TRUCK SERVICE MANUAL
FUEL SYSTEM
15. If maximum leakage of injector being checked
exceeds specified values, injector barrel must be
sent to a rebuild station for re-plungering.
16. If maximum leakage does not exceed values
Iisted In Table 6113, shift retraction lever back to
"A" position and loosen T-handle clamp. This
applies 200 lbs. (90.7 kg] load to plunger.
17. The cup-to-plunger seal is acceptable if no
bubbles occur in 10 seconds or if time interval
between consecutive bubbles of air observed in
bubble checker exceeds 5 seconds.
18. If the cup seat is damaged In any way and the
Injector does not pass the ST990 cup-to-plunger
seat test, the plunger may be lapped into the cup
to obtain good seating. This lapping is to be
done in a careful manner, without using the body
or barrel. Use no greater than 600 grade paste
lapping compound: Applying light pressure,
oscillate the plunger in the cup back and forth for
approximately one minute.
Caution: After lapping, both the cup and
plunger must be cleaned thoroughly,
lapping compound will damage fuel
system parts unless removed. The most
effective cleaning process is the use of
Fig. 6-1-62, F60176. Rotating plunger coupling and
an "ultrasonic cleaner" and an after rinse
spacer
in fuel oil.
19. With checks completed and retraction lever in
12. Rotate plunger top in clockwise direction by very
"A" position disconnect transfer line, exhaust the
air cylinder and remove injector.
small increment and observe flow meter reading.
20. Remove plunger, reinstall spring and plunger.
See Fig. 6162. Do not touch plunger top or any
Altitude Compensation
part of clamping mechanism while taking reading
The ST990 is an altitude sensitive device and when it Is
since external forces will disturb plunger from its
used to check the body or barrel to plunger leakage, unit
normal position and may affect barrel-to-plunger
readings should be corrected to allow for altitude
leakage in cup area.
variations. For every 1,000 feet [305 m] altitude rise
13. Continue to rotate plunger by small increments
above 600 feet [183 m], 0.2 units should be subtracted
to fin, highest reading on flow meter.
from the unit reading. For example, if your shop is 6,600
14. Maximum
readings on injectors which will
feet [2012 m] above sea level, you should subtract 1.2 (6
satisfactorily perform throughout another service
x 0.2) units from your unit measurement. The altitude
period are listed in Table 6-1-13.
does not affect the cup to plunger leakage check.
Maintenance
Table 6-1-13: ST990 Leakage Tester Data - Units of
1. ST990 injector leakage testers are checked for
Leakage
gauge and flow meter accuracy, leaks,
Injector Model
Size
New
Worn Limit
operation and compatibility with other ST990's
before release to the field.
PT (Type D)
3/8 Plunger 2.5
4.5
2. It is strongly recommended that each ST990
PT (Type D)
5/16 Plunger 2.2
4.5
location select a clean used injector which has a
maximum leakage in the 8 to 10 unit area and
record the maximum reading. When an injector
such as this is selected and carefully kept in a
secure place, it can be periodically used as an
indicator of test stand condition over extended
periods of use. The careful selection and safe
keeping of an injector such as this is a very
important troubleshooting tool in event that
discrepancies arise.
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