will lead to excessive leakage. Water will erode theorifices of injection nozzles until they will not spray thefuel properly, thus preventing proper atomization. Whenthis occurs, incomplete combustion and engine knocksresult.Air in the fuel system is another possible trouble thatmay prevent an engine from starting. Even if the enginewill start, air in the fuel system will cause the engine tomiss and knock, and perhaps to stall.When an engine fails to operate, stalls, misfires, orknocks, there may be air in the high-pressure pumps andlines. In many systems, the expansion and compressionof such air may take place even if the injection valvesdo not open. If this occurs, the pump is AIRBOUND. Todetermine if there is air in a fuel system, bleed a smallamount of fuel from the top of the fuel filter; if the fuelappears quite cloudy, there are probably small bubblesof air in the fuel.Insufficient Fuel SupplyAn insufficient fuel supply may result from adefective or inoperative part in the system. Such itemsas a closed inlet valve in the fuel piping or an emptysupply tank are more likely to be the fault of the operatorthan of the equipment. But an empty tank may be causedby leakage, either in the lines or in the tankLEAKAGE.-You can usually trace leakage in thelow-pressure lines of a fuel system to cracks in thepiping. Usually these cracks occur on threaded pipejoints at the root of the threads. Such breakage is causedby the inability of the nipples and pipe joints towithstand shock, vibration, and strains resulting fromthe relative motion between smaller pipes and theequipment to which they are attached.Metal fatigue can also cause breakage. Each systemshould have a systematic inspection of its fittings andpiping to determine if all the parts are satisfactorilysupported and sufficiently strong. In some instances,nipples may be connected to relatively heavy parts, suchas valves and strainers, which are free to vibrate. Sincevibration contributes materially to the fatigue of nipples,rigid bracing should be installed. When practicable,bracing should be secured to the unit itself, instead of tothe hull or other equipment.Breakage can also cause leakage in thehigh-pressure lines of a fuel system. The breakageusually occurs on either of the two end fittings of a lineand is caused by lack of proper supports or by excessivenozzle opening pressure. Supports are usually suppliedwith an engine and should not be discarded. Excessiveopening pressure of a nozzle-generally due to improperspring adjustment or to clogged nozzle orifices-mayrupture the high-pressure fuel lines. A faulty nozzleusually requires removal, inspection, and repair plus theuse of a nozzle tester.Leakage from fuel lines may also be caused byimproper replacement or repairs. When a replacementis necessary, always use a line of the same length anddiameter as the one you remove. Varying the length anddiameter of a high-pressure fuel line will change theinjection characteristics of the injection nozzle.In an emergency, you can usually repair ahigh-pressure fuel line by silver soldering a new fittingto the line. After making the silver solder repair, test theline for leaks and be certain no restrictions exist.Most leakage trouble occurs in the fuel lines, butleaks may occasionally develop in the fuel tank. Theseleaks must be eliminated immediately because ofpotential fire hazard.The principal causes of fuel tank leakage areimproper welds and metal fatigue. Metal fatigue isusually the result of inadequate support; excessivestresses develop in the tank and cause cracks.CLOGGED FUEL FILTERS-Another problemthat can limit the fuel supply to such an extent that anengine will not start is clogged fuel filters. Definite rulesfor filter replacement cannot be established for allengines. But instructions generally state that elementswill not be used longer than a specified time. Since thereare reasons that an element may not always functionproperly for its expected service life, it should bereplaced whenever it is suspected of being clogged.Filter elements may become clogged because ofdirty fuel, too small filter capacity, failure to drain thefilter sump, and failure to use the primary strainer.Usually, clogging is indicated by such symptoms asstoppage of fuel flow, increase in pressure drop acrossthe filter, increase in pressure upstream of the filter, orexcessive accumulation of dirt on the element (observedwhen the filter is removed for inspection). Symptoms ofclogged filters vary in different installations, and eachinstallation should be studied for external symptoms,such as abnormal instrument indications and engineoperation. If external indications are not apparent, visualinspection of the element will be necessary, especiallyif it is known or suspected that dirty fuel is being used.Fuel filter capacity should at least equal fuel supplypump capacity. A filter with a small capacity clogs more3-28
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