F i g u r e 4 A - 2 . C h e c k i n g Compression
Pressure
ing motor to obtain the compression
pressure.
6. Perform Steps 2 through 5 on each
cylinder. The compression pressure in
any one cylinder at a given altitude
should not be less than the minimum
shown in Table 4A-1. In addition, the
variation in compression pressure be-
tween cylinders must not exceed 25 psi
( 1 7 2 kPa) at 600 RPM.
Figure 4A-1. Locating a Misfiring
For example, if the compression pressure
Cylinder
readings were as shown in Table 4A-1, it
would be evident that No. 3 cylinder
Check the compression pressure as fol-
should be examined and the cause of the
lows:
low compression pressure be determined
and corrected.
1. Start the engine and run it at approx-
imately one-half rate load until normal
The pressures in Table 4A-1 are for an
operating temperature is reached.
engine operating at an altitude near sea
level.
All of the cylinder pressures are
2. Stop the engine and remove the fuel
above the low limit for satisfactory en-
pipes from the injector and fuel connec-
gine operation.
Nevertheless, the No. 3
tors of the No. 1 cylinder.
cylinder compression pressure indicates
that something unusual has occurred and
3. Remove the injector and install an
that a localized pressure leak has devel-
adaptor and pressure guage from Diag-
oped.
nosis Kit J 9531-01 (see Figure 4A-2).
Low compression pressure may result
4. Use one of the fuel pipes as a jumper
from any one of several causes:
connection between the fuel inlet and re-
turn manifold connectors. This will per-
A. Piston rings may be stuck or broken.
mit fuel from the inlet manifold to flow
To determine the condition of the
directly to the return manifold.
rings, remove the air box cover and
inspect them by pressing on the rings
5. Start the engine and run it at a 600
with a blunt tool (see Figure 4A-3 on
RPM.
Observe and record the compres-
page 4A-4). A broken or stuck ring
sion pressure indicated on the gauge.
will not have a "spring-like" action.
Do not crank the engine with the start-
4A-3
Engines
