1. Check the Obvious:
* Fuel Level: The most obvious cause is simply being out of gas! Make sure the tank actually has fuel.
* Loose Connections: Start by checking all the electrical connections related to the fuel gauge. This includes the connection at the sending unit (in the gas tank), the wiring harness, and the connection at the gauge itself. Look for corrosion, loose wires, or broken wires. Clean and tighten any connections you find problematic.
2. Fuel Sending Unit:
The most common culprit is a faulty fuel sending unit. This unit is located inside the fuel tank and sends a signal to the gauge based on the fuel level.
* Access the Sending Unit: This requires dropping the fuel tank. This is a significant undertaking involving draining the fuel (safely!), disconnecting fuel lines, and possibly removing other components for access. Consult a repair manual for your specific truck model.
* Test the Sending Unit: Once accessed, you can test the sending unit with a multimeter to see if it's sending the correct resistance values as the float arm moves. A repair manual will provide the correct resistance ranges. If the resistance is incorrect or erratic, the sending unit needs replacement.
* Replace the Sending Unit: Replace the sending unit with a new one. Be sure to get the correct replacement part for your specific truck.
3. Fuel Gauge Cluster:
The gauge itself could be malfunctioning.
* Test the Gauge: You can try testing the gauge with a multimeter, applying a known resistance value to simulate a fuel level. Again, a repair manual will show you how.
* Replace the Gauge: If the gauge itself is faulty, it may need replacement. This involves removing the instrument cluster from the dashboard.
4. Wiring and Grounds:
* Check Wiring: Inspect the wiring harness leading from the sending unit to the gauge for any breaks, chafing, or corrosion. Repair or replace any damaged wiring.
* Grounds: Ensure good ground connections throughout the circuit. Poor grounding can cause erratic gauge readings.
5. Other Considerations:
* Instrument Cluster Repair/Refurbishment: If the gauge is intermittent or the entire cluster is exhibiting problems, a professional instrument cluster repair shop might be an option for repair or refurbishment.
* Fuse: Check the fuse related to the fuel gauge circuit.
Important Safety Precautions:
* Fuel is Flammable: Always work in a well-ventilated area when dealing with fuel tanks and lines. Never work near an open flame or spark.
* Static Electricity: Ground yourself before working on the fuel system to prevent sparks.
* Repair Manual: Consult a repair manual specific to your 1987 Ford F-150. This is crucial for safe and effective repair. A Haynes or Chilton manual will be helpful.
If you're not comfortable working on this yourself, it's best to take your truck to a qualified mechanic. Replacing the fuel sending unit is a significant task. Incorrectly doing so could damage your fuel tank or create further issues.
