TM 5-3895-383-24
Problem 39: Air In Fuel
Probable Cause:
With air in the fuel system the engine will normally be difficult
to start, run rough and release a large amount of white smoke.
If air is in the system, it will generally get in on the suction side
of the fuel transfer pump. Check for leakage at the
connections between the fuel tank and the fuel transfer pump.
If leaks are found, tighten the connections or replace the lines.
NOTE:
The fuel priming pump (if equipped) may be used
to remove the air from the fuel filter and fuel
gallery in the cylinder head), and fill the fuel
system with fuel from the fuel tank before the
engine is started.
If there are no visual leaks, remove the fuel supply line from
the tank and connect it to an outside fuel supply. If this
corrects the problem, the suction line (standpipe) inside the
fuel tank has a leak.
If this does not correct the problem, install a sight tube in the
fuel return line and check the injectors to verify that they are
properly seated. This can be accomplished by moving each
injector (one at a time) to the "FUEL ON" position momentarily
and checking the eight tube for any increase in air bubbles.
Push on the rack bar on the exhaust manifold side of the
injector to move the injector to the "FUEL ON" position. If an
increase in air is found, then remove that injector. Check the
tip seal (O-ring) and replace if it is defective. Inspect the
injector sleeve for a smooth sealing surface for the injector to
seat on. If any defects are noted, the sleeve can be reamed or
it can be replaced if necessary.
The temperature of an exhaust manifold port can be an
indication of a cylinder that has air being delivered to it. Check
the exhaust manifold temperatures and compare the results. A
lower than normal cylinder temperature indicates that the
cylinder may be receiving air from the injector.
The color of the exhaust smoke can also indicate which
cylinder has a combustion leak. Move each injector one at a
time) to the "FUEL ON" position momentarily while checking
the color of the exhaust smoke. The cylinder that has air will
produce smoke that is gray or white in color.
Fuel System
Either too much fuel or not enough fuel for combustion can be
the cause of a problem in the fuel system.
Many times work is done on the fuel system when the problem
is really with some other part of the engine. The source of the
problem is difficult to find, especially when smoke comes from
the exhaust. Smoke that comes from the exhaust can be
caused by a bad fuel injector, but it can also be caused by one
or more of the reasons that follow:
Not enough air for good combustion.
An overload at high altitude.
Oil leakage into combustion chamber.
Not enough compression.
Fuel System Inspection
A problem with the components that send fuel to the engine
can cause low fuel pressure. This can decrease engine
performance.
Fuel System (Type I)
Fuel System (Type I)
(1) Tube assembly (return to tank from fuel passage in cylinder
head). (2) Fuel outlet port (to tank). (3) Tube assembly (from
transfer pump to fuel filter base). (4) Tube assembly (from fuel
filter base to fuel passage in cylinder head). (5) Fuel inlet port
(to fuel transfer pump). (6) Screen. (7) Fuel filter.
1.
Check the fuel level in the fuel tank. Look at the cap for
the fuel tank to make sure the vent is not filled with dirt.
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