TRUCK SERVICE MANUAL
4. Connect the fuel suction line to the suction side of the
Fuel Pump Trouble Shooting With ST775 or ST848
gear pump. Run a line from the gear pump bleed connection
to the fuel supply tank.
This portion of the manual should be fully understood by the
Fuel Pump Test Stand Operator, and through this knowledge
5. On gear pumps not tapped, use a damper with a
he should be able to produce a properly calibrated Fuel Pump.
tapped hole to make this connection.
Fuel Pump calibration on the ST848 Fuel Pump Test Stand
6. Shut off the fuel manifold pressure gauge to prevent
combined with injector calibration has produced widespread
damage from over pressure.
acceptance of fuel system accuracy. There have been reports
of erratic results, but investigations have revealed that the
7. The valve in the suction line and the valve controlling
cause for such problems fall into three basic categories: (1)
the main flow orifice are to be fully open. All other valves must
Mechanic and/or Testor Error, (2) Instrumentation Errors or (3)
be completely closed. With the valves in this position, fuel will
Maintenance Status.
be routed through the manifold orifice.
1. Mechanic and/or Testor Error
8. Start the pump drive in the proper direction and run
the pump at 400 to 450 RPM. The gear pump must pick up at
a. Misapplication of specifications and parts:
this speed without the aid of priming. Any gear pump which
fails to pick up fuel must be reworked or replaced.
The
(1) Calibrating a specific fuel pump model to the wrong
remaining steps of this check need not be made on any pump
value.
failing to pick up.
(2) Calibrating a specific injector model to the wrong flow
9. Increase pump speed for the following checks:
value.
Any gear pump with delivery below the listed value should be
(3) Lack of familiarity with this bulletin (4) Use of
reworked or replaced. Any gear pump which is wet to touch
camshaft and pistons other than those shown in fuel pump
after this check should be reworked or replaced.
calibration specifications.
Note:
Pistons, camshafts and injectors are sometimes
Table 5-1-9: Gear Pump Flow
superseded by others requiring a different calibration.
Gear Pump
RPM
Minimum Total
Flow
(5) Use of wrong injector assemblies in a specific engine
3/4 [19.05]
2100
850 Lbs./Hr.
model.
10. Any gear pump with a delivery above the listed
(6) Use of governor and torque springs other than those
minimum is to be considered acceptable for fuel pump
specified in fuel pump calibration data.
operation, and it can be installed on a fuel pump for proper
calibration.
b. Engine Test (1) Restricted intake air in engine.
(2) Excessive exhaust back pressure.
11. The following check should be made if the gear pump
delivery is just above the listed minimum described in Step 9.
(3) Restricted fuel supply to engine.
a. The unrestricted manifold pressure should be
(4) Aerate a fuel supply to engine.
checked for a 10/15% higher pressure at rated speed than the
manifold pressure. If it is too low or too high, it may be
(5) Excessive high oil level in engine crankcase.
necessary to change idle plunger (button) to give the correct
unrestricted pressure of 10/15% above final manifold pressure.
(6) Incorrect injector adjustment.
Note: The idle plunger (counter bore diameter) may be
(7) Dirt entering balance orifice.
decreased in size to increase the amount of pressure available
at the calibration set point.
2. Instrumentation Errors a. Erroneous fuel manifold
CAUTION
pressure gauges on engine and/or chassis dynamometer.
Step 11 is to be used only if gear pump is worn preventing
it from delivering enough pressure for proper calibration.
b. Erroneous flow meters.
Never change idle plunger to exceed the 10 to 15%
unrestricted manifold pressure.
c. Erroneous dynamometer load indicators.
d. Incorrect tachometer.
534
