1. Simple Checks - The Easiest First:
* Check the obvious: Is the gauge needle pegged at empty *all the time*? Or is it fluctuating erratically? A pegged-at-empty gauge usually points to a different problem than one that's wildly inaccurate.
* Test the Fuel Pump: Listen for the fuel pump's whine when you turn the ignition key to the "on" position (don't start the engine). If you hear nothing, the pump might be faulty and that could indirectly affect the gauge reading if it's not providing fuel.
* Top off the tank: Sometimes, low fuel levels can cause erratic readings due to the sensor's position in the tank. A full tank can sometimes resolve intermittent problems.
2. More Involved Troubleshooting:
* Check the Fuel Sender Unit (FSU): This is the most likely culprit. The FSU is located in the fuel tank and contains a float that moves with the fuel level, sending a signal to the gauge. This requires dropping the fuel tank (a significant job), but this is often where the problem lies. You'll need to:
* Safety First: Disconnect the battery's negative terminal before starting any work around the fuel system.
* Access the Fuel Tank: This is usually the most labor-intensive part of the process.
* Inspect the FSU: Check the float for damage, debris, or a broken arm. Also, inspect the electrical connections for corrosion or damage.
* Test the FSU Resistance: If you have a multimeter, you can measure the resistance of the FSU at different "fuel levels" (simulated by moving the float). This requires a wiring diagram to know which terminals to check. The resistance should change predictably as the float moves. If the resistance is not changing or is out of spec, the FSU is likely faulty.
* Check the Fuel Gauge Wiring: Inspect the wiring between the FSU and the gauge for breaks, shorts, or corrosion. A wiring diagram is extremely helpful here. Look for damaged insulation or loose connections.
* Check the Fuel Gauge itself: While less common, the gauge itself could be faulty. You can try swapping it with a known good gauge if you have access to one (a junkyard part might be an option).
* Check the Instrument Cluster: The problem could lie in the instrument cluster where the gauge resides. This is less likely but possible.
3. Getting Help:
* Wiring Diagram: A wiring diagram for your specific 1987 Ford E150 is invaluable. You can find these online (often for a fee) or at a local auto parts store.
* Repair Manual: A repair manual for your van will provide detailed instructions and diagrams for accessing and repairing the fuel system.
* Mechanic: If you're not comfortable working on your fuel system, it's best to take it to a qualified mechanic. Working with fuel is dangerous if you're not properly trained.
Important Safety Note: Gasoline is highly flammable and explosive. Always work in a well-ventilated area, and never smoke or use open flames near the fuel tank or fuel lines. Disconnect the negative battery terminal before beginning any work.
