* Faulty Fuel Sender Unit: This is the most common culprit. The fuel sender unit is located in the fuel tank and floats on the fuel. As the fuel level changes, the float moves, changing the resistance in a potentiometer which sends a signal to the gauge. If the float is stuck, broken, or the potentiometer is faulty, the gauge will give an inaccurate reading, often staying at full.
* Broken or Corroded Wiring: The wiring connecting the fuel sender to the gauge can become corroded, broken, or short-circuited. This will disrupt the signal and prevent the gauge from accurately reflecting the fuel level.
* Bad Fuel Gauge Cluster: The gauge itself could be malfunctioning. While less common than a faulty sender, the gauge itself could have a short or internal failure causing it to always read full.
* Low Fuel Level, but Gauge Not Responding: In some very rare instances, the sender might be working correctly but the needle is stuck at full. This might require investigating the gauge cluster itself.
Troubleshooting Steps (in order of likelihood and ease):
1. Check the Fuel Level (the obvious first step!): This might seem silly, but verify the car isn't actually running on fumes. Try to drive it until it sputters or stalls. If that doesn't happen, other issues are more likely.
2. Check the Fuel Sender: This usually requires dropping the fuel tank (a significant job). Inspect the float for any damage or obstructions, and check the potentiometer for continuity and proper resistance range (using a multimeter). This is best done by a mechanic familiar with classic car repair.
3. Check the Wiring: Inspect the wiring harness from the fuel tank to the gauge cluster for any obvious breaks, corrosion, or loose connections.
4. Test the Fuel Gauge: While this is more difficult, a mechanic with testing equipment could directly check the gauge's response to signals to determine if the gauge itself is faulty.
In short, while a variety of problems *could* cause this, a faulty fuel sender unit is the most likely suspect. You'll almost certainly need the help of a mechanic, especially for accessing and testing the sender, which requires dropping the fuel tank – a considerable undertaking.
