serrations of either the shaft or the hub have failed forany reason, replace both parts.Obstruction in the Combustion SpaceSuch items as broken valve heads and valve stemlocks or keepers that come loose because of a brokenvalve spring may cause an engine to come to an abruptstop. If an engine continues to run when suchobstructions are in the combustion chamber, the piston,liner, head, and injection nozzle will be severelydamaged.Piston SeizurePiston seizure may also cause an engine to stopsuddenly. The piston becomes galled and scuffed. Whenthis occurs, the piston may possibly break or extensivedamage may be done to other major engine parts. Theprincipal causes of piston seizure are insufficientclearance, excessive temperatures, or inadequatelubrication.Defective Auxiliary Drive MechanismsDefects in auxiliary drive mechanisms may causean engine to stop suddenly. Since most troubles in geartrains or chain drives require some disassembly, thisdiscussion 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 becauseof a broken or chipped gear. If you hear a metallicclicking noise in the vicinity of a gear housing, it isalmost a certain indication that a gear tooth has broken.Gears are most likely to fail because of improperlubrication, corrosion, misalignment, torsionalvibration, excessive backlash, wiped bearings andbushings,metal obstructions, or impropermanufacturing procedures.Gear shafts, bushings and bearings, and gear teethmust be checked during periodic inspections for scoring,wear, and pitting. All oil passages, jets, and spraysshould be cleaned to ensure proper oil flow. Allgear-locking devices must fit tightly to preventlongitudinal gear movement.Chains are used in some engines for camshaft andauxiliary drives; in other engines, chains are used todrive certain auxiliary rotating parts. Troubles in chaindrives are usually caused by wear or breakage. Troublesof 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 troublesthat may cause an engine to stall frequently or stopsuddenly. There may be some doubt as to the differencebetween stalling and stopping. In reality, there is noneunless we associate certain troubles with each. Ingeneral, troubles that cause FREQUENT STALLINGare those that can be eliminated with minor adjustmentsor maintenance. If such troubles are not eliminated, it isquite possible that the engine can be started, only to stallagain. Failure to eliminate some of the troubles thatcause frequent stalling may lead to troubles that causeSUDDEN STOPPING.ENGINE WILL NOT CARRY A LOADMany of the troubles that can lead to loss of powerin an engine may also cause the engine to stop and stallsuddenly or may even prevent it from starting. Comparethe list of some of the troubles that may cause a powerloss (fig. 3-41) with those in figures 3-36 and 3-40. Suchitems as insufficient air, insufficient fuel, and faultyoperation of the governor appear on all three charts.Many of the troubles listed are closely related, and theelimination of one may eliminate others.The operator of an internal-combustion engine maybe confronted with additional major difficulties, such asthose indicated in figure 3-42. Here, again, you can seethat many of these possible troubles are similar to thosethat have already been discussed in connection withstarting failures and with engine stalling and stopping.The discussion that follows covers only those troublesnot previously considered.ENGINE OVERSPEEDSWhen an engine overspeeds, the trouble is usuallycaused by either the governor mechanism or the fuelcontrol linkage, as previously discussed When you needinformation on a specific fuel system or speed controlsystem, check the manufacturer’s technical manual andthe special technical manuals for the particular system.These special manuals are available for the most widelyused 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 STOPSome troubles that may cause an engine to hunt, orvary its rpm at constant throttle setting, are similar tothose that may cause an engine to resist stopping.Generally, these two forms of irregular engine operation3-35
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