Fuel Pump: Description and Operation

Fuel Pump:

PURPOSE
To provide a constant fuel supply at a pressure greater than is needed by the injectors. Vapor lock problems are reduced using an in-tank fuel pump because the fuel is pushed from the tank under pressure rather than being pulled under a vacuum, a condition that produces vapor.

OPERATION
The fuel pump is a turbine type, low pressure electric pump, mounted in the fuel tank. Fuel is pumped at a positive pressure (above 62 kPa or 9 psi) from the pump through an in-line filter to the pressure regulator in the TBI unit. Excess fuel is returned to the fuel tank through the fuel return line.
When the key is first turned "ON" without the engine running, the control module turns the fuel pump relay "ON" for two seconds. This builds up the fuel pressure quickly. If the engine is not started within two seconds, the control module shuts the fuel pump "OFF" and waits for ignition reference pulses. As soon as the engine is cranked, the control module turns the relay "ON" and runs the fuel pump.
As a backup system to the fuel pump relay, the fuel pump is also turned "ON" by an oil pressure switch. When the engine oil pressure reaches about 28 kPa (4 psi), through cranking the oil pressure switch will close to complete the circuit to the fuel pump.
An inoperative fuel pump relay can result in long cranking times, particularly if the engine is cold.
An inoperative fuel pump would cause a no start condition. A fuel pump which does not provide enough pressure can result in poor performance.