serrations of either the shaft or the hub have failed for
any reason, replace both parts.
Obstruction in the Combustion Space
Such items as broken valve heads and valve stem
locks or keepers that come loose because of a broken
valve spring may cause an engine to come to an abrupt
stop. If an engine continues to run when such
obstructions are in the combustion chamber, the piston,
liner, head, and injection nozzle will be severely
Piston seizure may also cause an engine to stop
suddenly. The piston becomes galled and scuffed. When
this occurs, the piston may possibly break or extensive
damage may be done to other major engine parts. The
principal causes of piston seizure are insufficient
clearance, excessive temperatures, or inadequate
Defective Auxiliary Drive Mechanisms
Defects in auxiliary drive mechanisms may cause
an engine to stop suddenly. Since most troubles in gear
trains or chain drives require some disassembly, this
discussion will be limited to the causes of such troubles.
Gear failure is the principal trouble in gear trains.
Engine failure and extensive damage can occur because
of a broken or chipped gear. If you hear a metallic
clicking noise in the vicinity of a gear housing, it is
almost a certain indication that a gear tooth has broken.
Gears are most likely to fail because of improper
lubrication, corrosion, misalignment, torsional
vibration, excessive backlash, wiped bearings and
metal obstructions, or improper
Gear shafts, bushings and bearings, and gear teeth
must be checked during periodic inspections for scoring,
wear, and pitting. All oil passages, jets, and sprays
should be cleaned to ensure proper oil flow. All
gear-locking devices must fit tightly to prevent
longitudinal gear movement.
Chains are used in some engines for camshaft and
auxiliary drives; in other engines, chains are used to
drive certain auxiliary rotating parts. Troubles in chain
drives are usually caused by wear or breakage. Troubles
of this nature may be caused by improper chain tension,
lack of lubrication, sheared cotter pins, or misalignment.
Figure 3-40 is a summary of the possible troubles
that may cause an engine to stall frequently or stop
suddenly. There may be some doubt as to the difference
between stalling and stopping. In reality, there is none
unless we associate certain troubles with each. In
general, troubles that cause FREQUENT STALLING
are those that can be eliminated with minor adjustments
or maintenance. If such troubles are not eliminated, it is
quite possible that the engine can be started, only to stall
again. Failure to eliminate some of the troubles that
cause frequent stalling may lead to troubles that cause
ENGINE WILL NOT CARRY A LOAD
Many of the troubles that can lead to loss of power
in an engine may also cause the engine to stop and stall
suddenly or may even prevent it from starting. Compare
the list of some of the troubles that may cause a power
loss (fig. 3-41) with those in figures 3-36 and 3-40. Such
items as insufficient air, insufficient fuel, and faulty
operation of the governor appear on all three charts.
Many of the troubles listed are closely related, and the
elimination of one may eliminate others.
The operator of an internal-combustion engine may
be confronted with additional major difficulties, such as
those indicated in figure 3-42. Here, again, you can see
that many of these possible troubles are similar to those
that have already been discussed in connection with
starting failures and with engine stalling and stopping.
The discussion that follows covers only those troubles
not previously considered.
When an engine overspeeds, the trouble is usually
caused by either the governor mechanism or the fuel
control linkage, as previously discussed When you need
information on a specific fuel system or speed control
system, check the manufacturers technical manual and
the special technical manuals for the particular system.
These special manuals are available for the most widely
used models of hydraulic governors and overspeed trips,
and they contain specific details on testing, adjusting,
and repairing each controlling device.
ENGINE HUNTS OR WILL NOT STOP
Some troubles that may cause an engine to hunt, or
vary its rpm at constant throttle setting, are similar to
those that may cause an engine to resist stopping.
Generally, these two forms of irregular engine operation