TM 5-2420-230-24-2
B3.9 and B5.9 Series Engines
Piston (001-043)
Section 1 - Cylinder Block - Group 01
Page 1-83
Test (001-043-012)
Compression Gauge and Adapter
It is very time consuming and expensive to begin removing
internal engine components to diagnose failures. A com-
pression gauge can be used as an aid to detect failures.
Compression Check
If the air and fuel system are functioning correctly, perform
a compression check to determine whether the problem is:
Piston ring sealing
Valve sealing
Cylinder head gasket sealing or a crack in the cyl-
inder head.
NOTE: Due to variables such as; starter and battery con-
ditions that affect engine cranking speed, it is difficult to
establish an absolute value for compression pressure; how-
ever, the following values can be used as guidelines:
New engine (cranking speed @ 250 rpm) 2413 kPa
[350 psi]
Used engine (cranking speed @ 250 rpm) 2068 kPa
[300 psi].
It is recommended that the compression pressure be
checked and compared on all cylinders. All cylinders should
be within 690 kPa [100 psi] of each other.
Piston Ring Sealing
If the compression is low but can be increased significantly
by squirting lubricating oil into the cylinder, the cause of low
compression is inadequate sealing between the piston rings
and the cylinder walls.
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