Measurement, Inspection And Adjustment; Measuring The Compression Pressure - Yanmar ViO12-2A Service Manual

4-2 Measurement, Inspection and Adjustment

4-2-1 Measuring the Compression Pressure

1) Measurement Procedure
(1) Warm up the engine. Remove the fuel injection pipe
and the fuel injection valve from the cylinder to be
measured.
(2) Crank the engine before attaching the compression
gauge adapter.
Note :
Crank the engine when the regulator handle has been
turned to "STOP" position (no injection state).
(3) Attach the compression gauge adapter and the
compression gauge to the cylinder to be measured.
Note :
Never fail to attach the gasket to the tip of the adapter.
(4) Crank the engine with the starter motor until the
reading of the compression gauge is stabilized,
according to the above step (2) and Note.
Compression pressure at 250 rpm MPa
Standard
3.14 to 3.34
2) Trouble Diagnosis Through Measurement of Compression Pressure
Reduction in the compression pressure of the cylinder is one of the major causes of the increase in blow-by gas
(which causes contamination and increased consumption of lubricating oil, and other trouble) and the engine starting
failure. The compression pressure is influenced by the following factors :
(1) Degree of the clearance between the piston and the cylinder ;
(2) Degree of the clearance around the intake / exhaust valve seats ; and
(3) Gas leak through the nozzle gasket or the cylinder head gasket.
This means that the compression pressure decreases as the engine parts are worn and lose durability through the
use of the engine over a long period of time.
In addition, the compression pressure is also decreased by scratches on the cylinder or the piston which are caused
by dust or other foreign matters drawn in through the soiled filter element of the air cleaner, and wear or breakage of
any piston rings. Therefore, the condition of the engine should be diagnosed by measuring the compression pres-
sure.
4. ENGINE
[Engine compression pressure (Reference values)]
Limit
2.55
4-2-1
Compression gauge

Measuring the compression pressure

Variations of compression
pressure among cylinders
MPa
0.2 to 0.3