Fuel System Diagnosis. Isuzu S/T Isuzu

For 1990-2009 cars only

Makes 🢒 Isuzu 🢒 1999 🢒 S/T Isuzu - 2WD 🢒 Engine 🢒 Engine Controls - 2.2L 🢒 Fuel System Diagnosis


(1)	Fuel Pressure Regulator
(2)	Fuel Shut Off Adapter J 37287
(3)	Fuel Return Line
(4)	Quick Connect Adapter
(5)	Fuel Pressure Pinch Point
(6)	Fuel Pressure Pinch Point
(7)	Fuel Pressure Gauge Kit J 29658-D
(8)	Fuel Filter
(9)	Fuel Pressure Line

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 OFF the in-tank fuel pump after 2 seconds since the ignition switch is turned ON or after 2 seconds once 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 rail assembly. The fuel pump is designed to provide fuel at a pressure above the pressure that is needed by the fuel injectors. A fuel pressure regulator, attached to the fuel rail assembly, keeps the fuel available to the fuel injectors at a regulated pressure. The 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 gauge 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 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 flow 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 gauge 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

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

Go to Step 2

Go to Powertrain On Board Diagnostic (OBD) System Check

Turn the ignition switch OFF.
Turn the air conditioning system OFF.
Verify fuel is in the fuel tank.
Relieve the fuel pressure. Refer to the Fuel Pressure Relief .
Install fuel pressure gauge. (7 on illustration).
Turn the ignition switch ON.

Important: To avoid false fuel pressure readings, thoroughly bleed all air out of the fuel pressure gauge.

Bleed the air out of the fuel pressure gauge into an approved gasoline container.
Turn the ignition switch OFF for 10 seconds.
Turn the ignition switch ON.

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

Observe the fuel pressure indicated by the fuel pressure gauge 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 first specified value
Monitor the fuel pressure.

Does the fuel pressure drop more than the second 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 24

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.

•	Broken baffles inside the fuel tank.

•	The fuel pump electrical wiring for high resistance.

Was a problem found in any of these areas?

Go to Step 24

Go to Step 20

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

Is the fuel pressure within the specified limits?

214-303 kPa (31-44 psi)

Go to Symptoms

Go to Step 9

Disconnect the vacuum hose from the fuel pressure regulator.
With the engine idling, apply 41-47 kPa (12-14 in. Hg) of vacuum to the fuel pressure regulator.

Is the fuel pressure within the specified limits and remain constant?

214-303 kPa (31-44 psi)

Go to Step 19

Go to Step 20

Turn the ignition switch OFF.
Relieve the fuel pressure . Refer to Fuel Pressure Relief .
Place the bleed hose of the fuel pressure gauge into an approved gasoline container.
Turn the ignition switch ON.
Bleed the air out of the fuel pressure gauge.
Using a scan tool pressurize the fuel system.

Important: Repeat pressurizing the fuel system with the scan tool as necessary to achieve the highest possible fuel pressure.

Wait for the fuel pressure to build.
Slowly pinch the gauge inlet hose (5 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.
Close the valve in the fuel pipe shut-off adapter that is connected to the fuel return pipe (2 on illustration).

Does the fuel pressure indicated by the fuel pressure gauge 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 gauge into an approved gasoline container and opening the relief valve.
Disconnect the fuel return hose quick-connect fitting, refer to Quick Connect Fitting(s) Service (Metal Collar) .
Place the open end of the flexible hose into an approved gasoline container.
Turn the ignition switch OFF for approximately 10 seconds.
Turn the ignition switch ON.
Observe the fuel pressure gauge 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 gauge into an approved gasoline container and opening the relief valve.
Using a scan tool, pressurize the fuel system.
Slowly pinch the fuel pressure gauge outlet hose (6 on illustration) while the fuel pump is running.

Does the fuel pressure indicated by the fuel pressure gauge rise above the first specified value?

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

Go to Step 20

Go to Step 7

Refer to Fuel Pump Electrical Circuit Diagnosis 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.

•	Broken baffles inside the fuel tank.

•	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).

Refer to Fuel Injector Replacement .

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