Fuel Pump: Description and Operation

PURPOSE
To pump fuel from the fuel tank to the TBI or CPI unit through an in-line filter and fuel supply line.

OPERATION
The fuel pump is a turbine type, low pressure electric pump, mounted in the fuel tank. Fuel is pumped at a positive pressure from the fuel pump through the in-line filter to the pressure regulator. Excess fuel is returned to the fuel tank through the fuel return line.
The fuel pump is attached to the fuel gage sender assembly. A fuel strainer is attached to the fuel pump inlet line and prevents dirt particles from entering the fuel line and tends to separate water from the fuel.
When the key is first turned "ON" without the engine running, the control module turns a 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 until the engine starts. As soon as the engine is cranked, the control module turns the relay "ON" and runs the fuel pump.
When the engine is cranking or running, the control module receives distributor ignited reference pulses which in turn energize the fuel injectors.
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.

NOTE
- Vapor lock problems are reduced when using an electric pump because the fuel is pushed from the tank under pressure rather than being pulled under vacuum, a condition that produces vapor.
- An inoperative fuel pump relay can result in long cranking times, particularly if the engine is cold. The oil pressure switch will turn "ON" the fuel pump as soon as oil pressure reaches about 28 kPa (4 psi).