Fuel Pump: Description and Operation

When the ignition switch is turned to the "ON" position (before engaging starter), the Engine Control Module (ECM) turns the FP relay "ON" for two seconds and the FP builds up pressure to the TBI unit. If the engine is not started within two seconds, the ECM shuts the FP "OFF" and waits until the engine starts. As soon as the engine is cranked, the ECM turns the relay "ON" and runs the FP.
As a back up system to the FP relay, the FP also can be turned "ON" by the oil pressure switch. The oil pressure sender has two internal circuits. One operates the oil pressure indicator or gage in the instrument cluster, and the other is a normally open switch which closes when oil pressure reaches about 28 kPa (4 psi). If the FP relay fails, a Diagnostic Trouble Code (DTC) 54 is set in the ECM, and the oil pressure switch runs the FP.
An inoperative FP relay can result in long cranking times, particularly if the engine is cold. The oil pressure switch will turn "ON" the FP as soon as oil pressure reaches about 28 kPa (4 psi).