15.2
Trouble Shooting (Engine)
DETROIT DIESEL 53
Air Box Pressure
connected to the oil level dipstick opening in the
cylinder block. Check the readings obtained at various
Proper air box pressure is required to maintain
engine speeds with the Engine Operating Conditions in
sufficient air for combustion and scavenging of the
Section 13.2.
burned gases. Low air box pressure is caused by a high
air inlet restriction, damaged blower rotors, an air leak
from the air box (such as leaking end plate gaskets) or
a clogged blower air inlet screen Lack of power or
Exhaust Back Pressure
black or grey exhaust smoke are indications of low air
box pressure.
A slight pressure in the exhaust system is normal.
However, excessive exhaust back pressure seriously
High air box pressure can be caused by partially
plugged cylinder liner ports.
affects engine operation. It may cause an increase in
the air box pressure with a resultant loss of efficiency
Check the air box pressure with a manometer
of the blower. This means less air for scavenging
connected to an air box drain tube.
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,
Air Inlet Restriction
an exhaust pipe which is too long or too small in
diameter, an excessive number of sharp bends in the
Excessive restriction of the air inlet wiII affect the flow
exhaust system, or obstructions such as excessive
of air to the cylinders and result in poor combustion
carbon formation or foreign matter in the exhaust
and lack of power. Consequently the restriction must
system.
be kept as low as possible considering the size and
capacity of the air cleaner. An obstruction in the air
inlet system or dirty or damaged air cleaners will
Check the exhaust back pressure. measured in inches
result in a high blower inlet restriction.
of mercury, with a manometer. Connect the manome-
ter to the exhaust manifold (except on turbocharged
Check the air inlet restriction with a water manometer
engines) by removing the 1/8 " pipe plug which is
connected to a fitting in the air inlet ducting located
provided for that purpose. If no opening is provided,
2 " above the air inlet housing. When practicability
prevents the insertion of a fitting at this point. the
drill an 11/32 " hole in the exhaust manifold
manometer may be connected to the engine air inlet
companion flange and tap the hole to accommodate a
housing. The restriction at this point should be
1/8 " pipe plug.
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.
The difference between the two readings, with and
without the air cleaner and ducting, is the actual
restriction caused by the air cleaner and ducting.
Check the readings obtained at various speeds (at no-
Check the normal air inlet vacuum at various speeds
load) with the Engine Operating Conditions in
(at no-load) and compare the results with the Engine
Section 13.2.
Operating Conditions in Section 13.2.
PROPER USE OF MANOMETER
fluid travels up to the amount the other column travels
The U-tube manometer is a primary measuring device
down to obtain the pressure (or vacuum) reading.
indicating pressure or vacuum by the difference in the
height of two columns of fluid.
The height of a column of mercury is read differently
than that of a column of water. Mercury does not wet
Connect the manometer to the source of pressure,
vacuum or differential pressure. When the pressure is
the inside surface; therefore, the top of the column
imposed, add the number of inches one column of
has a convex meniscus (shape). Water wets the surface
SEC. 15.2 Page 3
November, 1973
