DETROIT DIESEL 53
Trouble Shooting (Engine) 15.2
connected to the oil level dipstick opening in the cylinder block. Check the readings obtained at various engine speeds
Exhaust Back Pressure
A slight pressure in the exhaust system is normal. However, excessive exhaust back pressure seriously affects engine
operation. It may cause an increase in the air box pressure with a resultant loss of efficiency of the blower. This means
less air for scavenging which results in poor combustion and higher temperatures.
Causes of high exhaust back pressure are usually a result of an inadequate or improper type of muffler, an exhaust pipe
which is too long or too small in diameter, an excessive number of sharp bends in the exhaust system, or obstructions
such as excessive carbon formation or foreign matter in the exhaust system.
Check the exhaust back pressure, measured in inches of mercury, with a manometer. Connect the manometer to the
exhaust manifold (except on turbocharged engines) by removing the 1/8 " pipe plug which is provided for that purpose. If
no opening is provided, drill an 11/32 " hole in the exhaust manifold companion flange and tap the hole to accommodate
a 1/8 " pipe plug.
Check the readings obtained at various speeds (at no-load) with the Engine Operating Conditions in Section 13.2.
Air Box Pressure
Proper air box pressure is required to maintain sufficient air for combustion and scavenging of the 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 black or grey exhaust smoke are
indications of low air box pressure.
High air box pressure can be caused by partially plugged cylinder liner ports.
Check the air box pressure with a manometer connected to an air box drain tube.
Check the readings obtained at various speeds with the Engine Operating Conditions in Section 13.2.
Air Inlet Restriction
Excessive restriction of the air inlet will affect the flow of air to the cylinders and result in poor combustion and lack of
power. Consequently the restriction must 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 result in a high blower inlet restriction.
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 the insertion of a fitting at this 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.
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 normal air inlet vacuum at various speeds (at no-load) and compare the results
PROPER USE OF MANOMETER
The U-tube manometer is a primary measuring device indicating pressure or vacuum by the difference in the height of
two columns of fluid.
Connect the manometer to the source of pressure, vacuum or differential pressure. When the pressure is imposed, add
the number of inches one column of fluid travels up to the amount the other column travels down to obtain the pressure
(or vacuum) reading.
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
November, 1973 SEC. 15.2