Even when the starting equipment is in an operating condition, an engine may fail to start. Most troubles that prevent an engine from starting are associated with fuel and the fuel system. However, defective or inoperative parts or assemblies may be the source of some trouble. Failure to follow instructions may be the cause of an engine failing to start. The corrective action is obvious for such items as leaving the fuel throttle in the OFF position and leaving the cylinder indicator valves open. If an engine fails to start, follow the prescribed starting instructions and recheck the procedure.

Foreign Matter in the Fuel Oil System

In the operation of an internal-combustion engine, cleanliness is of paramount importance. This is especially true in the handling and care of diesel fuel oil. Impurities are the prime source of fuel pump and injection system troubles. Sediment and water cause wear, gumming, corrosion, and rust in a fuel system. Even though fuel oil is generally delivered clean from the refinery, handling and transferring increase the chances that fuel oil will become contaminated. Corrosion often leads to replacement or at least to repair of the part. You must continually take steps to prevent water from accumulating in a fuel system, not only to eliminate the cause of corrosion but also to ensure proper combustion in the cylinders. Centrifuge all fuel, and drain the fuel filter cases periodically to prevent excessive collection of water.

Water in fuel will cause irreparable damage to the entire fuel system in a short time. It corrodes the fuel injection pump, where close clearances must be maintained, and also corrodes and erodes the injection nozzles. The slightest corrosion can cause a fuel injection pump to bind and seize which, if not corrected, will lead to excessive leakage. Water will erode the orifices of injection nozzles until they will not spray the fuel properly, thus preventing proper atomization. When this occurs, incomplete combustion and engine knocks result.

Air in the fuel system is another possible trouble that may prevent an engine from starting. Even if the engine will start, air in the fuel system will cause the engine to miss and knock, and perhaps to stall.

When an engine fails to operate, stalls, misfires, or knocks, there may be air in the high-pressure pumps and lines. In many systems, the expansion and compression of such air may take place even if the injection valves do not open. If this occurs, the pump is AIRBOUND. To determine if there is air in a fuel system, bleed a small amount of fuel from the top of the fuel filter; if the fuel appears quite cloudy, there are probably small bubbles of air in the fuel.

Insufficient Fuel Supply

An insufficient fuel supply may result from a defective or inoperative part in the system. Such items as a closed inlet valve in the fuel piping or an empty supply tank are more likely to be the fault of the operator than of the equipment. But an empty tank may be caused by leakage, either in the lines or in the tank LEAKAGE.-You can usually trace leakage in the low-pressure lines of a fuel system to cracks in the piping. Usually these cracks occur on threaded pipe joints at the root of the threads. Such breakage is caused by the inability of the nipples and pipe joints to withstand shock, vibration, and strains resulting from the relative motion between smaller pipes and the equipment to which they are attached.

Metal fatigue can also cause breakage. Each system should have a systematic inspection of its fittings and piping to determine if all the parts are satisfactorily supported and sufficiently strong. In some instances, nipples may be connected to relatively heavy parts, such as valves and strainers, which are free to vibrate. Since vibration contributes materially to the fatigue of nipples, rigid bracing should be installed. When practicable, bracing should be secured to the unit itself, instead of to the hull or other equipment.

Breakage can also cause leakage in the high-pressure lines of a fuel system. The breakage usually occurs on either of the two end fittings of a line and is caused by lack of proper supports or by excessive nozzle opening pressure. Supports are usually supplied with an engine and should not be discarded. Excessive opening pressure of a nozzle-generally due to improper spring adjustment or to clogged nozzle orifices-may rupture the high-pressure fuel lines. A faulty nozzle usually requires removal, inspection, and repair plus the use of a nozzle tester.

Leakage from fuel lines may also be caused by improper replacement or repairs. When a replacement is necessary, always use a line of the same length and diameter as the one you remove. Varying the length and diameter of a high-pressure fuel line will change the injection characteristics of the injection nozzle.

In an emergency, you can usually repair a high-pressure fuel line by silver soldering a new fitting to the line. After making the silver solder repair, test the line for leaks and be certain no restrictions exist. Most leakage trouble occurs in the fuel lines, but leaks may occasionally develop in the fuel tank. These leaks must be eliminated immediately because of potential fire hazard.

The principal causes of fuel tank leakage are improper welds and metal fatigue. Metal fatigue is usually the result of inadequate support; excessive stresses develop in the tank and cause cracks. CLOGGED FUEL FILTERS-Another problem that can limit the fuel supply to such an extent that an engine will not start is clogged fuel filters. Definite rules for filter replacement cannot be established for all engines. But instructions generally state that elements will not be used longer than a specified time. Since there are reasons that an element may not always function properly for its expected service life, it should be replaced whenever it is suspected of being clogged. Filter elements may become clogged because of dirty fuel, too small filter capacity, failure to drain the filter sump, and failure to use the primary strainer.

Usually, clogging is indicated by such symptoms as stoppage of fuel flow, increase in pressure drop across the filter, increase in pressure upstream of the filter, or excessive accumulation of dirt on the element (observed when the filter is removed for inspection). Symptoms of clogged filters vary in different installations, and each installation should be studied for external symptoms, such as abnormal instrument indications and engine operation. If external indications are not apparent, visual inspection of the element will be necessary, especially if it is known or suspected that dirty fuel is being used. Fuel filter capacity should at least equal fuel supply pump capacity. A filter with a small capacity clogs more rapidly than a larger one, because the space available for dirt accumulation is more limited. There are two standardized sizes of fuel filter elements-large and small. The small element is the same diameter as the large but is only one-half as long. This construction permits substitution of two small elements for one large element.

You can increase the interval of time between element changes by using the drain cocks on a filter sump. Removal of dirt through the drain cock will make room for more dirt to collect.

If new filter elements are not available for replacement and the engine must be operated, you can wash some types of totally clogged elements and get limited additional service. This procedure is for emergencies only. An engine must never be operated unless all the fuel is filtered; therefore, a washed filter is better than none at all.

Fuel must never flow from the supply tanks to the nozzles without passing through all stages of filtration. Strainers, as the primary stage in the fuel filtration system, must be kept in good condition if sufficient fuel is to flow in the system. Most strainers have a blade mechanism that can be turned by hand. If you cannot readily turn the scraper by hand, disassemble and clean the strainer. This minor preventive maintenance will prevent the scraping mechanism from breaking.

TRANSFER PUMPS.If the supply of fuel oil to the system is to be maintained in an even and uninterrupted flow, the fuel transfer pumps must function properly. These pumps may become inoperative or defective to the point that they fail to discharge sufficient fuel for engine starting. Generally, when a pump fails to operate, some parts have to be replaced or reconditioned. For some types of pump, it is customary to replace the entire unit. However, for worn packing or seals, satisfactory repairs may be made. If plunger-type pumps fail to operate because the valves have become dirty, submerge and clean the pump in a bath of diesel oil.

Repairs of fuel transfer pumps should be made according to maintenance manuals supplied by the individual pump manufacturers.