Direct Connection Vacuum Test (V6 Engines); Indirect Connection Vacuum Test (V6 Engine); Electrical Pump (Fuel Injected Vehicles); Tbi, Mpi Pressure Test - Jeep cherokee 1989 Owner's Manual

4-2
PRESSURE TEST
3.28 All system pressure tests on TBI-equipped vehicles require
the removal of the pressure test port plug from the throttle body
-
in its place, install a special pressure test fitting (available at a
dealer parts department)
1 1 Position the flexible hose and restrictor so fuel can be discharged
into a graduated container.
12 Reattach the coil primary lead wires (the engine must be operated
for the following test).
13 Start the engine (or have your assistant start it), let it run at curb
idle rpm, then discharge the fuel into the container by momentarily
opening the hose restrictor.
1 4 Close the hose restrictor, allow the pressure to stabilize and note
the pressure. The gauge should indicate 4-to-5 psi for four-cylinder
engines and 6-to-7.5 psi for V6 engines.
a) If the pump pressure is not within specification, and the fuel lines
are in satisfactory condition, the pump is defective and should
be replaced.
ing tests for capacity and vacuum.
Capacity test
15 Operate the engine at curb idle rpm.
16 Open the hose restrictor and allow fuel to discharge into a
graduated container for 3 0 seconds, then close the restrictor. A t least
one pint of fuel should have been discharged.
a) If the pump volume is less than one pint, repeat the test with
an auxiliary fuel supply and, for four-cylinder engines, a replace-
ment fuel filter.
the auxiliary fuel supply, look for a restriction in the fuel supply
line from the tank and check the tank vent to make sure it's work-
ing properly.
Direct connection vacuum test
17 You will need a vacuum gauge to perform the direct connection
vacuum test. In this test, the vacuum test gauge is connected directly
to the fuel pump inlet t o test the pump's ability to create a vacuum.
18 Detach the fuel inlet line at the fuel pump.
19 Attach a vacuum gauge to the fuel pump inlet.
2 0 Operate the engine at curb idle speed and note the vacuum gauge
reading. It should indicate a vacuum of 1 0 in-Hg (the gauge will not
indicate a vacuum until the fuel in the carburetor float bowl has been
consumed and the pump begins to operate at full capacity).
21 If the pump vacuum is not within specification, the pump is defec-
tive. Replace it (see Section 4).
Indirect connection vacuum test
22 You will need a vacuum gauge and a T-fitting to perform the indirect
connection vacuum test. In this test, a vacuum gauge is connected
by a T-fitting into the pump inlet to determine whether an obstruction
The Motor Manual Guy
Chapter 4 Fuel and exhaust systems
(V6 engines)
(V6
engine)
ADJUSTING
SCREW
3.31 If the TBI system fuel pressure indicated on the test gauge
is incorrect. adjust it by turning the regulator adjusting screw
turn the screw in (clockwise) t o increase pressure or out
(counterclockwise) t o decrease pressure
exists in the fuel line or the in-tank fuel filter.
23 Detach the fuel inlet line at the fuel pump.
2 4 Install a T-fitting between the disconnected fitting and the fuel
pump inlet. Connect a vacuum gauge to the T-fitting.
25 Operate the engine at a speed of 1500 rpm for 3 0 seconds and
note the reading on the vacuum gauge (again, the gauge will not indi-
cate any vacuum until the fuel in the carburetor float bowl has been
consumed and the pump begins t o operate at full capacity). The indi-
cated vacuum should not exceed 3 in-Hg.
26 If the indicated vacuum exceeds the specified vacuum, check the
fuel line for a restriction. A partially clogged in-tank fuel filter can also
cause excess vacuum.

Electrical pump (fuel injected vehicles)

TBI pressure test
Refer to illustrations 3.28 and
27 Detach the cable from the negative battery terminal.
28 Remove the pressure test port plug from the throttle body (see
illustration).
29 Install a pressure test fitting (available at a Jeep parts department)
in place of the test port plug.
3 0 Attach a 0-to-30 psi fuel pressure gauge to the pressure test fitting
(don't use a carburetor type 0-to-1 5 psi gauge).
31 Start the engine and let it idle. The pressure gauge should read
14-to-1 5 psi. If the pressure is incorrect, adjust it by turning the regu-
lator adjusting screw (see illustration) to obtain the correct fuel pressure.
Turn the screw at the bottom of the regulator in (clockwise) to increase
pressure or out (counterclockwise) to decrease pressure.
3 2 If the fuel pressure is considerably higher than specified, and adjust-
ing the regulator fails to lower it to the specified level, inspect the fuel
return line for blockage.
3 3 If the fuel pressure is considerably below specification and adjusting
the regulator fails to raise it to the specified level, momentarily pinch
off the fuel return line and recheck the pressure.
Section 13).
MPI pressure test
Refer to illustrations 3.35, 3.36 and 3.4 1
3 4 The MPI fuel system employs a vacuum-assisted pressure regu-
3.3 1
-