The fuel pressure on my 1998 5.4 Navigator is 30 psi running w/vacuum line connected 40 psi dis-connected goes to 0 when engine is shut off. What should it be?

You should have a test port on the Fuel Supply rail, use a tool from a local auto parts "Tool Loaner program" where you can borrow tools for free,w/a deposit of course,then follow the steps provided;

MPI Fuel System Pressure Test

The MPI fuel system used in vehicles equipped with a 4.OL engine employs a vacuum balanced pressure regulator. Fuel pressure should be approximately 55-69 kPa (8-10 psi) lower with the vacuum line attached to the regulator than with the vacuum line disconnected. System fuel should be 214 kPa (31 psi) with the vacuum line connected to the regulator and 269 kPa (39 psi) with the vacuum line disconnected. CAUTION: Some fuel may be discharged when connecting fuel gauge to fuel rail.

1. Connect a 0-414 kPa (0-60 psi) fuel pressure gauge to test port pressure fitting on fuel rail (Fig. 7).
2. Remove vacuum line from pressure regulator.
3. Start the vehicle.
4. Note gauge reading. With vacuum line disconnected, fuel pressure should be approximately 269 kPa (39 psi).
5. Connect vacuum line to pressure regulator. Note gauge reading. Fuel pressure should be approximately 214 kPa (31 psi).
6. If fuel pressure is not approximately 55-69 kPa (8-10 psi) higher with vacuum line removed from regulator, inspect pressure regulator vacuum line for leaks, kinks or blockage. CAUTION: Fuel pressure will rise to as much as 655 kPa (95 psi) when the fuel return tine is pinched shut, shut engine down immediately after pinching oft fuel return line.
7. If fuel pressure is low, momentarily pinch shut the hose section of the fuel return line. If fuel pressure remains low, inspect the fuel supply line, fuel filter, and fuel rail inlet for blockage. If fuel pressure rises replace fuel pressure regulator.
8. If fuel pressure is above specifications, inspect the fuel return line for kinks and blockage.

Capacity Test

1. Remove the cap from the pressure test port in the fuel rail.
2. Connect a 0-414 kPa (0-60 psi) fuel pressure gauge to the pressure fitting on the fuel rail (Fig. 7).
3. Start the vehicle. Pressure should be approxi- mately 214 kPa (31 psi) with the vacuum hose connected to the pressure regulator and 269 kPa (39 psi) with the vacuum hose removed from the pressure regulator.
4. If the pressure is not to specification, check the following before replacing the fuel pressure regulator:

• 4a - Inspect the fuel supply and return lines/hoses for kinks or restricting bends
• 4b - Check the fuel pump flow rate. A good fuel pump will deliver at least 1 liter of fuel per minute with the fuel return line pinched off. If the fuel pump does not pump adequately, then inspect the fuel system for a plugged fuel filter or fuel pump inlet filter (sock). Fuel pump flow rate can be done by connecting one end of an old A/C gauge hose to the fuel test port on the fuel rail and inserting the other end of the hose into a container of at least 1 liter capacity. Run the fuel pump by installing a jumper wire into diagnostic connector terminals D1-5 and D1-6. Be sure to pinch off the fuel return line or most of the fuel will be returned to the fuel tank.

1.6L DOHC (same way as the ISUZU impulse ) same car.
4XE1 engine twin cam.
has MPI injection with fuel rail, regulator and loop
disconnecting the feed line and stuffing it with guage only does 1 of 5 tests. done on MPI. it does JUST the shut test.
the correct way it to check it at the filter or rail tests ports on most cars.
in some cases you must add it.
they sell banjo fittings with test ports at summit racing dot com.
so the first question on all MPI cars, is , where is the stock fittings.
other cars the rail has a schrader valve or a plugged hole to add one. (test port) did you look first for this?
to test the FPR, unplug the vacuum line running
and pressure will jump up 6 to 10 psi.
this simulates wide open throttle pressure, a key fact in getting full power from engine, i test cranking presssure, idle, 3000rpm
and wot pressures to be sure the PFR is working, not just the pump.



the spec at the rail is.

Late 1986 to 1993 I-TEC MPFI system (after VIN 0908197)
Regulator vacuum line connected-36 psi
Regulator vacuum line disconnected-42.5 to 45 psi

the shunt test can hit 60 pis easy. do so for only 5sec.
as this stresses the pump .

the only rail drawing , shows no fittings (cheap buggers)



not even a cheap plug...

below is the twin cam, 1.6L
buy a TEE feeting, and put it at upper, red arrow.
use clamps rated for fuel injection, not chezzy screw band clamps as seen on carb cars. use the correct parts.
or you will get leak and it will not be pretty,
the filter as at 14 ID. this is the pressure feed side of the loop.
30 to 60psi , so all parts must be rated over 60, psi, (100psi is a nice goal)



now the words directly from the IMPULSE FSM.

Testing

1. Relieve the fuel pressure.
2. Disconnect the fuel line near the engine and install fuel pressure gauge T-connector in the line.
3. Connect the fuel pressure gauge to the T-connector.
4. Start the engine and check the fuel pressure, it should be:
5. 4XC1-Turbo and DOHC I-Mark, G200Z Impulse: 35.6 psi with the vacuum hose of the pressure regulator disconnected and plugged. 28.4 psi with the vacuum hose connected and at idle.
6. 4ZC1-Turbo, 4ZD1 Impulse and 4ZE1 engine: 42.6 psi with the vacuum hose at the pressure regulator disconnected and plugged. 35.6 psi with the vacuum hose connected at idle. Disconnect the VSV before checking pressure.
7. V6 engine: 9.13 psi at the fuel pressure line after the fuel filter.
8. 4XE1 SOHC and DOHC engine: Flow test, disconnect the hose from the EFI fuel feed line at the engine and place in a suitable container. Apply battery voltage to the fuel pump. Pump should supply 1/4 quart (0.24L) within 15 seconds. Pressure test, connect a pressure gauge in the fuel pressure line after the fuel filter, located at the driver's side frame rail. Disconnect the vacuum hose from the pressure regulator. Apply battery voltage to the pump. After the pump stabilizes, the pressure should be 35-38 psi (245-256 kPa) and hold steady.

sounds like you need to replace your fuel pressure regulator.

Normal fuel pressure at idle is 30-40 psi. You should be able to disconnect the vac line to the regulator and see pressure go up.
If you think pressure is low, the fuel filter would be the first suspect.
The pressure regulator could also be faulty.

What is liter size of engine? The fuel pressure specs that I looked at for your model year, all were over 40 psi and some close to 60 psi depending on the engine.

The fuel pressure should be 40-47 psi.This is with the key on and engine not running.Should stay at least 40 for about 5 or 10 minutes.Hope this helps.Good luck.

SOUND LIKE VACUUM LEAK AT INTAKE MANIFOLD LOOK FOR STUCK OPEN PCV VALVE OR LEAKING EGR VALVE GASKET.CHECK MAP SENSOR VACUUM HOSE MAKE SURE ITS CONNECTED AND GETTING VACUUM OR CHECK FOR A DISCONNECTED ELECTRICAL CONNECTOR AT MAP SENSOR.IF ALL LOOK GOOD CHECK FUEL PRESSURE SHOULD BE 40 - 50 PSI FOR 98 TO 2001 CADILLACS WITH 4.6 ENGINES CADILLACS 92 TO 97 MODELS WITH ENGINES 4.9 TO 3.8 FUEL PRESSURE 41 - 47 PSI. THATS YOUR FUEL PRESSURES WITH NEW FUEL FILTERS.IF FUEL PRESSURE WAY LOW FUEL FUEL PUMP WEAK.IF FUEL PRESSURE GOES HIGHER THAN NORMAL OR SEE GAS IN FUEL PRESSURE REGULATOR VACUUM LINE.FUEL PRESSURE REGULATOR IS FAULTY.WEAK IGNITION COIL OR COILS BAD PLUGS WORN DISTRIBUTOR CAP AND ROTOR WILL CAUSE SPUTTERING.

28-45 when running
35-45 key engine off
check your fuel filter for blockage

Disconnect the electrical connection to relieve the system's pressure. Start the Navigator and let the engine run until it stalls from lack of gasoline. Turn off the ignition, reconnect the pump's electrical connection and disconnect the negative battery cable.

Step 2 Detach the fuel pressure sensor vacuum hose. Check for fuel in the line. If there is fuel present in the line, the fuel delivery problem is likely caused by a faulty fuel pump pressure regulator. If there is no fuel in the line, reattach the vacuum hose and continue with the fuel delivery diagnostic.

Step 3 Twist off the fuel pressure test port cap and secure a pressure gauge to the fuel pressure valve. Reconnect the negative battery cable and start the engine. The pressure gauge needs to read 45 psi when at wide open throttle. High pressure readings indicate a malfunctioning pressure regulator. Low pressure readings indicate a clogged fuel filter or a broken fuel pump.

Step 4 Cut the engine. Relieve system pressure and remove the negative battery cable again so you can safely remove the fuel filter to check for clogs.

Step 5 Loosen the filter fittings and unhook the fuel lines from the filter. Undo and remove the bracket on the fuel filter.

Step 6 Inspect the filter for clogs and replace it with a new one if necessary. If there are no clogs, the fuel delivery problem is probably a faulty fuel pump.

30psi seems pretty low, should be around 45 or so. It sounds like your fuel pump is done again. What did you replace the pump with, OEM or a parts store part?