TM 5-3810-300-24&P-3
15.2 Trouble Shooting (Engine)
the insertion of a fitting at this point, the manometer may
be connected to the engine air inlet housing. The
Check the readings obtained at various speeds with the
restriction at this point should be checked at a specific
Engine Operating Conditions in Section 13.2.
engine speed. Then the air cleaner and ducting should
be removed from the air inlet housing and the engine
Air Inlet Restriction
again operated at the same speed while noting the
manometer reading.
Excessive restriction of the air inlet will affect the flow of
air to the cylinders and result in poor combustion and
The difference between the two readings, with and
lack of power. Consequently the restriction must be kept
without the air cleaner and ducting, is the actual
as low as possible considering the size and capacity of
restriction caused by the air cleaner and ducting.
the air cleaner. An obstruction in the air inlet system or
dirty or damaged air cleaners will result in a high blower
Check the normal air inlet vacuum at various speeds (at
inlet restriction.
no-load) and compare the results with the Engine
Operating Conditions in Section 13.2.
Check the air inlet restriction with a water manometer
connected to a fitting in the air inlet ducting located 2 "
above the air inlet housing. When practicability prevents
PROPER USE OF MANOMETER
PRESSURE CONVERSION CHART
1" water
=
.0735" mercury
1" water
=
.0361 psi
1" mercury
=
.4919 psi
1" mercury
=
13.6000" water
1 psi
=
27.7000" water
1 psi
=
2.0360" mercury
TABLE 3
The height of a column of mercury is read differently than
that of a column of water. Mercury does not wet the
inside surface; therefore, the top of the column has a
convex meniscus (shape). Water wets the surface and
therefore has a concave meniscus. A mercury column is
Fig. 4 - Comparison of Column Height for
read by sighting horizontally between the top of the
Mercury and Water Manometers
convex mercury surface (Fig. 4) and the scale. A water
manometer is read by sighting horizontally between the
The U-tube manometer is a primary measuring device
bottom of the concave water surface and the scale.
indicating pressure or vacuum by the difference in the
height of two columns of fluid.
Should one column of fluid travel further than the other
column, due to minor variations in the inside diameter of
Connect the manometer to the source of pressure,
the tube or to the pressure imposed, the accuracy of the
vacuum or differential pressure. When the pressure is
reading obtained is not impaired.
imposed, add the number of inches one column of fluid
travels up to the amount the other column travels down
Refer to Table 3 to convert the manometer reading into
to obtain the pressure (or vacuum) reading.
other units of measurement.
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