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For 1990-2009 cars only
Makes 🢒 Pontiac 🢒 1996 🢒 Firebird 🢒 Engine 🢒 Engine Controls - 4.3L (Caprice Only) and 5.7L 🢒 Fuel System Diagnosis

Fuel System Diagnosis Fuel System Diagnosis


Object Number: 21294 Size: MF
(1)J34730-1A Fuel Pressure Gauge
(2)Fuel Pressure Gauge Bleed Hose
(3)Return Pipe
(4)Feed Pipe
(5)J 37287 Fuel Line Shut-off Adapters

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-pipe fuel filter to the fuel rail 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 fuel rail, 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. The fuel pump test connector is located in the engine compartment near the passenger side shock tower.

Test Description

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

  1. 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 spring pressure inside the fuel pressure regulator.

    2.

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

    3. • The fuel pump check valve.
    • The fuel pump flex pipe.
    • The valve or valve seat within the fuel pressure regulator.
    • The fuel injector(s).

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

    4.

  4. 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 heated oxygen sensors(s) will stop toggling. The heated oxygen sensor output voltage(s) will drop below 300 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.

  1. 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 the 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.

    2.

  2. A rich condition may result from the fuel pressure being above 325 kPa (47 psi). A rich condition may cause DTC P0132, DTC P0152, DTC P0172 or DTC P0175 to set. Driveability conditions associated with rich conditions can include hard starting (followed by black smoke) and a strong sulfur smell in the exhaust.

    3.

  3. 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.

    4.

  4. A lean condition may result from the fuel pressure being below 284 kPa (41 psi). A lean condition may cause DTC P0131, DTC P0151, DTC P0171 or DTC P0174 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.

    5.

  5. Notice: Do not allow the fuel pressure to exceed 414 kPa (60 psi). Fuel Pressure in excess of 414 kPa (60 psi) may damage the fuel pressure regulator.

    Restricting the fuel return pipe with the J 37287 fuel pipe shut-off adapter causes the fuel pressure to rise above the regulated fuel pressure. With battery voltage applied to the fuel pump test connector, the fuel pressure should rise above 325 kPa (47 psi) as the valve on the fuel pipe shut-off adapter connected to the fuel return pipe becomes partially closed.

  6. 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. 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:

    7. 1. Remove the fuel rail, but leave the fuel pipes connected to the fuel rail. Refer to Fuel Injection Fuel Rail Assembly Replacement .
    2. Lift the fuel rail just enough to leave the fuel injector nozzles in the fuel injector ports.

    Caution: In order to reduce the risk of fire and personal injury that may result from fuel spraying on the engine, verify that the fuel rail is positioned over the fuel injector ports. Also verify that the fuel injector retaining clips are intact.

    3. Pressurize the fuel system by using the scan tool fuel pump enable.
    4. Visually and physically inspect the fuel injector nozzles for leaks.

Step

Action

Value(s)

Yes

No

1

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

--

Go to Step 2

Go to Powertrain On Board Diagnostic (OBD) System Check

2
  1. Turn the ignition OFF.
  2. Turn the air conditioning system OFF.

    3. 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.

  3. Install the J 34730 fuel pressure gauge (refer to (1) in the illustration).
  4. Place the bleed hose (refer to (2) in the illustration) of the fuel pressure gauge into an approved gasoline container.
  5. Turn the ignition ON.
  6. Bleed the air out of the fuel pressure gauge.
  7. Turn the ignition OFF for 10 seconds.
  8. Turn the ignition ON.

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

  9. Observe the fuel pressure 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

3

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 remain constant.

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

4

Relieve the fuel pressure to the first specified value.

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 19

Go to Step 5

5

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

--

Go to Step 6

Go to Step 8

6

Visually and physically inspect the following items for a restriction:

    • The fuel filter.
    • The fuel feed pipe (refer to (4) in the illustration).

Was a restriction found?

--

Go to Step 24

Go to Step 7

7
  1. Remove the fuel sender assembly. Refer to Fuel Sender Assembly Replacement .
  2. Visually and physically inspect the following items:
    3. • 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.

Was a problem found in any of these areas?

--

Go to Step 24

Go to Step 19

8
  1. Start the engine.
  2. 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

9
  1. Disconnect the vacuum hose from the fuel pressure regulator.
  2. With the engine idling, apply 12-14 inches of vacuum to the fuel pressure regulator manifold vacuum hose connection.

Does the fuel pressure drop by the amount specified?

21-69 kPa (3-10 psi)

Go to Step 20

Go to Step 21

10
  1. Relieve the fuel pressure. Refer to the Fuel Pressure Relief Procedure .
  2. Disconnect the fuel feed pipe (refer to (4) in the illustration) and the fuel return pipe (refer to (3) in the illustration) from the fuel rail. Refer to Servicing Quick Connect Fittings .
  3. Install the J 37287 fuel pipe shut-off adapters (refer to (5) in the illustration) between the fuel feed pipe and the fuel return pipe and the fuel rail.
  4. Open the valves on the fuel pipe shut-off adapters.
  5. Turn the ignition OFF.
  6. Pressurize the fuel system using a scan tool.
  7. Place the bleed hose (refer to (2) in the illustration) of the fuel pressure gauge (refer to (1) in the illustration) into an approved gasoline container.
  8. Bleed the air out of the fuel pressure gauge.
  9. Wait for the fuel pressure to build.
  10. 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 19

Go to Step 11

11
  1. Open the valve in the fuel pipe shut-off adapter (refer to (5) in the illustration) that is connected to the fuel feed pipe (refer to (4) in the illustration).
  2. Pressurize the fuel system using a scan tool.
  3. Wait for the fuel pressure to build.
  4. Close the valve in the fuel pipe shut-off adapter that is connected to the fuel return pipe (refer to (3) in the illustration).

Does the fuel pressure remain constant?

--

Go to Step 21

Go to Step 22

12

Is the fuel pressure above the specified limit?

325 kPa (47 psi)

Go to Step 13

Go to Step 15

13
  1. Relieve the fuel pressure. Refer to the Fuel Pressure Relief Procedure .
  2. Disconnect the fuel return pipe (refer to (3) in the illustration) from the fuel rail. Refer to Servicing Quick Connect Fittings .
  3. Attach a length of flexible fuel hose to the fuel rail outlet passage.
  4. Place the open end of the flexible fuel hose into an approved gasoline container.
  5. Turn the ignition OFF for 10 seconds.
  6. Turn the ignition ON.
  7. Observe the fuel pressure 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

14

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

Was a restriction found?

--

Go to Step 24

Go to Step 21

15

Is the fuel pressure above the specified value?

0 kPa (0 psi)

Go to Step 16

Go to Step 17

16
  1. Relieve the fuel pressure. Refer to the Fuel Pressure Relief Procedure .
  2. Disconnect the fuel return pipe (refer to (3) in the illustration) from the fuel rail. Refer to Servicing Quick Connect Fittings .
  3. Install the J 37287 fuel pipe shut-off adapter (refer to (5) in the illustration) between the fuel return pipe and the fuel rail.
  4. Open the valve on the fuel pipe shut-off adapter.
  5. Turn the ignition OFF.
  6. Pressurize the fuel system using a scan tool.
  7. Place the bleed hose (refer to (2) in the illustration) of the fuel pressure gauge (refer to (1) in the illustration) into an approved gasoline container.
  8. Bleed the air out of the fuel pressure gauge.

    9. Notice: Do not allow the fuel pressure to exceed 414 kPa (60 psi). Fuel Pressure in excess of 414 kPa (60 psi) may damage the fuel pressure regulator.

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

Does the fuel pressure rise above the specified value?

325 kPa (47 psi)

Go to Step 21

Go to Step 7

17

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

18

Visually and physically inspect the following items:

    • The fuel filter for obstructions.
    • The fuel feed pipe (refer to (4) in the illustration) 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 19

19

Replace the fuel pump.

Is the action complete?

--

System OK

--

20

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

Is the action complete?

--

System OK

--

21

Replace the fuel pressure regulator.

Is the action complete?

--

System OK

--

22

Locate and replace any leaking fuel injector(s).

Is the action complete?

--

System OK

--

23

Locate and correct the restriction in the fuel return pipe (refer to (3) in the illustration).

Is the action complete?

--

System OK

--

24

Repair the problem as necessary.

Is the action complete?

--

System OK

--