In any discussion of a fuel system, the importance of each of its parts cannot be overlooked.

The first requirement for trouble-free operation of a fuel system is clean fuel. Accordingly, the filters, the strainers, the tanks, the transfer pumps, and the lines must be maintained according to prescribed instructions. Even when these parts function properly, the principal elements of the injection systempressure pump, injection valves, and injection nozzlesare subject to troubles. The following discussion covers some of these troubles, their symptoms and causes, and provides general information concerning maintenance and repair of this equipment. As you study this information, keep in mind the dif-ferences which may exist between the various systems. (A system, for example, may be of the jerk pump or common rail type, or the pressure pump and the injector may be separate or combined.)

Damaged Plunger

In the plunger and barrel assembly of a high-pressure pump and in the plunger and bushing assembly of a unit injector, the symptoms and causes of damage are similar.

Damage may become apparent through erratic engine operation. Symptoms vary widely and may include failure of the engine to develop full power, low exhaust temperature, low firing pressure for the affected cylinder, difficulty in balancing (calibrating) the pumps or injectors, and failure of one or more cylinders of the engine to fire. Damage to a plunger and the part in which it slides may also be recognized by testing the unit on a test stand. However, the best way to determine the extent of damage is to disassemble the unit, clean it thoroughly, and then carefully inspect each part.

Cleaning of the units can be best accomplished by use of an approved solvent. Clean diesel fuel may be used when more effective cleaners are not available. A brush must be used with diesel fuel and even then, removal of gummy deposits is dif-ficult. Keep each plunger and barrel (bushing) together during the inspection to avoid improper assembly, as they are manufactured in matched sets.

The use of a magnifying glass during the examination of a plunger will facilitate the detec-tion of damage. Inspect for fine scratches, dull surface appearance, cracks, pit marks (usually accompanied by dark discoloration), and erosion and roughness at the edge of the helix or at the end of the plunger. An example of a badly scored Bosch plunger is illustrated in A of figure 3-14. A plunger with the lapped surface and helix edge in good condition is shown in B of figure 3-14. Surface irregularities in the region illustrated are serious because they affect metering and, consequently, engine operation.

When examining a barrel or bushing, search for erosion of the ports or scoring of the lapped surfaces. Pay particular attention to the lapped plane surface at the end of a pump barrel. Rust or pit marks on this surface must be removed by lapping before reassembly.

Figure 3-14.A damaged and serviceable plunger.

Damage to the plunger of a fuel injection pump or injector may be caused by such different factors as entry of dirt into the equipment, careless handling while the equipment is disassembled, corrosion, and improper assembly and disassembly procedures.

Dirt and water are responsible for practically all trouble encountered with fuel injection equip-ment. If the units are not properly protected, they can be damaged beyond repair within a very short period of operation. Remember that the clearances between the lapped surfaces are so small that occasionally extremely fine particles, such as dust from the atmosphere, are capable of scoring these surfaces. Then small amounts of water that may collect from condensation will cor-rode these surfaces.

An engine should never be operated unless the fuel has been properly filtered before reaching the injection equipment. Although regular filters and strainers are present in all fuel systems, in some systems special safety filters or screens are incor-porated to further reduce the possibility of foreign matter mixing with the fuel as it reaches the pump and the injector. The location of these additional safety devices depends upon the system. In one system a screen is placed between the fuel transfer pump and the fuel distributor, while in another a filter is mounted directly on the pump.

During the overhaul of fuel injection equipment, a spotlessly clean working space is essential for the protection of all parts. Ideally, the area should also be air conditioned. All air should be thoroughly filtered before it enters the space. Benches should have smooth tops. Metal-topped benches should be covered with linoleum or lint-free rags. Ample quantities of approved cleaning solvent, of clean fuel oil, and of compressed air to blow parts dry, should be used to help ensure cleanliness during overhaul. Never use rags or waste to clean injectors, as lint particles from them may damage the injector parts.

From the time a unit is removed from the engine until it is replaced on the engine, extreme care must be exerted to keep dust and dirt away from all its parts. Before any connections are loosened, all dirt should be removed from the unit, tubing, and fittings by washing. After removal of the unit from the engine, all opening (pump, nozzle, tubing, or injectors) should be covered with approved caps or coverings. Because many surfaces of the parts of pumps and injectors are lapped to extremely accurate finishes, it is essential that they be HANDLED WITH GREAT CARE. Parts that are dropped may be bent, nicked, dented, or otherwise ruined. All work should be done well over the center of the bench. The use of a linoleum cover-ing will reduce casualties caused by dropping parts on the bench. Never leave parts uncovered on the bench, but keep them immersed in diesel fuel until handled. Never handle lapped surfaces when they are dry, as the perspiration on your hands may cause corrosion. Before a lapped surface is handled, it should be immersed in clean diesel fuel, and the hands rinsed in clean fuel. Since the mating parts of pumps and injectors are fitted to one another, such parts as plunger and barrel should be kept together to avoid interchanging. Since water in the fuel, or improper storage of parts, can also cause CORROSION of the parts of a pump or an injector, all fuel should be cen-trifuged, and filter and strainer cases drained periodically to prevent excessive collection of water. Information on proper stowage procedures should be obtained from the appropriate technical manual.

Special care must be exercised in

DISASSEMBLING and ASSEMBLING the parts of a fuel injection system, since any damage to these finely finished surfaces will necessitate replacement of the parts. When work is being done on any part of a fuel injection system, the procedure outlined in the engine technical manual, or the manufacturers fuel system technical manual, must be followed.

Remember that the damage to a plunger and barrel assembly of a fuel pressure pump or to the plunger and bushing assembly of a unit injector generally requires replacement of the parts. A damaged part may not be replaced individually. A plunger and its mating part (barrel, bushing, or bore) must be installed as a complete assembly.