Special care must be exercised inDISASSEMBLING and ASSEMBLING the partsof a fuel injection system, since any damage tothese finely finished surfaces will necessitatereplacement of the parts. When work is beingdone on any part of a fuel injection system, theprocedure outlined in the engine technical manual,or the manufacturer’s fuel system technicalmanual, must be followed.Remember that the damage to a plunger andbarrel assembly of a fuel pressure pump or to theplunger and bushing assembly of a unit injectorgenerally requires replacement of the parts.Adamagedpartmaynotbereplacedindividually.A plunger and its mating part (barrel, bushing,or bore) must be installed as a complete assembly.External LeakageTrouble caused by external leakage from aninjection pump or an injector may become suffi-ciently serious to cause an engine to misfire. Itis of extreme importance that signs of externalleakage be detected as soon as possible. Leakageoutside of the combustion space may be suffi-ciently large not only to affect engine operationbut also to create a fire hazard. External leakageof a unit injector can cause fuel dilution of theengine lube oil, reduce lubrication, and increasethe possibility of a crankcase explosion.In general, external leakage from pumps andinjectors is caused by improper assembly, looseconnections, faulty gaskets, damaged threads andsealing surfaces, broken springs, or cracked hous-ings or bodies. While leakage from pumps isgenerally visible during engine operation, leakagefrom an injector may not become apparent untilappropriate tests are performed.You can stop the external leakage from apump or injector either by tightening loose con-nections or by replacing the damaged parts.Before the equipment is inspected for leakage,thoroughly clean all parts. On some equipment,you may eliminate mild roughness or discolora-tion of the sealing surfaces by lapping.Stuck PlungerWhen the cylinder of an engine fails to fire,it is an indication that the injection pump plungeris stuck. Misfiring may occur intermittently if theplunger sticks and releases at intervals. Upondisassembly, it may be difficult to remove theplunger. Sometimes the plunger may stick whenthe pump or the injector is assembled, but willwork smoothly when the unit is disassembled. Attimes, the plunger will not stick until some timeafter the unit has been removed from the engine.This is particularly true when the plunger andmating part have been stored under conditionsthat cause corrosion, or when the parts have beenmishandled after removal.A unit injector may be checked, after removalfrom the engine, by performing the bindingplunger test. This test is performed by depress-ing the plunger, either by hand or by using the“popping” fixture of a test stand, and noting thereturn action of the plunger. The plunger shouldreturn with a definite snap. This test should beperformed at three successive rack settings. Asluggish return action indicates a sticky plunger.A sticking plunger may be caused by dirt,gummy deposits in the unit, or distortion of theplunger and its adjacent part.The movement of a plunger may be restrictedor entirely prevented by small particles of dirtwhich may lodge between the plunger and itsmating surface. Lacquer-like deposits, from fuel,will also interfere with the movement of theplunger.The greatest care must be taken whenhandling the parts of a pump or injector. Becauseof the extremely close clearances between plungerand mating surfaces, a slight distortion of eitherwill cause binding. Distortion may result fromdropping, from striking the plunger and a matingpart, or from improper assembly.Stuck plungers in fuel pumps or injectorsshould be freed or replaced. Sometimes a littlecleaning may eliminate the need for a replace-ment. The plunger and barrel or bushing assemblyshould be soaked in an approved cleaning fluid.The assembly should be soaked overnight, orlonger if necessary. Cleaning fluids approved forthis purpose will immediately soften and removeany paint or enamel with which they come in con-tact. These fluids should be used with care, sincethey will damage rubber gaskets.The specific procedures for cleaning fuelinjection equipment, although similar, vary tosome degree, depending upon the unit involvedChapter 3—ENGINE MAINTENANCE3-19
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