* Fuel Pump Itself: The most obvious possibility is a failed fuel pump. Relays simply switch power; a bad pump won't run even with power. This is the most common cause.
* Wiring Harness: Look for broken, corroded, or frayed wires in the harness leading to the fuel pump. This is especially common in older vehicles exposed to the elements. A visual inspection is crucial, and a multimeter can check for continuity.
* In-Tank Fuel Pump Module (if applicable): Some 1991 F-150s might have an in-tank fuel pump module. This module contains the pump, sending unit, and other components. A faulty module can cause the pump to not function.
* Fuel Pump Inertia Switch: This safety device cuts power to the fuel pump after a collision to prevent fuel leaks. It's typically located in the passenger compartment, often under the dash. Check if it's tripped (push the button to reset it).
* Cranking Sensor/Cam Sensor/Crankshaft Position Sensor (CKP): The fuel pump is often controlled by the PCM (Powertrain Control Module) and these sensors. If the PCM doesn't receive the correct signals telling it the engine is cranking, it may not energize the fuel pump. A bad sensor can prevent the pump from working.
* PCM (Powertrain Control Module): While less likely, a faulty PCM can prevent the fuel pump from receiving power. This is a more advanced diagnosis usually requiring specialized tools.
Troubleshooting Steps:
1. Check the Fuel Pump Relay Socket: Even with new relays, ensure the socket itself isn't corroded or damaged. Clean it thoroughly.
2. Listen for the Pump: Have someone crank the engine while you listen near the fuel tank. You should hear a whirring sound if the pump is trying to operate. The absence of sound strongly suggests a pump issue.
3. Check for Power at the Fuel Pump: Use a multimeter to check for power at the fuel pump connector while someone cranks the engine. You should have power there if the relay and wiring are good.
4. Check the Fuel Pump Inertia Switch.
5. Inspect the Wiring Harness: Carefully examine the wiring leading to the fuel pump for damage.
If you're not comfortable working with car electrical systems, it's best to take your truck to a qualified mechanic. They have the tools and experience to diagnose the problem accurately and safely. Incorrectly diagnosing and fixing electrical problems can be dangerous.
