Trouble caused by external leakage from an injection pump or an injector may become suffi-ciently serious to cause an engine to misfire. It is of extreme importance that signs of external leakage be detected as soon as possible. Leakage outside of the combustion space may be suffi-ciently large not only to affect engine operation but also to create a fire hazard. External leakage of a unit injector can cause fuel dilution of the engine lube oil, reduce lubrication, and increase the possibility of a crankcase explosion. In general, external leakage from pumps and injectors is caused by improper assembly, loose connections, faulty gaskets, damaged threads and sealing surfaces, broken springs, or cracked hous-ings or bodies. While leakage from pumps is generally visible during engine operation, leakage from an injector may not become apparent until appropriate tests are performed.

You can stop the external leakage from a pump 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 Plunger

When the cylinder of an engine fails to fire, it is an indication that the injection pump plunger is stuck. Misfiring may occur intermittently if the plunger sticks and releases at intervals. Upon disassembly, it may be difficult to remove the plunger. Sometimes the plunger may stick when the pump or the injector is assembled, but will work smoothly when the unit is disassembled. At times, the plunger will not stick until some time after the unit has been removed from the engine. This is particularly true when the plunger and mating part have been stored under conditions that cause corrosion, or when the parts have been mishandled after removal.

A unit injector may be checked, after removal from the engine, by performing the binding plunger 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 the return action of the plunger. The plunger should return with a definite snap. This test should be performed at three successive rack settings. A sluggish return action indicates a sticky plunger. A sticking plunger may be caused by dirt, gummy deposits in the unit, or distortion of the plunger and its adjacent part.

The movement of a plunger may be restricted or entirely prevented by small particles of dirt which may lodge between the plunger and its mating surface. Lacquer-like deposits, from fuel, will also interfere with the movement of the plunger.

The greatest care must be taken when

handling the parts of a pump or injector. Because of the extremely close clearances between plunger and mating surfaces, a slight distortion of either will cause binding. Distortion may result from dropping, from striking the plunger and a mating part, or from improper assembly.

Stuck plungers in fuel pumps or injectors should be freed or replaced. Sometimes a little cleaning may eliminate the need for a replace-ment. The plunger and barrel or bushing assembly should be soaked in an approved cleaning fluid. The assembly should be soaked overnight, or longer if necessary. Cleaning fluids approved for this purpose will immediately soften and remove any paint or enamel with which they come in con-tact. These fluids should be used with care, since they will damage rubber gaskets. The specific procedures for cleaning fuel injection equipment, although similar, vary to some degree, depending upon the unit involved and the manufacturer. The following brief description of the procedures for equipment made by two different manufacturers emphasizes some of these similarities, and further emphasizes the need for following only the procedures indicated in the appropriate manufacturers technical manual.

A plunger of a Bosch fuel injection pump can be loosened by cleaning. However, if the plunger does not slide freely in the barrel, both the plunger and barrel should be cleaned with an approved cleaning fluid, rinsed in clean fuel oil, and blown dry with compressed air. A small quantity of mut-ton tallow should then be placed on the plunger. Working the plunger back and forth and rotating it in the barrel should remove all gummy deposits. Instructions for Bosch fuel injection equipment state that such items as hard or sharp tools or abrasives of any kind should never be used in cleaning the pumps.

Freeing the sticking plunger in a GM unit in-jector may be done in much the same manner as in a Bosch pump.

Stains on plungers may be removed by the use of a limited quantity of jewelers rouge on a piece of soft tissue paper. It is important to remember that the plunger should not be lapped to the bushing with an abrasive such as jewelers rouge. After a plunger has been cleaned with jewelers rouge, it must be cleaned thoroughly with diesel fuel before being placed in the bushing. If after repeated cleanings, the plunger still does not slide freely, you may assume that either the plunger or the bushing is distorted.

The principal difference in the cleaning pro-cedures for these two units of equipment is in the use of abrasives. If the recommended cleaning procedure for these units fails to loosen the plunger so it will slide freely, the plunger and its mating part will have to be replaced.