TM 5-3895-346-14
TROUBLESHOOTING (ENGINE)
ENGINE OVERHAUL
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 turbo- charged engines) by removing the 1/8 inch pipe plug which is
provided for that purpose. If no opening is provided, drill an 11/32 inch hole in the exhaust manifold companion
flange and tap the hole to accommodate a 1/8 inch pipe plug.
On turbocharged engines, check the exhaust-back pressure in the exhaust piping 6 to 12 inches from the
turbine outlet (Fig. 1, Engine Operating Conditions Specifications). The tapped hole must be in a
comparatively straight pipe area for an accurate measurement.
Check the readings obtained at various speeds (at no-load) with the Engine Operating Conditions
Specifications.
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 Specifications.
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
inches above the air inlet housing (nonturbocharged engines) or the compressor inlet (turbocharged engines).
When practicability prevents the insertion of a fitting at this point (nonturbocharged engines), 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.
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