Mercury 305 CID, Service Manual, Page: 573 / 967

90-823225--1 1096 5E-32 - ELECTRONIC FUEL INJECTION (MULTI-PORT AND THROTTLE BODY)
Multi Port Injector Balance Test
(Refer to the following figure for test set-up.)
The injector balance tester is a tool used to turn the
injector ON for a precise amount of time, thus spray-
ing a measured amount of fuel into the manifold. This
causes a drop in fuel rail pressure that we can record
and compare between each injector. All injectors
should have the same amount of pressure drop. Any
injector with a pressure drop that is 1.5 PSI (or more)
greater or less than the average drop of the other in-
jectors should be considered faulty and replaced. In-
jector testers are available for various manufactur-
ers. For 7.4L / 454 and 502 engines: the tester must
be capable of selecting an injector pulse width in the
range of 200-400 milliseconds (m sec). The recom-
mended starting point for these engines is approxi-
mately 300 m sec. In any case a pulse width that
drops the fuel rail pressure to half the normal operat-
ing pressure, should be used.
STEP 1
Engine cool down period (ten minutes) is necessary
to avoid irregular readings due to “hot soak” fuel boil-
ing. Relieve fuel pressure in the fuel rail as outlined
in “Fuel Pressure Relief Procedure” in “Repair Proce-
dures.” Remove plenum as outlined in “Repair Pro-
cedures.” With ignition OFF, connect fuel pressure
gauge to fuel pressure tap.
Disconnect harness connectors at all injectors, and
connect injector tester to one injector. Use adaptor
harness furnished with injector tester to energize in-
jectors. Follow manufacturer’s instructions for use of
adaptor harness. Ignition must be OFF at least ten
seconds to complete ECM shutdown cycle. Fuel
pump should run about two seconds after ignition is
turned ON.
At this point, insert clear tubing attached to vent valve
into a suitable container and bleed air from gauge
and hose to ensure accurate gauge operation. Re-
peat this step until all air is bled from gauge.
STEP 2
Turn ignition OFF for ten seconds and then ON again
several times to get fuel pressure to its maximum.
Record this initial pressure reading. Energize tester
one time and note pressure drop at its lowest point.
(Disregard any slight pressure increase after drop
hits low point.) By subtracting this second pressure
reading from the initial pressure, we have the actual
amount of injector pressure drop.
STEP 3
Repeat Step 2 on each injector and compare the
amount of drop. Usually, good injectors will have
virtually the same drop. Retest any injector that has
a pressure difference of 1.5 PSI (10 kPa), either more
or less than the average of the other injectors on the
engine. Replace any injector that also fails the retest.
If the pressure drop of all injectors is within 1.5 PSI
(10 kPa) of this average, the injectors appear to be
flowing properly. Reconnect them and review “Trou-
bleshooting.”
NOTE: The entire test should not be repeated more
than once without running the engine to prevent
flooding. (This includes any retest on faulty injectors.)