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Chapter 13 Preventive Maintenance and Troubleshooting
Testing Differential Pressure
A differential pressure test checks the con-
dition of an engine by measuring leakage from
the cylinder to other parts of the engine. This test
device and procedure can identify a specifi c worn
or damaged component in the engine that may or
may not be directly related to the cylinder condi-
tion or rings. The device is designed so that spe-
cifi c leakages can be detected and isolated before
disassembling parts of the engine.
The differential pressure tester requires the
application of air pressure to the cylinder being
tested with the piston at top dead center on the
compression stroke.
A schematic diagram of the differential pres-
sure tester is shown in Figure 13-29. As the reg-
ulated air pressure is applied to one side tester
with the air shutoff valve closed, both gauges will
read the same. However, when the air shutoff
valve is opened and leakage through the cylinder
increases, the cylinder pressure gauge will record
a proportionally lower reading.
The differential pressure test can be applied to
single cylinder engines or multiple cylinder engines.
Each cylinder is tested separately and in the same
manner. Use the following procedure to test:
1. Run engine until it is warmed up to provide
uniform lubrication to cylinder walls and rings.
2. Remove the spark plug wire(s) and spark plug(s).
3. Rotate the engine crankshaft until the piston
of the cylinder being tested is at top dead
center of the compression stroke. Both valves
must be closed.
4. Install the holding fi xture supplied with the
tester to prevent the crankshaft from turning
during the test.
5. Thread the adaptor into the spark plug hole.
6. Connect the adaptor on the rubber hose to the
quick disconnect fi tting on the tester.
7. Connect an air source of at least 90 psi to the
tester adaptor.
Make sure all personnel and objects are clear of the
engine shaft, belts, pulleys, or other devices connected
to the engine shaft.
8. Slowly adjust the regulated pressure gauge on
the left with the regulator knob to read 80 psi.
9. The right-hand gauge indicates the relative
pressure of the cylinder being tested. Due to
standard engine clearances and normal wear,
no cylinder is expected to maintain a perfect
80 psi. Generally, a reading of 60 psi or above
is acceptable.
It is important that all cylinders on multi-cylinder
engines have a somewhat consistent reading. Good
judgment should be used as to the allowable tolerance
between cylinders. Recheck a cylinder that has a
reading that is signifi cantly lower (15 psi or more) than
the other cylinder(s). If a second reading is still low,
the cylinder should be suspected of being defective.
10. By listening for the sound of escaping air, you
can determine leakage points in the engine
and the cause of the low pressure reading. A
hissing at the carburetor intake indicates a
leaking intake valve, a hissing in the exhaust
suggests a leaking exhaust valve, and hissing
from the crankcase breather indicates leaking
piston rings. See Figure 13-30.
If the test indicates a valve problem, fi rst check valve
clearance to make sure the valve is not being held
open due to inadequate
Warning
Note
I t t i s important p p t n t ha at a ll cylin nd ers on multi-cylinder
engines e n n s have v a somewhat consistent readin g. Good
jud ju dg ment m n n should o be used a a s t o t h e allowable tolerance
between be e w w n n cylin y nd ers. r Rechec ck a cylinder that h as a
reading re ad i th at a i s s signifi g cantly y lowe r (15 psi or more) than
t th h e e other o o e e cylinder(s). y n r I f a second e readin g is still low,
t th h e e cylinder c c n n r should be suspected p o f bein g defective.
Note
I If t t h e e e test s s indicates n e a valve problem, fi rst check valve
clearance cl e r c c t o make m k su re t he e valve is not bein g held
open op p n n due u u t o o inadequate n e cclearance.ce.aranle e
Figure 13-29.
This schematic diagram shows a differential pressure
tester connected to an engine cylinder. (Eastern
Technology Corp.)
Regulated
pressure gauge
Air shutoff
valve
Cylinder
pressure gauge
Piston at
top dead
center
Metering
orifice
To air
compressor