Description & Operation
Throttle Body Injection (TBI) uses a low pressure electric fuel pump to supply fuel to the injectors. The fuel pump is mounted in the fuel tank, attached to sending unit. A relay controls pump operation.
When ignition switch is turned to "ON" position, the Electronic Control Module (ECM) turns on the fuel pump relay for 2 seconds. If engine is not started within this two second period, the ECM shuts off the fuel pump. As soon as the engine is cranked again, the ECM turns the relay on and runs the fuel pump.
On the 5.7L engine in the G van and all other 5.7L or 7.4L engines in vehicles over 8500 GVW, a fuel module will override the ECM and the fuel pump will run for approximately twenty seconds. The fuel module corrects a hot restart vapor lock during a high ambient condition.
As a back-up system to the fuel pump relay, the fuel pump can also be turned on by the oil pressure switch. When the engine oil pressure reaches about 4 psi (.28 kg/cm2 ) through cranking and the fuel pump relay does not complete the circuit, the oil pressure switch will close to complete the circuit to run the fuel pump. An inoperative fuel pump relay can result in long cranking time, particularly when the engine is cold.