3.16 0.1 (31 1)
3/4TNV84 3.24 0.1 (33 1)
3/4TNV84T 2.94 0.1 (30 1)
TNV88 106(T) 3.43 0.1 (35 1)
Compression pressure
at 250 min
MPa (kgf/cm
Standard Limit
2.45 0.1 (25 1)
2.55 0.1 (26 1)
2.45 0.1 (25 1)
2.75 0.1 (28 1)
Deviation among cylinders
-1
2
)
0.2 0.3 (2 3)
57
3. Troubleshooting
2
MPa (kgf/cm )