Quantcast Engine tuning

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TM 5-685/NAVFAC MO-912
(5) For the 1ow-speed engine, about one-third of
Check air filter. Change air filter oil if oil
the fuel charge is present, while two-thirds of the
bath type.
charge is injected at a controlled rate after ignition
(g) Check high idle speed.
occurs. In practice, the lower speed engines use a
(h) Check low idle speed
lower octane fuel. Since such engines are usually
(i) Check engine for correct horsepower. Use
large, a relatively coarse atomization is used, re-
sulting in greater ignition delay. In low-speed en-
(j) Visually check engine for leaks.
gines, actual fuel timing is usually in the range
(k) In addition to these items, some engines
from 7 to 12 degrees before top dead center.
may require additional adjustment or checking be-
(6) The med'ium-speed engines usually are
fore the tune-up is complete.
timed from 10 to 18 degrees before top dead center
(2) Major tune-up includes the following:
while high-speed units will range as much as 35 to
(a) Retorque cylinder head.
40 degrees before top dead center. Generally, the
(b) Adjust tappet clearance.
duration of injection decreases with speed.
(c) Clean and adjust injectors and/or injec-
b. Timing procedure. Timing is established by
tion nozzles.
setting the fuel injection cam with the control sys-
(d) Check pump static timing.
tem in the maximum fuel position. Since the fuel
(e) Change fuel filters and strainers. Drain
cam is usually symmetrical, lost motion affects the
engine coolant.
opening and closing times equally. For example, if
(f) Service air cleaner.
an engine were timed at full load for opening 10
(g) Check and overhaul injection pump if
degrees before top dead center and closing 10 de-
grees after top dead center, at half load, the timing
(h) Check high idle speed.
might be 6 degrees before top dead center to 6 de-
(i) Check low idle speed.
grees after top dead center. By lowering fuel pres-
(j) Check engine for correct horsepower. Use
sure, the injection period can be lengthened to ap-
proach the full load values. Balance is secured by
(k) Visually check engine for leaks.
adjusting the lost motion device for each of the cyl-
(3) During the tune-up, check for any loose
inders. It is important to maintain all fuel nozzle
bolts or hose clamps that may be a potential trouble
tips in good condition, and to have carefully
spot. Also, replace all gaskets, such as tappet cover
matched orifices on the nozzle. The nozzle orifice
gaskets, pump gaskets, timing cover gaskets, and
and duration of injection are the only balancing
any other gaskets that have been disturbed during
adjustments. Since duration should be similar for
the tune-up.
all cylinders, matched orifices must be used. Always
G-4. Engine failure and repairs.
install new fuel nozzle orifices in full sets for a
common rail engine.
a. Failure identification. A well planned and ex-
ecuted preventive maintenance program reduces
G-3. Engine tuning.
the possibilities of experiencing a catastrophic en-
a. General Tuning of diesel engines is necessary
gine failure. However, it is not completely possible
whenever the engine is not running normally, has
to prevent or anticipate such a failure. Indication of
lost power, or has operated the number of hours
some of these failures are as follows:
that constitute a tune-up interval.
(1) Crankcase explosions. If, during operation,
b. Tune-up categories. There are two categories of
explosions can be heard in the crankcase, shut the
tune-up, minor and major. Refer to the time interval
engine down immediately. Allow the engine to cool
specified by the manufacturer for minor and major
before removing any cover plates for inspection.
tune-ups. The specific manufacturer's literature
(2) Runaway engine. May be caused by a stuck
should be consulted for tune up details related to
fuel pump rack or defective engine safety stop. Lu-
the engine in use.
bricate the control linkage when the engine is at
(1) Minor tune-up includes the following:
rest or shut off the fuel supply to the engine, as
(a) Retorque cylinder head. This is optional;
follow manufacturer's instructions.
(3) Sudden stop. May be caused by overload,
(b) Adjust tappet clearance.
low lubricating oil, seized engine components, or
(c) Adjust injector timing or setting on en-
empty fuel tank. Inspect to identify the problem.
gine using unit injectors.
Allow the engine to cool before removing any cover
(d) Check pump static timing on engines us-
ing a pump-nozzle combination.
(4) Unusual noises. Can be caused by fuel in-
(e) Change fuel filters and strainers.
jection equipment troubles, a loose or broken con-


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