Component Diagnostic Charts
Chart A-7 - Fuel System Diagnosis (Part 1 of 4):
Fuel System Diagnosis:
CHART A-7 - FUEL SYSTEM DIAGNOSIS (PART 1 OF 4)
Circuit Description:
When the ignition switch is turned "ON," the Electronic Control Module (ECM) will turn "ON" the in-tank fuel pump. It will remain "ON" as long as the engine is cranking or running, and the ECM is receiving reference pulses. If there are no reference pulses, the ECM will shut "OFF" the fuel pump within 2 seconds after ignition "ON" or engine stops.
^ The pump will deliver fuel to the fuel rail and injectors, then to the pressure regulator, where the system pressure is controlled to about 234 to 325 kPa (34 to 47 psi). Excess fuel is then returned to the fuel tank.
Test Description: Numbers below refer to circled numbers on the diagnostic chart.
1. Wrap a shop towel around the fuel pressure connector to absorb any small amount of fuel leakage that may occur when installing the gage. Ignition "ON," pump pressure should be 280-325 kPa (40.5-47 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. Pressure that continues to fall is caused by one of the following:
^ In-tank fuel pump check valve not holding
^ Pump coupling hose or pulsator leaking
^ Fuel pressure regulator valve leaking
^ Injector(s) sticking open
4. An injector sticking open can best be determined by checking for a fouled or saturated spark plug(s). If a leaking injector can not be determined by a fouled or saturated spark plug the following procedure should be used.
^ Remove Plenum, and fuel rail bolts, but leave fuel lines connected.
^ Lift fuel rail out just enough to leave injector nozzles in the ports.
CAUTION: Be sure injector(s) are not allowed to spray on engine and that injector retaining clips are intact. This should be carefully followed to prevent fuel spray on engine which would cause a fire hazard.
^ Pressurize the fuel system and observe injector nozzles.
Chart A-7 - Fuel System Diagnosis (Part 2 of 4):
Wiring Diagram for Chart A-7 - Fuel System Diagnosis:
CHART A-7 - FUEL SYSTEM DIAGNOSIS (PART 2 OF 4)
CONTINUED FROM A3 OR A7
Test Description: Numbers below refer to circled numbers on the diagnostic chart.
1. If the fuse is blown, this test will confirm a short to ground on CKT 120. To prevent misdiagnosis, be sure fuel pump is disconnected before test.
2. Determines if the pump circuit is Electronic Control Module (ECM) controlled. The ECM will turn "ON" the pump relay. Engine is not cranking or running so the ECM will turn "OFF" the relay within 2 seconds after ignition is turned "ON".
3. Turns "ON" the fuel pump if CKT 120 wiring is OK. If the pump runs, it is a basic fuel delivery problem which the following steps will locate.
4. Checks for battery voltage at the pump relay.
5. Checks relay ground CKT 450.
Chart A-7 - Fuel System Diagnosis (Part 3 of 4):
CHART A-7 - FUEL SYSTEM DIAGNOSIS (PART 3 OF 4)
Test Description: Numbers below refer to circled numbers on the diagnostic chart.
6. Checks for Electronic Control Module (ECM) control of relay through CKT 465.
7. The fuel pump voltage control circuit includes an engine oil pressure switch with a separate set of normally open contacts. The switch closes at about (4 lbs) 27 kPa of oil pressure and provides a second battery feed path to the fuel pump. If the relay fails, the pump will continue to run using the battery feed supplied by the closed oil pressure switch.
A failed pump relay may result in extended engine crank time, because of the time required to build enough oil pressure to close the oil pressure switch and turn "ON" the fuel pump.
There may be instances when the relay has failed but the engine will not crank fast enough to build enough oil pressure to close the switch. This or a faulty oil pressure switch can result in "Engine Cranks But Will Not Run".
8. Checks the oil pressure switch to be sure it provides battery feed to the fuel pump should the pump relay fail.
9. Checks for open oil pressure switch with ignition "OFF." Should the switch stick closed, the fuel pump will continue to run and discharge the battery.
Chart A-7 - Fuel System Diagnosis (Part 4 of 4):
Fuel System Diagnosis:
CHART A-7 - FUEL SYSTEM DIAGNOSIS (PART 4 OF 4)
Test Description: Numbers below refer to circled numbers on the diagnostic chart.
1. Pressure but less than 280 kPa (40.5 psi) falls into two areas:
^ Regulated pressure but less than 280 kPa (40.5 psi). Amount of fuel to injectors OK but pressure is too low. System will be lean running and may set Code 44. Also, hard starting cold and overall poor performance.
^ Restricted flow causing pressure drop - Normally, a vehicle with a fuel pressure of less than 165 kPa (24 psi) at idle will not be driveable. However, if the pressure drop occurs only while driving, the engine will normally surge then stop as pressure begins to drop rapidly.
2. Restricting the fuel return line allows the fuel pump to develop its maximum pressure (dead head pressure). When battery voltage is applied to the pump test terminal, pressure should be above 414 kPa (60 psi).
3. This test determines if the high fuel pressure is due to a restricted fuel return line or a pressure regulator problem.