* Faulty Fuel Sender Unit: This is the most common cause. Located in the fuel tank, it's a float that moves with the fuel level and sends a signal to the gauge. It could be malfunctioning, corroded, or simply have a broken float.
* Wiring Issues: Broken, corroded, or loose wires between the sender unit, the gauge, and the instrument cluster are frequent culprits. This includes connectors and grounds.
* Bad Fuel Gauge: The gauge itself could be defective.
* Instrument Cluster Problems: The instrument cluster (the whole panel with the gauges) might have a faulty circuit related to the fuel gauge.
* Low Fuel: While seemingly obvious, an extremely low fuel level can sometimes interfere with the sender unit's ability to send a reliable signal.
* Corrosion in the Fuel Tank: Severe corrosion in the fuel tank can interfere with the sender unit's operation.
To diagnose the problem, you'd need to systematically check each of these possibilities. A mechanic experienced with Jeeps, or a good DIY guide specific to the 1991 Wrangler's fuel system, would be your best resources.
