Depress the plunger to the bottom of its travel while
working the control rack back and forth. If the rack
sticks, binds, or fails to move at all, it indicates that the
internal parts of the injector are either dirty or damaged.
VALVE OPENING/PRESSURE TEST. The
purpose of this testis to determine the pressure at which
the valve opens and injection begins. Place the injector
in the tester (fig. 5-28). Operate the pump handle until
all air is purged from the injector tester and the injector.
Then close the outlet clamp. If you are testing an injector
that has just been removed from the engine, the flow of
fuel through the injector on the tester should be the same
as it was on the engine. If required, reverse the fuel
connections on the tester to obtain the proper fuel flow.
With the injector rack in the full-fuel position, pump
the handle of the injector tester with smooth, even
strokes (fig. 5-29), and record the injector valve opening
pressure. The specified valve opening pressure for the
crown or high valve injector is 450 to 850 psi. For the
needle valve injector, the specified opening pressure is
2,000 to 3,200 psi. If the pressure is not within limits,
check the manufacturers maintenance manuals for
probable causes and corrections.
Figure 5-28.-Injector installed in tests.
Figure 5-29.-Checking injector valve opening pressure.
HOLDING PRESSURE TEST. This test is used
to determine whether the various lapped surfaces in the
injector are sealing properly.
To conduct this test, bring the pressure up to a point
just below the valve opening pressure (450 psi for
crown, needle, and high valve injectors). Then close off
the fuel shutoff valve. These actions cause the pressure
to drop. The time for the pressure to drop from 450 psi
to 250 psi must NOT be less than 40 seconds. If the
injector pressure drops from 450 psi to 250 psi in less
than 40 seconds, dry the injector thoroughly and make
the following checks:
1. Open the injector tester fuel valve and operate
the pump to maintain the test pressure.
2. Check for a leak at the injector rack opening. A
leak indicates a poor bushing-to-body fit.
3. Check for leaks around the spray tip or seal ring.
Leaks in these areas are usually caused by a loose
injector nut, a damaged seal ring, or a burned surface on
the injector nut or spray tip.
4. Check for leaks at the filter cap, which would
indicate a loose filter cap gasket.
5. If you find a dribble at the spray tip orifices, it
indicates a leaking valve assembly due to either a
damaged surface or dirt. Leakage at the tip will cause
preignition in the engine.