Mpi Fuel Pressure Leak Down Test; Capacity Test - Jeep cherokee 1989 Owner's Manual

3.35 To test the MPI fuel system pressure, you will need
to attach a 0-to-60 psi pressure gauge to the test port
pressure fitting on the fuel rail
vacuum line attached to the regulator than with the vacuum line discon-
nected. Fuel system pressure should be 31 psi with the vacuum line
attached to the regulator and 3 9 psi with the line detached.
3 5 Attach a 0-to-60 psi fuel pressure gauge to the test port pressure
fitting on the fuel rail (see illustratimf.
36 Detach the vacuum tube from the fuel pressure regulator (see il-
lustration).
37 Start the engine.
38 Note the gauge reading. With the vacuum line detached, fuel
pressure should be about 39 psi.
3 9 Attach the vacuum line to the pressure regulator. Note the gauge
reading. Fuel pressure should be about 31 psi.
4 0 If the indicated fuel pressure is not about 8-to-1 0 psi higher with
the vacuum line removed from the regulator, inspect the pressure
regulator vacuum line for leaks, kinks and blockage.
41 If the fuel pressure is below the specified level, momentarily pinch
off the hose section of the return line (see illustration). The fuel pressure
will rise to 95 psi when the fuel return line is pinched shut, so turn
off the engine immediately after pinching off the fuel return line.
a) If the fuel pressure remains below specifications, inspect the fuel
supply line, fuel filter (see Chapter I ) and fuel rail inlet (see Sec-
tion 14) for blockage.
line for kinks and blockage.
down
MPI fuel pressure leak
4 2 If an abnormally long cranking period is required to restart a hot
engine after the vehicle has been shut down for a short period of time,
the fuel pressure may be leaking past the fuel pressure regulator or
the check valve in the outlet end of the fuel pump.
43 With the engine off, attach a gauge capable of reading 0-to-1 0 0
psi to the pressure test port fitting on the fuel rail.
4 4 Start the vehicle and let the engine idle. Check the fuel pressure
reading on the gauge. The fuel pressure should be within the specifica-
tions noted above.
4 5 Shut the engine off. Note the fuel pressure reading on the gauge.
Leave the fuel pressure gauge connected. Allow the engine to sit for
3 0 minutes, then compare the fuel pressure reading on the gauge to
the reading you took when you shut down the engine. A drop of 0-to-20
psi (to the 19-to-39 psi range) is acceptable. If the fuel pressure drop
is within specification, the fuel pump outlet check valve and the fuel
pressure regulator are both operating normally. If the fuel pressure drop
is greater than 20 psi, restart the vehicle, let the engine idle and momen-
The Motor Manual Guy
Chapter 4 Fuel and exhaust systems
test
3.36 Detach the vacuum tube from the fuel pressure regulator,
start the vehicle and note the gauge reading
line detached, fuel pressure should be about
3.41 If the fuel pressure is below the specified level,
momentarily pinch off the hose section of the fuel return
-
line fuel pressure will rise t o
line is pinched, so turn the engine off immediately after
pinching off the fuel return line
will rise to 9 5 psi when the fuel return line is pinched off, so shut the
engine down immediately after pinching off the fuel return line. Note
the pressure reading on the gauge. Allow the engine to sit for 3 0
minutes. Take another reading and compare it to the reading you took
when you first shut down the engine.
pressure regulator.
fuel pressure is bleeding off past the outlet check valve in the
fuel pump. Replace the pump.

Capacity test

4 6 Remove the cap from the pressure test port in the fuel rail (see
illustration 2.5).
47 Attach a 0-to-60 psi fuel pressure gauge to the pressure fitting.
4 8 Start the engine. Pressure should be about 31 psi with the vacuum
hose attached to the pressure regulator and 39 psi with the vacuum
hose removed from the pressure regulator (see illustration 3.36). If
4-3
-
with the vacuum
39 psi
95 psi when the fuel return