Fuel System Diagnosis. Isuzu S/T Isuzu

For 1990-2009 cars only

Makes 🢒 Isuzu 🢒 1997 🢒 S/T Isuzu - 2WD 🢒 Engine 🢒 Engine Controls - 2.2L 🢒 Fuel System Diagnosis


(1)	Fuel Pressure Regulator
(2)	Fuel Pressure Pinch Point
(3)	Fuel Pressure Gage Kit J 29658-D
(4)	Fuel Pressure Pinch Point
(5)	Fuel Pressure Line
(6)	Flexible Hose
(7)	Fuel Pressure Pinch Point
(8)	Fuel Return Line
(9)	Fuel Injectors

Circuit Description

When the ignition switch is turned ON, the Powertrain Control Module (PCM) will turn ON the in-tank fuel pump. The in-tank fuel pump will remain ON as long as the engine is cranking or running and the PCM is receiving reference pulses. If there are no reference pulses, the PCM will turn the in-tank fuel pump OFF 2 seconds after the ignition switch is turned ON or 2 seconds after the engine stops running. The in-tank fuel pump is an electric pump within an integral reservoir. The in-tank fuel pump supplies fuel through an in-line fuel filter to the fuel passage within the lower manifold assembly. The fuel pump is designed to provide fuel at a pressure above the pressure needed by the fuel injectors. A fuel pressure regulator, attached to the lower manifold assembly, keeps the fuel available to the fuel injectors at a regulated pressure. Unused fuel is returned to the fuel tank by a separate fuel return pipe.

Test Description

Number(s) below refer to the step number(s) on the Diagnostic Table.

To relieve the fuel pressure, refer to Fuel Pressure Relief . When the ignition switch is ON and the fuel pump is running, the fuel pressure indicated by the fuel pressure gage should be 284-325 kPa (41-47 psi). This pressure is controlled by the amount of pressure the spring inside the fuel pressure regulator can provide.

A fuel system that drops more than 5 psi in 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 valve seat within the fuel pressure regulator.

•	The fuel injector(s).

Fuel pressure that drops-off during acceleration, cruise or hard cornering may cause a lean condition. A lean condition can cause a loss of power, surging, or misfire. A lean condition can be diagnosed using a scan tool. If an extremely lean condition occurs, the oxygen sensors will stop toggling. The oxygen sensor output voltage(s) will drop below 500 mV. Also, the fuel injector pulse width will increase.

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

When the engine is at idle, the manifold pressure is low (high vacuum). This low pressure (high vacuum) is applied to the fuel pressure regulator diaphragm. The low pressure (high vacuum) will offset the pressure being applied to the fuel pressure regulator diaphragm by the spring inside fuel pressure regulator. When this happens, the result is lower fuel pressure. The fuel pressure at idle will vary slightly as the barometric pressure changes, but the fuel pressure at idle should always be less than the fuel pressure noted in Step 2 with the Engine OFF.

A rich condition may result from the fuel pressure being above 325 kPa (47 psi). A rich condition may cause a DTC P0132 or a DTC P0172 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 the fuel pressure being below 284 kPa (41 psi). A lean condition may cause a DTC P0131 or a DTC P0171 to set. Driveability conditions associated with lean conditions can include hard starting (when the engine is cold), hesitation, poor driveability, lack of power, surging, and misfiring.

Restricting the fuel return pipe causes the fuel pressure to rise above the regulated fuel pressure. Using a scan tool to pressurize the system, the fuel pressure should rise above 325 kPa (47 psi) as the gage outlet hose is pinched.

Check the spark plug associated with a particular fuel injector for fouling or saturation in order to determine if that particular fuel injector is leaking.

Fuel System Diagnosis

Step

Action

Value(s)

Yes

Was the Powertrain On-Board Diagnostic (OBD) System Check performed?

Go to Step 2

Go to Powertrain On Board Diagnostic (OBD) System Check

Turn the ignition OFF.
Turn the air conditioning system OFF.
Relieve the fuel pressure. Refer to the Fuel Pressure Relief .
Install fuel pressure gage. Refer to illustration.
Turn the ignition ON.
Bleed the air out of the fuel pressure gage into an approved gasoline container.
Turn the ignition OFF for 10 seconds.
Turn the ignition ON.

Important: The fuel pump will run for approximately 2 seconds. Cycle the ignition as necessary in order to achieve the highest possible fuel level

Observe the fuel pressure indicated by the fuel pressure gage with the fuel pump running.

Is the fuel pressure within the specified limits?

284-325 kPa (41-47 psi)

Go to Step 3

Go to Step 12

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

Note the fuel pressure after the fuel pump stops running.

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

34 kPa (5 psi)

Go to Step 10

Go to Step 4

Relieve the fuel pressure to the specified value
Monitor the fuel pressure.

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

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

Go to Step 21

Go to Step 5

Is the fuel pressure suspected of dropping-off during acceleration, cruise, or hard cornering?

Go to Step 6

Go to Step 8

Visually and physically inspect the following items for a restriction:

•	The in-line fuel filter.

•	The fuel feed pipe.

Was a restriction found?

Go to Step 23

Go to Step 7

Remove the modular sender assembly. Refer to Fuel Sender Assembly Replacement
Visually and physically inspect the following items:

•	The fuel pump strainer for a restriction.

•	The fuel pump flex pipe for leaks.

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

•	The fuel pump electrical wiring for high resistance.

Was a problem found in any of these areas?

Go to Step 23

Go to Step 20

Start the engine.
Allow the engine to idle at normal operating temperature.

Does the fuel pressure drop by the amount specified?

21-69 kPa (3-10 psi)

Go to Symptoms

Go to Step 9

Disconnect the vacuum hose from the fuel pressure regulator.
With the engine idling, apply 12-14 inches of vacuum to the fuel pressure regulator.

Does the fuel pressure indicated by the fuel pressure gage drop by the amount specified?

21-6 kPa (3-10 psi)

Go to Step 19

Go to Step 20

Turn the ignition OFF.
Place the bleed hose of the fuel pressure gage into an approved gasoline container.
Turn the ignition ON.
Bleed the air out of the fuel pressure gage.
Using a scan tool pressurize the fuel system.

Important: The fuel pump will run for approximately 2 seconds. Repeat Step 5 as necessary in order to achieve the highest possible fuel pressure.

Wait for the fuel pressure to build.
Pinch the gage inlet hose (refer to 4 on illustration).

Does the fuel pressure remain constant?

Go to Step 21

Go to Step 11

Using a scan tool pressurize the fuel system.
Wait for the fuel pressure to build.
Pinch the fuel return hose (refer to 7 on illustration).

Does the fuel pressure indicated by the fuel pressure gage remain constant?

Go to Step 20

Go to Step 22

Is the fuel pressure above the specified limit?

325 kPa (47 psi)

Go to Step 13

Go to Step 15

Relieve the fuel pressure by placing the bleed hose of the fuel pressure gage into an approved gasoline container and opening the relief valve.
Disconnect the fuel return hose quick-connect fitting, refer toServicing Quick Connect Fittings (Metal Collar)
Place the open end of the flexible hose into an approved gasoline container.
Turn the ignition OFF for approximately 10 seconds.
Turn the ignition ON.
Observe the fuel pressure gage with the fuel pump running.

Is the fuel pressure within the specified limits?

284-325 kPa (41-47 psi)

Go to Step 23

Go to Step 14

Visually and physically inspect the fuel outlet passages for a restriction.

Was a restriction found?

Go to Step 24

Go to Step 20

Is the fuel pressure above the specified value?

0 kPa (0 psi)

Go to Step 16

Go to Step 17

Notice: Do not allow the fuel pressure to exceed the specified value because damage to the fuel pressure regulator or the fuel pressure gage may result.

Relieve the fuel pressure by placing the bleed hose of the fuel pressure gage into an approved gasoline container and opening the relief valve.

Important:: The fuel pump will run for approximately 2 seconds. Repeat Step 2 as necessary in order to achieve the highest possible fuel pressure.

Using a scan tool, pressurize the fuel system.
Bleed the air out of the fuel pressure gage.
Wait for the fuel pressure to build.
Slowly pinch the gage outlet hose. Refer to 2 on illustration.

Does the fuel pressure indicated by the fuel pressure gage rise above the specified value?

325 kPa (47 psi) 414 kPa (60 psi)

Go to Step 20

Go to Step 7

Use the Engine Cranks But Will Not Run table to diagnose the fuel pump electrical circuit.

Was a problem found with the fuel pump electrical circuit?

Go to Step 24

Go to Step 18

Visually and physically inspect the following items:

•	The in-line fuel filter for obstructions.

•	The fuel feed pipe for a restriction.

•	The fuel pump strainer for obstructions.

•	The fuel pump flex pipe for leaks.

Was a problem found in any of these areas?

Go to Step 24

Go to Step 21

Locate and repair the loss of vacuum to the fuel pressure regulator.

Is the action complete?

System OK

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

Is the action complete?

System OK

Replace the modular fuel sender. Refer to Fuel Sender Assembly Replacement .

Is the action complete?

System OK

Locate and replace any leaking fuel injector(s).

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