Chart A-5 A-6 & A-7 Fuel System Diagnosis
Chart A-5 - Fuel System Diagnosis:
Wiring Diagram For Chart A-5, A-6 & A-7 - Fuel System Diagnosis:
CHART A-5, A-6, & A-7 - FUEL SYSTEM DIAGNOSIS
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 HEI distributor reference pulses.
If there are no reference pulses, the ECM will shut "OFF" the fuel pump within 2 seconds after key "ON".
The pump will deliver fuel to the TBI unit, where the system pressure is controlled to 62 to 90 kPa (9 to 13 psi). Excess fuel is then returned to the fuel tank. The fuel pump test terminal is located near the fuel pump relay of the engine compartment. When the engine is stopped, the pump can be turned "ON" by applying battery voltage to the test terminal.
Improper fuel system pressure will result in one or all of the following symptoms:
^ Cranks but won't run.
^ Code 44
^ Code 45
^ Cuts out, may feel like ignition problem
^ Poor fuel economy, loss of power.
^ Hesitation.
Test Description:
1. Determines if the pump circuit is 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".
2. Turns "ON" the fuel pump if circuit 120 wiring is OK. If the pump runs, it is a basic fuel delivery problem which the following steps will locate.
3. Checks for battery voltage at the pump relay.
Chart A-6 - Fuel System Diagnosis:
Test Description (Cont'd):
4. Checks relay ground CKT 450.
5. Checks for ECM control of relay through CKT 465.
6. 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 27 kPa (4 psi) 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 will 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".
7. Checks the oil pressure switch to be sure it provides battery feed to the fuel pump should the pump relay fail.
8. 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:
Fuel Pressure Gauge Test Point For Chart A-7:
Test Description (Cont'd):
9. Pressure but, less than 62 kPa (9 psi), falls into two areas:
^ Regulated pressure but less than 62 kPa (9 psi) - Amount of fuel to injector OK but, pressure is too low. System will be lean running and may set Code 44. Also, hard starting cold and poor overall performance.
^ Restricted flow causing pressure drop - Normally, a vehicle with a fuel pressure of less than 62 kPa (9 psi) at idle will not be driveable.
10. 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 from 90 to 124 kPa (13 to 18 psi).
11. This test determines if the high fuel pressure is due to a restricted fuel return line or a throttle body pressure regulator problem.