Fuel System Diagnosis Table
Circuit Description
When you turn ON the ignition switch, the Vehicle Control Module (VCM) turns ON the in-tank fuel pump. 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 34730-1A Fuel Pressure Gage in J 34730-E or |
| • | J 34730-1A Fuel Pressure Gage with J 34730-250 Fuel Pressure Adapter Kit. |
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, the fuel pressure should be 415-455 kPa (60-66 psi) for the Central SFI. 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 fuel pump check valve not holding. |
| • | A partially disconnected fuel pulse dampener (pulsator). |
| • | The fuel pressure regulator valve leaking. |
| • | A central SFI injector and poppet valve leaking. |
-
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 very somewhat depending on the barometric pressure; however, the pressure idling should be less indicating the pressure regulator control.
-
The fuel pressure less than 415 kPa (60 psi) for Central SFI, falls into the following 3 areas:
| • | A regulated pressure but less than 415 kPa (60 psi) for Central SFI. Amount of fuel to injector OK, but pressure is too low. System will run lean and may set a DTC. Also, hard starting cold and overall poor performance or may not start at all. 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 undriveable. 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. |
Step | Action | Value(s) | Yes | No | ||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
1 |
Important: Before clearing DTCs, use the scan tool in order to record the freeze frame and the failure records for reference because the Clear Info function will lose the data. Was the Powertrain On-Board Diagnostic (OBD) System Check performed? | -- | ||||||||||
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) | |||||||||||
3 |
Does the fuel pressure hold steady within the specified value? | Central SFI - 415-455 kPa (60-66 psi) | ||||||||||
Does the fuel pressure hold steady within the specified value? | Central SFI - 415-455 kPa (60-66 psi) | |||||||||||
Is the fuel pressure at the specified value? | Central SFI - 415-455 kPa (60-66 psi) | Go to Driveability Symptoms | ||||||||||
6 | Check for a partially disconnected fuel pulse dampener (Pulsator). Was a problem found? | -- | ||||||||||
7 | Repair connection at the fuel pulse dampener (pulsator). Refer to Wiring Repairs in Engine Electrical. Is the action complete? | -- | -- | |||||||||
8 | Inspect the fuel feed line between the pinch and the throttle body. Was a problem found? | -- | ||||||||||
9 |
Is fuel pressure present? | -- | ||||||||||
10 |
Listen for the fuel pump running. Does the fuel pump run? | -- | ||||||||||
11 | Perform the following checks:
Was a problem found? | -- | ||||||||||
12 |
Is the action complete? | -- | ||||||||||
13 | Replace the in-tank fuel pump. Refer to Fuel Sender Assembly Replacement (2nd design) . Is the action complete? | -- | -- | |||||||||
14 |
Is the fuel pressure above the specified value? | Central SFI - 455 kPa (66 psi) | ||||||||||
15 |
Is the fuel pressure above the specified value? | Central SFI - 415 kPa (60 psi) | ||||||||||
Is the fuel pressure below the specified value | Central SFI - 415 kPa (60 psi) | |||||||||||
17 | Check for a restricted fuel line from the fuel pressure regulator to the point where the fuel line was disconnected. Is the fuel line restricted? | -- | ||||||||||
18 | Repair the restriction in the fuel line from the fuel pressure regulator to the point where the fuel line was disconnected. Is the action complete? | -- | -- | |||||||||
19 | Repair the restriction in the fuel return line to the fuel tank. Is the action complete? | -- | -- | |||||||||
20 | Perform the following items:
Was a problem found? | -- | ||||||||||
21 | Repair as necessary or replace as necessary the following items:
Is the action complete? | -- | -- | |||||||||
Notice: DO NOT allow the fuel pressure to exceed 517 kPa (75 psi). Excessive pressure may damage the fuel system. Is the fuel pressure above the specified value? | Central SFI - 455 kPa (66 psi) | |||||||||||
23 | Replace the fuel pressure regulator. Refer to Fuel Pressure Regulator Replacement . Is the action complete? | -- | -- | |||||||||
24 | Is the fuel pressure less then the specified value? | Central SFI - 415 kPa (60 psi) | ||||||||||
25 | Perform the following items:
Was a problem found? | -- | ||||||||||
26 | Repair or replace as necessary:
Is the action complete? | -- | -- | |||||||||
27 | Replace the fuel feed line. Is the action complete? | -- | -- | |||||||||
28 |
Does the engine start and continue to run? | -- | ||||||||||
29 |
Are any DTCs displayed? | -- | Go to The Applicable DTC Table | |||||||||
30 | Using the scan tool, select the Capture Info and the Review Info. Are any DTCs displayed that have not been diagnosed? | -- | Go to The Applicable DTC Table | System OK |
