Fuel System Diagnosis. GMC Truck GMC P42 Commercial Chassis 1998

For 1990-2009 cars only

Makes 🢒 GMC_Truck 🢒 1998 🢒 GMC P42 Commercial Chassis 🢒 Engine 🢒 Engine Controls - 5.0L and 5.7L 🢒 Fuel System Diagnosis

Circuit Description

When the ignition switch is turned ON, the Vehicle Control Module (VCM) energizes the fuel pump relay which powers the in-tank fuel pump ON. The pump remains ON as long as the engine is cranking or running and the VCM is receiving reference pulses. If there are no reference pulses, the VCM shuts the fuel pump OFF within 2 seconds after the ignition was switched to the ON position or if the engine stops. The VCM will also turn ON the fuel pump for 2 seconds when the ignition is turned to the OFF position.

An electric fuel pump pumps the fuel through an in-line filter to the Central SFI unit. The pump is attached to the fuel level meter assembly inside of the fuel tank. The pump is designed to provide fuel pressure above what is needed by the fuel injectors. The pressure regulator keeps the fuel available to the injectors at a regulated pressure. Unused fuel is returned to the fuel tank by the fuel return pipe.

Diagnostic Aids

Tools Required

•	J 34730-E Fuel Pressure Gauge.

•	J 34730-1A Fuel Pressure Gauge with J 34730-250 Fuel Pressure Adapter Kit.

•	J 42873-1 and the J 42873-2 .

Test Description

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

Wrap a shop towel around the fuel pressure connection to absorb any small amount of fuel leakage that may occur when installing the fuel pressure gauge. Turn ON the ignition leaving the engine OFF, the fuel pressure should be 415-455 kPa (60-66 psi). This pressure is controlled by a spring pressure within the regulator assembly.

The fuel pressure that continues to fall is caused by one of the following items:

•	The in-tank modular fuel sender is not holding pressure.

•	The fuel pressure regulator valve is leaking.

•	A Central SFI injector is leaking.

•	Fuel line is leaking.

If the fuel pressure is less than 415 kPa (60 psi), it falls into the following 3 areas:

•	The pressure is regulated but less than 415 kPa (60 psi).The amount of fuel reaching the injector is sufficient, but the pressure is too low. The system will run lean, hard starting cold, no start, overall poor performance, and may set a DTC. Refer to Engine Cranks but Does Not Run

•

A restricted flow causing pressure drop -- Normally, a vehicle with a fuel pressure of less than 300 kPa (44 psi) at idle is inoperable. However, if the pressure drop occurs only while driving, the engine will normally surge then stop running as the pressure begins to drop rapidly. This is most likely caused by a restricted fuel line or plugged filter.

•	A leaking or contaminated pressure regulator valve or seat interface may not allow the regulated pressure to be achieved.

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

Restricting the fuel return line allows the fuel pressure to build above regulated pressure. With the battery voltage applied to the fuel pump test terminal, the pressure should rise above 455 kPa (66 psi), as the valve in the return line is partially closed.

When the engine is idling, the manifold pressure is low (high vacuum) and is applied to the fuel pressure regulator diaphragm. This will offset the spring and result in a lower fuel pressure. This idle pressure will vary somewhat depending on the barometric pressure; however, the pressure at idle should be less indicating the pressure regulator control.

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 OFF the ignition for 10 seconds with the A/C OFF.
Relieve the fuel pressure. Refer to Fuel Pressure Relief .

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 fuel pressure gauge J 34730-1A or the equivalent.
Turn ON the ignition leaving the engine OFF.
The fuel pump should operate for 2 seconds.
Bleed the air out of the fuel pressure gauge into an approved gasoline container.
Turn OFF the ignition for 10 seconds.
Turn ON the ignition leaving the engine OFF.
Monitor the fuel pressure with the fuel pump running. It should measure within the specified value.

Important: The ignition may have to be cycled ON more than once in order to obtain the maximum fuel pressure.

Is the fuel pressure within the specified value?

Central SFI - 415-455 kPa (60-66 psi)

Go to Step 3

Go to step 8

Turn OFF the ignition for 10 seconds with the A/C OFF.
Turn ON the ignition leaving the engine OFF. The fuel pump should operate for 2 seconds.
Monitor the fuel pressure after the pump stops running. The pressure may vary a few pounds then the pressure should stabilize and hold steady.

Does the fuel pressure hold steady within the specified value?

Central SFI - 415-455 kPa (60-66 psi)

Go to Step 5

Go to Step 4

Relieve the fuel pressure. Refer to Fuel Pressure Relief .

Caution: Place a dry chemical (Class B) fire extinguisher nearby before performing any on-vehicle service procedures. Failure to follow these precautions may result in personal injury.

Caution: Provide proper ventilation when working with fuel in enclosed areas where fuel vapors can collect. The lack of adequate ventilation may result in personal injury.

Disconnect the fuel filter.
Drain any remaining fuel from the fuel pipes into an approved gasoline container.
Install the J 42873-1 in the place of the fuel filter.
Open the valve on the fuel pipe shut-off adapter.
Turn OFF the ignition for 10 seconds with the A/C OFF.
Turn ON the ignition leaving the engine OFF.
Check for any leaks at the fuel shut-off adapter fittings.
Start the engine and let idle for 30 seconds.
Turn OFF the ignition for 10 seconds with the A/C OFF.
Turn ON the ignition leaving the engine OFF.
Wait for the fuel pressure to build.
Close the valve in the fuel feed pipe shut-off. The pressure should remain constant.

Does the fuel pressure remain constant within the specified value?

Central SFI - 415-455 kPa (60-66 psi)

Go to Step 12

Go to Step 6

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

Go to Step 27

Go to Step 26

Inspect the fuel feed line between the shut-off adapter and the engine fuel pipes.

Was a problem found?

Go to Step 25

Go to Step 7

Open the valve in the fuel feed pipe shut-off adapter.
Releive the fuel pressure. Refer to Fuel Pressure Relief .

Caution: Place a dry chemical (Class B) fire extinguisher nearby before performing any on-vehicle service procedures. Failure to follow these precautions may result in personal injury.

Caution: Provide proper ventilation when working with fuel in enclosed areas where fuel vapors can collect. The lack of adequate ventilation may result in personal injury.

Disconnect the fuel return pipe union located near the fuel filter.
Drain any remaining fuel from the fuel pipes into an approved gasoline container.
Install the J 42873-2 in the place of the fuel return pipe union.
Open the valve in the fuel return pipe shut-off adapter.
Turn OFF the ignition for 10 seconds with the A/C OFF.
Turn ON the ignition leaving the engine OFF.
Check for any leaks at the fuel shut-off adapter fittings.
Start the engine and let idle for 30 seconds.
Turn OFF the ignition for 10 seconds with the A/C OFF.
Turn ON the ignition leaving the engine OFF.
Wait for the fuel pressure to build.
Close the valve in the shut-off adapter that is connected to the fuel return pipe.

Does the fuel pressure remain constant?

Go to Step 22

Go to Step 29

Turn OFF the ignition for 10 seconds with the A/C OFF.
Turn ON the ignition leaving the engine OFF.The fuel pump should operate for 2 seconds and then turn off.
Monitor the fuel pressure after fuel pump stops running.

Is the fuel pressure present?

Go to Step 13

Go to Step 9

Turn OFF the ignition.
Connect a 10 amp fused jumper wire from B+ to the fuel pump test terminal.
Listen for the fuel pump running.

Does the fuel pump run?

Go to Step 10

Go to Fuel Pump Electrical Circuit Diagnosis

Check for the following conditions:

•	Plugged in-line fuel filter

•	Plugged fuel pump strainer

•	Restricted fuel line

Was a problem found?

Go to Step 11

Go to Step 12

Repair or replace as necessary.

Is the action complete?

Go to Step 30

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

Is the action complete?

Go to Step 30

Turn OFF the ignition for 10 seconds with the A/C OFF.
Turn ON the ignition leaving the engine OFF. The fuel pump should operate for 2 seconds.
Monitor the fuel pressure.

Is the fuel pressure above the specified value?

Central SFI - 455 kPa (66 psi)

Go to Step 14

Go to Step 15

Relieve the fuel pressure. Refer to Fuel Pressure Relief .

Caution: Place a dry chemical (Class B) fire extinguisher nearby before performing any on-vehicle service procedures. Failure to follow these precautions may result in personal injury.

Caution: Provide proper ventilation when working with fuel in enclosed areas where fuel vapors can collect. The lack of adequate ventilation may result in personal injury.

Disconnect the engine compartment fuel return line
Attach a flex hose to the fuel return line.
Insert the other end into an approved gasoline container.
Monitor the fuel pressure within 2 seconds after the ignition is turned ON.

Is the fuel pressure within the specified value?

Central SFI - 415-455 kPa (60-66 psi)

Go to Step 16

Go to Step 17

Turn OFF the ignition for 10 seconds with the A/C OFF.
Turn ON the ignition leaving the engine OFF. The fuel pump should operate for 2 seconds.
Monitor the fuel pressure.

Is the fuel pressure below the specified value

Central SFI - 415 kPa (60 psi)

Go to Step 19

Go to Step 30

Locate and correct the restriction in the fuel return line.

Is the action complete?

Go to Step 30

Go to Step 17

Visually and physically inspect the engine fuel return outlet pipe (where the fuel line was disconnected) for restriction.

Was a restriction found?

Go to Step 18

Go to Step 22

Replace the engine fuel pipes. Refer to Fuel Hose/Pipes Replacement - Engine Compartment .

Is the action complete?

Go to Step 30

Perform the following items:

•	Check for a restricted fuel line.

•	Check for a restricted in-line fuel filter

Was a problem found?

Go to Step 20

Go to Step 21

Repair or replace as necessary the following items:

•	The restricted fuel line

•	The restricted fuel line filter

Is the action complete?

Go to Step 30

Relieve the fuel pressure. Refer to Fuel Pressure Relief .

Caution: Place a dry chemical (Class B) fire extinguisher nearby before performing any on-vehicle service procedures. Failure to follow these precautions may result in personal injury.

Caution: Provide proper ventilation when working with fuel in enclosed areas where fuel vapors can collect. The lack of adequate ventilation may result in personal injury.

Disconnect the fuel return pipe union located near the fuel filter.
Drain any remaining fuel from the fuel pipes into an approved gasoline container.
Install the J 42873-2 in the place of the fuel return pipe union.
Open the valve on the fuel return pipe shut-off adapter.
Turn OFF the ignition for 10 seconds with the A/C OFF.
Turn ON the ignition leaving the engine OFF.
Bleed the air out of the fuel pressure gauge into an approved gasoline container.
Connect a 10 amp fused jumper wire from B+ to the fuel pump test terminal.

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

Slowly close the valve on the fuel return pipe shut-off adapter. The pressure should rise above the specified value.

Does the fuel pressure rise above the specified value?

Central SFI - 455 kPa (66 psi)

Go to Step 22

Go to Step 23

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

Is the action complete?

Go to Step 30

Perform the following items:

•	Check for low fuel pump voltage due to high resistance connections

•	Check for high fuel pump voltage due to internal pump binding

•	Check for a poor fuel pump ground

•	Check for a partially restricted fuel pump strainer

•	Check for an incorrect fuel pump

•	Check for a faulty fuel pump

Was a problem found?

Go to Step 24

Go to Step 12

Repair or replace as necessary.

Is the action complete?

Go to Step 30

Replace the fuel feed line.

Is the action complete?

Go to Step 30

Start the engine.
Idle until the engine reaches the normal operating temperature.
Open the throttle quickly.
Monitor the fuel pressure.
The fuel pressure should approach the top of the specified range.

Does the fuel pressure rise quickly?

Central SFI - 415-455 kPa (60-66 psi)

Go to Symptoms

Go to Step 27

Visually and physically inspect the following items for a restriction:

The in-line fuel filter.
The fuel feed pipe.
The fuel pump strainer must not be damaged or mispositioned.
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 28

Go to Step 12

Repair the problem as necessary.

Is the action complete?

Go to Step 30

Locate and replace any leaking injector(s).

Is the action complete?

Go to Step 30

Go to Fuel Injector Balance Test

Use the scan tool in order to display the Capture Information and the Review Capture Information function.

Are there any DTCs displayed that have not been diagnosed?

Go to the applicable DTC table

System OK