Fuel System Diagnosis. Isuzu S/T Isuzu

For 1990-2009 cars only

Makes 🢒 Isuzu 🢒 2000 🢒 S/T Isuzu - 2WD 🢒 Engine 🢒 Engine Controls - 4.3L 🢒 Fuel System Diagnosis

Circuit Description

When you turn ON the ignition, the vehicle control module (VCM) energizes the fuel pump relay, which powers ON the fuel pump. The pump remains ON as long as the engine is cranking or running and the VCM receives reference pulses. If there are no reference pulses, the VCM shuts OFF the fuel pump within 2 seconds after the ignition was switched ON, or if the engine stops. The VCM also turns ON the fuel pump for 2 seconds when you turn OFF the ignition.

An electric fuel pump pumps the fuel through an in-line filter to the fuel meter body assembly. The pump attaches to the fuel level sender assembly inside the fuel tank. The fuel pump provides more fuel pressure than is needed by the fuel injectors. The pressure regulator keeps the fuel available to the injectors at a regulated pressure. Unused fuel returns to the fuel tank via the fuel return pipe.

Test Description

The numbers below refer to the step numbers on the diagnostic table.

When the ignition is ON and the fuel pump is running, the fuel pressure gauge should indicate 414-455 kPa (60-66 psi). The spring pressure inside the fuel pressure regulator controls the fuel pressure.

A fuel system that drops more than 34 kPa (5 psi) during 10 minutes has a leak in one or more of the following areas:

•	The fuel pump check valve

•	The fuel pump flex pipe

•	The valve or the valve seat within the fuel pressure regulator

•	The fuel injectors

•	The fuel pipe

A fuel system that drops more than 14 kPa (2 psi) during 10 minutes after being relieved to 69 kPa (10 psi) indicates a leaking fuel pump check valve.

Fuel pressure that drops during acceleration, during cruise, or during hard cornering may cause a lean condition. A lean condition may cause a loss of power, surging, or misfires. Use a scan tool in order to diagnose a lean condition. If an extremely lean condition occurs, the heated oxygen sensors will stop toggling. The heated oxygen sensor output voltages will be less than 300 mV. The fuel injector pulse width will increase.

Important: Ensure that sure the fuel system is not operating in the Fuel Cut-Off Mode. This can cause false indications from the scan tool.

When you quickly open the throttle, the manifold pressure increases, indicating a low vacuum. The fuel pressure regulator increases the fuel pressure as the engine vacuum drops in order to compensate for the engine load.

A rich condition may result from the fuel pressure that is more than 455 kPa (66 psi). A rich condition may cause a DTC to set. Driveability conditions associated with rich conditions can include hard starting followed by black smoke and a strong sulfur smell in the exhaust.

This test determines if the high fuel pressure is due to a restricted fuel return pipe or if the high fuel pressure is due to a faulty fuel pressure regulator.

A lean condition may result from fuel pressure that is less than 414 kPa (60 psi). A lean condition may cause a DTC to set. Driveability conditions associated with lean conditions include hard starting when the engine is cold, hesitation, unsatisfactory driveability, lack of power, surging, and misfiring.

Notice: DO NOT allow the fuel pressure to exceed 517 kPa (75 psi). Excessive pressure may damage the fuel system.

Restricting the fuel return pipe with the fuel pipe shut-off adapter causes the fuel pressure to rise above the regulated fuel pressure. Use a scan tool in order to pressurize the fuel system. The fuel pressure should be more than 455 kPa (66 psi) as the valve on the fuel pipe shut-off adapter connected to the fuel return pipe becomes partially closed.

If the injectors are not grounded, check the spark plug that is associated with a particular fuel injector for fouling or for saturation in order to determine if that particular fuel injector is leaking. If checking the spark plug associated with a particular fuel injector for fouling or saturation does not determine that a particular fuel injector is leaking, use the following procedure:

1.	Remove the fuel meter body assembly. Refer to Fuel Meter Body Assembly Replacement .

2.	Re-install the engine compartment fuel feed and the return pipes to the fuel meter body and to the chassis fuel pipes. Refer to Fuel Hose/Pipes Replacement - Engine Compartment .

3.	Lift the fuel meter body assembly just enough to leave the fuel injector poppet nozzles in the ports.

4.	Pressurize the fuel system by using the scan tool fuel pump enable.

5.	Inspect the fuel injector poppet nozzles for leaks.

Step

Action

Values

Yes

Did you perform the Powertrain On-Board Diagnostic (OBD) System Check?

Go to Step 2

Go to Powertrain On Board Diagnostic (OBD) System Check

Caution: The fuel pump will operate for 2 seconds after you have turned off the ignition. Make sure that all the fuel pipes and fittings are securely fastened in order to prevent personal injury.

Turn OFF the ignition.
Turn OFF the air conditioning system.

Caution: Wrap a shop towel around the fuel pressure connection in order to reduce the risk of fire and personal injury. The towel will absorb any fuel leakage that occurs during the connection of the fuel pressure gage. Place the towel in an approved container when the connection of the fuel pressure gage is complete.

Install the J 34730-1A fuel pressure gauge to the fuel pressure test connection to the engine compartment.
Place the bleed hose of the fuel pressure gauge into an approved gasoline container.
Turn ON the ignition, with the engine OFF.
Use a scan tool in order to command ON the fuel pump.
Bleed the air from the fuel pressure gauge.
Use a scan tool in order to command ON the fuel pump.

Important:: The fuel pump runs for approximately 2 seconds each time you use the scan tool in order to command ON the fuel pump. Enable the fuel pump as necessary in order to achieve the highest possible fuel pressure.

Monitor the fuel pressure with the fuel pump running.

Is the fuel pressure within the specified range?

414-455 kPa (60-66 psi)

Go to Step 3

Go to Step 11

Important:: The fuel pressure may vary slightly when the fuel pump stops running. After the fuel pump stops running, the fuel pressure should stabilize and it should remain constant.

Does the fuel pressure drop more than the specified value during 10 minutes?

34 kPa (5 psi)

Go to Step 8

Go to Step 4

Relieve the fuel pressure to the first specified value.

Does the fuel pressure drop more than the second specified value during 10 minutes?

69 kPa (10 psi) 14 kPa (2 psi)

Go to Step 20

Go to Step 5

Is the fuel pressure dropping during :

•	Acceleration

•

Cruise

•	Hard cornering?

Go to Step 7

Go to Step 6

Start the engine.
Allow the engine to idle at the normal operating temperature.
Quickly open the throttle.
Monitor the fuel pressure. The fuel pressure should approach the top of the specified range.

Does the fuel pressure rise quickly?

414-455 kPa (60-66 psi)

Go to Symptoms

Go to Step 7

Inspect the following:

•	The fuel filter for restriction

•	The fuel feed pipe for restriction

•	The fuel pump electrical wiring for high resistance

Did you find a problem?

Go to Step 24

Go to Step 16

Relieve the fuel pressure. Refer to Fuel Pressure Relief .
Raise the vehicle. Refer to Lifting and Jacking the Vehicle in General Information.
Disconnect the fuel feed pipe from the fuel filter outlet. Refer to Plastic Collar Quick Connect Fitting Service .
Drain any remaining fuel from the fuel pipe into an approved gasoline container.
Install the J 42964-1 fuel pipe shut-off adapter onto the in-line fuel feed pipe.
Open the valve on the fuel pipe shut-off adapter.
Install the J 34730-1A fuel pressure gauge on the J 42964-1 pressure test connection of the fuel feed pipe shot-off adapter that is farthest from the fuel tank.
Reconnect the negative battery cable.
Turn ON the ignition, leaving the engine OFF.
Use a scan tool in order to command ON the fuel pump.
Inspect for any leaks at the fuel pipe shut-off adapter fittings.
Place the bleed hose of the fuel pressure gauge into an approved gasoline container.
Bleed the air from the fuel pressure gauge.
Use a scan tool in order to command ON the fuel pump.
Wait for the fuel pressure to build.
Close the valve in the fuel pipe shut-off adapter that is connected to the fuel feed pipe.

Does the fuel pressure remain constant?

Go to Step 20

Go to Step 9

Inspect the fuel feed pipe for a leak between the shut-off adapter and the engine fuel pipe.

Did you find a fuel leak?

Go to Step 24

Go to Step 10

Open the valve in the fuel pipe shut-off adapter that is connected to the fuel feed pipe.
Relieve the fuel pressure. Refer to the Fuel Pressure Relief .
Disconnect the fuel return pipe quick connect fitting that is located near the fuel filter. Refer to Plastic Collar Quick Connect Fitting Service .
Drain any remaining fuel from the fuel pipe into an approved gasoline container.
Install the J 42964-2 fuel pipe shut-off adapter onto the in-line fuel return pipe.
Open the valve on the fuel return pipe shut-off adapter.
Reconnect the negative battery cable.
Turn ON the ignition leaving the engine OFF.
Use a scan tool in order to command ON the fuel pump.
Wait for the fuel pressure to build.
Close the valve in the fuel pipe shut-off adapter that is connected to the fuel return pipe.

Does the fuel pressure remain constant?

Go to Step 21

Go to Step 22

Is the fuel pressure more than the specified limit?

455 kPa (66 psi)

Go to Step 12

Go to Step 14

Relieve the fuel pressure. Refer to Fuel Pressure Relief .
Raise the vehicle. Refer to Lifting and Jacking the Vehicle in General Information.
Disconnect the fuel feed pipe from the fuel filter outlet and from the fuel return pipe quick connect fitting that is located near the fuel filter. Refer to Plastic Collar Quick Connect Fitting Service .
Drain any remaining fuel from the fuel pipes into an approved gasoline container.
Install the J 42964-1 fuel pipe shut-off adapter onto the in-line fuel feed pipe.
Open the valve on the fuel feed pipe shut-off adapter.
Install the J 34730-1A fuel pressure gauge on either of the J 42964-1 pressure connections for the fuel feed pipe shut-off adapter.
Attach a length of flexible fuel hose to the chassis fuel return pipe.
Place the open end of the flexible fuel hose into an approved gasoline container.
Reconnect the negative battery cable.
Use a scan tool in order to command ON the fuel pump.
Observe the fuel pressure with the fuel pump running.

Is the fuel pressure within the specified limits?

414-455 kPa (60-66 psi)

Go to Step 23

Go to step 13

Inspect the following for restrictions:

•	The engine compartment fuel return pipe

•	The chassis fuel return pipe

Did you find a restriction?

Go to Step 24

Go to Step 21

Is the fuel pressure more than the specified value?

0 kPa (0 psi)

Go to Step 15

Go to Step 17

Relieve the fuel pressure. Refer to Fuel Pressure Relief .
Raise the vehicle. Refer to Lifting and Jacking the Vehicle in General Information.
Disconnect the fuel feed pipe from the fuel filter outlet and from quick connect fitting for the fuel return pipe, located near the fuel filter. Refer to Plastic Collar Quick Connect Fitting Service .
Drain any remaining fuel from the fuel pipes into an approved gasoline container.
Install the J 42964-1 and the J 42964-2 fuel pipe shut-off adapters onto the fuel feed pipe and the in-line fuel return pipe.
Open the valves on the fuel pipe shut-off adapters.
Install the J 34730-1A fuel pressure gauge on the J 42964-1 pressure connection for the fuel feed pipe shut-off adapter that is closest to the fuel tank.
Reconnect the negative battery cable.
Use a scan tool in order to command ON the fuel pump.
Inspect for any leaks at the fuel pipe shut-off adapter fittings.
Place the bleed hose of the fuel pressure gauge into an approved gasoline container.
Bleed the air from the fuel pressure gauge.
Use a scan tool in order to command ON the fuel pump.

Notice: DO NOT allow the fuel pressure to exceed 517 kPa (75 psi). Excessive pressure may damage the fuel system.

Slowly close the valve in the fuel pipe shut-off adapter that is connected to the fuel return pipe.

Is the fuel pressure more than the specified value?

455 kPa (66 psi)

Go to Step 21

Go to Step 16

Perform the following:

•	Inspect for low fuel pump voltage due to high resistance connections.

•	Inspect for a faulty fuel pump ground.

•	Inspect the fuel strainer for a restriction.

•	Inspect the fuel pump flex pipe for leaks.

•	Verify that the fuel pump is the correct fuel pump for this vehicle

Did you find any problems?

Go to Step 24

Go to Step 20

Turn ON the ignition, leaving the engine OFF.
Use a scan tool in order to command ON the fuel pump.
Listen for the fuel pump running.

Does the fuel pump run?

Go to Step 18

Go to Fuel Pump Electrical Circuit Diagnosis

Inspect for:

•	No fuel in the fuel tank. Do not rely on the fuel gauge reading.

•	Obstructions in the fuel filter

•	Restrictions in the fuel feed pipe

•	Obstructions in the fuel strainer

•	Leaks in the fuel pump flex pipe

Did you find any problems?

Go to Step 24

Go to Step 19

Inspect for a leak in the fuel feed pipe between the shut-off adapter and the fuel sender.

Did you find a fuel leak?

Go to Step 24

Go to Step 20

Replace the fuel pump. Refer to Fuel Tank Fuel Pump Module Replacement .

Is the action complete?

System OK

Replace the fuel pressure regulator. Refer to Fuel Pressure Regulator Replacement .

Is the action complete?

System OK

Locate and replace any leaking or grounded fuel injectors. For leaking injectors refer to the Test Description for this step. For grounded injector diagnosis refer to Fuel Injector Circuit Diagnosis .

Is the action complete?

System OK

Locate and correct the restriction in the fuel return pipe.

Is the action complete?

System OK

Repair the problem as necessary.

Is the action complete?

System OK