Fuel Pump: Description and Operation

DESCRIPTION
The fuel pump is a roller vane, high 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 in the Central MFI unit . 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. 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.

OPERATION
In order to properly control the fuel supply, the fuel pump is operated by the control module through the fuel pump relay and oil pressure switch.

When the key is first turned "ON," the Powertrain Control Module (PCM) energizes the fuel pump for two seconds to build up the fuel pressure quickly. If the engine is not started within two seconds, the PCM shuts "OFF" the fuel pump and waits until the engine starts. When the engine is cranked and the fuel control reference signal has been detected by the PCM, the PCM supplies 12 volts to the fuel pump relay to energize the electric in-tank fuel pump.

As a backup system to the fuel pump relay, the fuel pump can also be energized via the oil pressure switch. The normally open oil pressure switch closes when oil pressure reaches about 28 kPa (4 psi). If the fuel pump relay fails, the oil pressure switch will close and run the fuel pump.

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).