TM 5-3895-359-14&P
Minimum Compression Pressure at 600 rpm
Altitude above
Air
4-71N Engine
Sea Level
Density
psi
kPa
feet
meters
515
3551
500
152
0.715
480
3310
2,500
762
0.663
440
3034
5,000
1,524
0.613
410
2827
7,500
2,286
0.567
380
2620
10,000
3,048
0.525
Air density at 500 ft. altitude based on 85F (29.4C) and 29.38 in Hg (99.49 kPa) wet barometer.
TABLE 1
Cylinder
Gage Reading
6.
Perform Steps 2 through 5 on each cylinder. The
psi
kPa
compression pressure in any one cylinder at a given
1
445
3066
altitude above sea level should not be less than the
2
440
3032
minimum shown in Table 1. In addition, the variation in
3
405
2791
compression pressures between cylinders must not
4
435
2997
exceed 25 psi (172 kPa) at 600 rpm.
TABLE 2
EXAMPLE: If the compression pressure readings were
as shown in Table 2, it would be evident that No. 3
between the fuel inlet and return manifold connectors.
cylinder should be examined and the cause of the low
This will permit fuel from the inlet manifold to flow
compression pressure be determined and corrected.
directly to the return manifold.
The pressures in Table 2 are for a 71E engine operating
5.
Start the engine and run it at a 600 rpm.
at an altitude near sea level. Note that all of the
Observe and record the compression pressure indicated
cylinder pressures are above the low limit for
on the gage. Do not crank the engine with the starting
satisfactory engine operation. Nevertheless, the No. 3
motor to obtain the compression pressure.
cylinder compression pressure indicates that something
unusual has occurred and that a localized pressure leak
has developed.
Low compression pressure may result from any one of
several causes:
A. Piston rings may be stuck or broken. To
determine the condition of the rings, remove the
air box cover and inspect them by pressing on
the rings with a blunt tool (Fig. 3). A broken or
stuck ring will not have a "spring-like" action.
B. Compression pressure may be leaking past the
cylinder head gasket, the valve seats, the
injector tube or a hole in the piston.
Fig. 2 - Checking Compression Pressure
10-11-10
