* Faulty Fuel Sender Unit: This is the most common cause. The fuel sender unit is located inside the fuel tank. It's a float that moves up and down with the fuel level, sending a corresponding electrical signal to the gauge. The float can crack, the sending unit's arm can become stuck, the electrical connections can corrode, or the unit itself can fail.
* Bad Fuel Gauge: The gauge itself in the instrument cluster could be defective. It might be internally shorted or have a faulty mechanism.
* Wiring Issues: Corrosion, broken wires, or loose connections in the wiring between the fuel sender and the gauge are frequent problems. This is especially likely in an older car exposed to the elements.
* Ground Issues: A poor ground connection can prevent the gauge from receiving the correct signal.
* Faulty Fuel Pump Relay (Less Likely): While less direct, a problem with the fuel pump relay *could* affect the fuel gauge if the relay is also involved in supplying power to the fuel sender. This is less common than the other issues.
* Instrument Cluster Issues (Less Likely): Problems within the instrument cluster itself (beyond just the gauge) can sometimes affect the fuel gauge.
Troubleshooting Steps:
1. Check the Fuel Gauge for Accuracy at Extremes: Fill your tank completely full. Does the gauge read full? Then, let the car run until the gauge reads nearly empty. Does it accurately reflect nearly empty? Inconsistent readings at the extremes point to the fuel sender.
2. Inspect the Wiring: Carefully examine the wiring harness running from the fuel tank to the instrument cluster for any visible damage, corrosion, or loose connections.
3. Check for Power at the Fuel Sender: You'll need a multimeter to test for power and ground at the fuel sender. (This requires disconnecting the fuel sender, so be cautious and consult a repair manual for proper procedure.) This will determine if the sender is receiving power and if it's properly grounded.
4. Test the Fuel Gauge (Most Difficult): Testing the gauge itself requires removing the instrument cluster, a somewhat involved process best left to someone experienced with automotive electrical systems.
Recommendations:
* Start with the simplest and most likely cause: The fuel sender unit is your best bet first. Replacing it is often a relatively inexpensive fix compared to other components and is fairly accessible. You'll need to drop the fuel tank, which is more involved but usually a straightforward task.
* Consult a Repair Manual: A repair manual specific to your 1986 Oldsmobile Cutlass Supreme will provide detailed diagrams and instructions for troubleshooting and replacing the fuel sender, gauge, and other components.
* Consider Professional Help: If you're uncomfortable working on your car's electrical system, taking it to a qualified mechanic is the safest option. They have the tools and experience to diagnose and repair the problem efficiently.
Remember to always disconnect the negative battery terminal before working on any electrical components in your vehicle. Safety first!
