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 damaged.

Piston Seizure

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 lubrication.

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 bushings, metal obstructions, or improper manufacturing procedures.

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 SUDDEN STOPPING.