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
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