GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 1998 🢒 GMC C-Series Medium Duty Truck B7 🢒 Engine 🢒 Engine Controls - 6.0L and 7.0L 🢒 Fuel System Diagnosis

Circuit Description

When the fuel pump is running, the system delivers the fuel to the injectors and then to the regulator. The regulator controls the system pressure to about 179 to 220 kPa (26 to 32 psi). The system returns the excess fuel to the fuel tank.

Diagnostic Aids

The fuel system is under pressure. In order to avoid fuel spillage, refer to the fuel system procedures for testing or making the repairs requiring disassembly of the fuel pipes or the fittings.

The fuel pressure drops to almost 0 psi after the pump shuts OFF.

Test Description

The numbers below refer to the step numbers on the Diagnostic Table.

  1. Pressure less than 179 kPa (26 psi) falls into 2 areas:

    2. • Regulated pressure less than 179 kPa (26 psi) -- Amount of fuel to the injector is OK but the pressure is too low. The system will be run lean and may set DTC 44. Also, hard starting cold and poor overall performance.
    • Restricted flow causing pressure drop -- Normally, a vehicle with a fuel pressure of less than 179 kPa (26 psi) at idle will not be drivable. However, if the pressure drop occurs only while driving, the engine could surge and stop when pressure is too low.

  1. This test determines if the high fuel pressure is due to a restricted fuel return pipe or a throttle body pressure regulator problem.

Fuel System Diagnosis

Step

Action

Value(s)

Yes

No

1

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

--

Go to Step 2

Go to Powertrain On Board Diagnostic (OBD) System Check

2
  1. Turn the ignition OFF for the specified time.
  2. Turn the ignition ON leaving the engine OFF.
  3. Listen for the in-tank fuel pump.

Did the fuel pump operate for the specified time?

10 seconds

2 seconds

Go to Step 3

Go to Fuel Pump Electrical Circuit Diagnosis

3
  1. Turn the ignition OFF. Check for an adequate fuel level in the main fuel tank.
  2. Relieve the fuel pressure.
  3. Install a fuel pressure gauge.
  4. Disconnect the fuel pump oil pressure switch.
  5. Connect a fused jumper wire between the battery feed circuit terminal and the fuel pump signal circuit terminal at the harness connector.
  6. Observe the fuel pressure within the specified time.

Is the fuel pressure within the specified range?

2 seconds

179-220 kPa (26-32 psi)

Fuel System OK

Go to Step 4

4

Does the engine require a long cranking times before the engine starts?

--

Go to Step 6

Go to Step 5

5

Is the fuel pressure below the specified value?

179 kPa (26 psi)

Go to Step 8

Go to Step 15

6
  1. Turn the ignition OFF.
  2. Connect a fused jumper between the battery positive terminal and the fuel pump test terminal.
  3. Wait until the fuel pressure stabilizes.
  4. Disconnect the jumper wire.
  5. Pinch off the fuel return line.

Does some of the pressure hold for the specified time?

Minimum 30 seconds

Go to Symptoms

Go to Step 7

7
  1. Check for a leaking fuel pump feed hose in the fuel tank.
  2. If no leaks have been detected, replace the fuel pump. Refer to Fuel Pump Strainer Replacement .

Is the action complete?

--

Go to Step 18

--

8
  1. Install a fuel pressure gauge on the inlet side of the fuel filter.
  2. Reinstall the outlet fuel filter line.
  3. Disconnect the fuel pump oil pressure switch.
  4. Connect a fused jumper wire between the battery feed circuit terminal and the fuel pump signal circuit terminal at the harness connector.
  5. Observe the fuel pressure.

Is the fuel pressure above the specified value?

179 kPa (26 psi)

Go to Step 9

Go to Step 10

9

Replace the fuel filter. Refer to In-Line Fuel Filter Replacement .

Is the action complete?

--

Go to Step 3

--

10
  1. Turn the ignition OFF.
  2. Disconnect the fuel injector connector.
  3. Disconnect the fuel pump oil pressure switch.
  4. Connect a fused jumper wire between the battery feed circuit terminal and the fuel pump signal circuit terminal at the harness connector.
  5. Gradually pinch the pressure gauge outlet hose.
  6. Observe the fuel pressure.

Is the fuel pressure above the specified value?

220 kPa (32 psi)

Go to Step 11

Go to Step 12

11

Replace the pressure regulator assembly. Refer to Fuel Meter Body Assembly Replacement .

Is the action complete?

--

Go to Step 18

--

12
  1. Check for a restricted line from the fuel tank to the fuel pressure gauge.
  2. If you find a problem, repair as necessary.

Did you find a problem?

--

Go to Step 18

Go to Step 13

13
  1. Check for a faulty coupling hose or a faulty fuel pump strainer.
  2. If you find a problem, repair as necessary.

Did you find a problem?

--

Go to Step 18

Go to Step 14

14

Check the fuel pump for being the wrong application. If OK, replace the fuel pump. Refer to Fuel Pump Replacement .

Is the action complete?

--

Go to Step 18

--

15

  1. Remove the jumper wire from the fuel pump oil pressure switch connector.
  2. Disconnect the fuel injector connector.
  3. Disconnect the fuel return line flexible hose.
  4. Attach a 5/16 I.D. flex hose to the throttle body side of the return line.
  5. Insert the other end into an approved gasoline container.
  6. Turn the ignition ON leaving the engine OFF.
  7. Observe the fuel pressure in the specified time.

Is the fuel pressure within the specified range?

2 seconds

179-220 kPa (26-32 psi)

Go to Step 16

Go to Step 17

16

Locate and repair the restriction in the fuel return line.

Is the action complete?

--

Go to Step 18

--

17
  1. Check for a restricted fuel line between the fuel pressure gauge and the return line flexible hose.
  2. If you find a problem, repair as necessary.

Did you find a problem?

--

Go to Step 18

Go to Step 11

18
  1. Clear the DTCs. Refer to Clearing DTCs.
  2. Attempt to start the engine.

Does the engine start and continue to run?

--

Go to Step 19

Go to Step 2

19

Allow the engine to idle until it reaches the normal operating temperature.

Are any DTCs stored?

--

Go to the Applicable DTC Table

System OK