DETROIT DIESEL 53
Trouble Shooting Engine 15.2
burned gases. Low air box pressure is caused by a high
The crankcase pressure may be checked with a
air inlet restriction, damaged blower rotors, an air leak
manometer. The manometer should be connected to
from the air box, such as leaking end plate gaskets, or a
the oil level dipstick opening in the cylinder block.
clogged blower air inlet screen. Lack of power or black
Check the readings obtained at various engine speeds
or grey exhaust smoke are indications of low air box
with the Engine Operating Conditions in Section 13.2.
pressure.
Exhaust Back Pressure
High air box pressure can be caused by partially
A slight pressure in the exhaust system is normal.
plugged-cylinder liner ports.
However, excessive exhaust back pressure seriously
affects engine operation. It may cause an increase in
To check the air box pressure, connect a manometer to
the air box pressure with a resultant loss of efficiency of
an air box drain tube.
the blower. This means less air for scavenging which
results in poor combustion and higher temperatures.
Check the readings obtained at various speeds with the
Engine Operating Conditions in Section 13.2.
Causes of high exhaust back pressure are usually a
result of an inadequate or improper type of muffler, an
Air Inlet Restriction
exhaust pipe which is too long or too small in diameter,
Excessive restriction of the air inlet will affect the flow of
an excessive number of sharp bends in the exhaust
air to the cylinders and result in poor combustion and
system, or obstructions such as excessive carbon
lack of power. Consequently the restriction must be
formation or foreign matter in the exhaust system.
kept as low as possible considering the size and
capacity of the air cleaner. An obstruction in the air inlet
The exhaust back pressure, measured in inches of
system or dirty or damaged air cleaners will result in a
mercury, may be checked with a manometer in the
high blower inlet restriction.
engine diagnosis test kit J 9531-01. Connect the
manometer to the exhaust manifold by removing the
The air inlet restriction may be checked with a water
1/8" pipe plug which is provided for that purpose. If
manometer connected to a fitting in the' air intake
there is no opening provided, drill an 11/32" hole in the
ducting located 2" above the air inlet housing. When
exhaust manifold companion flange; then tap the hole to
practicability prevents the insertion of a fitting at this
accommodate a 1/8' pipe plug.
point, the manometer may be connected to the engine
air inlet housing. The restriction at this point should be
checked at a specific engine speed. Then, the air
cleaner and ducting should be removed from the air
inlet housing and the engine again operated at the same
speed while noting the manometer reading.
Check the readings obtained at various speeds (at
noload) with the specifications in Section 13.2.
The difference between the two readings, with and
without the air cleaner and ducting, is the actual
Air Box Pressure
restriction caused by the air cleaner and ducting.
Proper air box pressure is required to maintain sufficient
air for combustion and scavenging of the
Check the normal air inlet vacuum at various speeds (at
no-load) and compare the results with the Engine
Operating Conditions in Section 13.2.
PROPER USE OF MANOMETER
The U-tube manometer is a primary measuring device
The height of a column of mercury is read differently
indicating pressure or vacuum by the difference in the
than that of a column of water. Mercury does not wet
height of two columns of fluid.
the inside surface; therefore, the top of the column has
a convex meniscus (shape). Water wets the surface
Connect the manometer to the source of pressure,
and therefore has a concave meniscus. A mercury
vacuum or differential pressure. When the pressure is
column is read by sighting horizontally between the top
imposed, add the number of inches one column of fluid
of the convex mercury surface (Fig. 2) and the scale. A
travels up to the amount the other column travels down
water manometer is read by sighting horizontally
to obtain the pressure (or vacuum) reading.
between the bottom of the concave water surface and
the .scale.
1970 General Motors Corp.
October, 1970
SEC. 15.2 Page 3
