Chart A-6 Fuel System Diagnosis

Chart A-6 Fuel System Diagnosis - Central MFI (1 Of 2):

Chart A-6 Fuel System Diagnosis - Central MFI (2 Of 2):

Fuel System Circuit:

Central Multiport Fuel Injection (CMFI) System:

TOOLS REQUIRED
J 34730-1A Fuel pressure gage.
OR
J 34730-1 Fuel pressure gage with J 34730-250 fuel pressure adapter kit.

CIRCUIT DESCRIPTION
When the ignition switch is turned "ON" the Powertrain Control Module (PCM) will turn "ON" the in-tank fuel pump. It 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 shut "OFF" the fuel pump within 2 seconds after ignition "ON" or engine stops.
The Central Multiport Fuel Injection (CMFI) pump will deliver system pressure that is controlled to about 380-420 kPa (55 to 61 psi). Excess fuel is then returned to the fuel tank.

CHART TEST DESCRIPTION
Number(s) below refer to circled number(s) on the diagnostic chart:

1. Wrap a shop towel around the fuel pressure connection to absorb any small amount of fuel leakage that may occur when installing the gage. Ignition "ON" pump pressure should be 380-420 kPa (55-61 psi). This pressure is controlled by spring pressure within the regulator assembly.

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

3. Fuel pressure that continues to fall is caused by one of the following:
^ In-tank fuel pump check valve not holding.
^ A partially disconnected fuel pulse dampener (pulsator).
^ Fuel pressure regulator valve leaking.
^ CMFI injector and poppet valve(s) leaking.

4. Fuel pressure but less than 380 kPa (55 psi) falls into three areas:

^ Regulated pressure but less than 380 kPa (55 psi). Amount of fuel to injector OK, but pressure is too low. System will be lean running and may set Diagnostic Trouble Code (DTC) 44. Also, hard starting cold and overall poor performance or may not start at all.

^ Restricted flow causing pressure drop - Normally, a vehicle with a fuel pressure of less than 300 kPa (44 psi) at idle will not be driveable. However, if the pressure drop occurs only while driving, the engine will normally surge then stop running as pressure begins to drop rapidly. This is most likely caused by a restricted fuel line or plugged filter.

^ Leaking or contaminated pressure regulator valve/seat interface may not allow regulated pressure to be achieved.

5. Restricting the fuel return line allows fuel pressure to build above regulated pressure, with battery voltage applied to the pump "test" terminal, pressure should rise above 420 kPa (61 psi) as the valve in the return line is partially closed.

CAUTION: Do NOT allow pressure to exceed 517 kPa (75 psi), as damage to the regulator may result.

6. This test determines if the high fuel pressure is due to a restricted fuel return line or a pressure regulator problem.