* Faulty Fuel Level Sending Unit: This is the most likely culprit. The sending unit is the component that floats in the tank and sends a signal to the instrument cluster indicating the fuel level. Even with a new tank, the sending unit might be faulty or improperly installed. It's possible the old sending unit was transferred to the new tank, and *that's* the problem.
* Wiring Issues: Check the wiring harness connecting the sending unit to the instrument cluster. Look for broken wires, corroded connections, or loose terminals. A break in the wire or a poor connection will prevent an accurate reading.
* Instrument Cluster Problem: Less likely, but possible. The gauge itself in the instrument cluster could be malfunctioning. This would need a diagnostic check at an auto repair shop.
* Incorrect Installation: If the new fuel tank or sending unit wasn't installed correctly, the float might be obstructed or the connection points might be improperly seated.
* Ground Issue: A poor ground connection can affect the fuel gauge reading.
Troubleshooting Steps:
1. Verify Sending Unit: If you replaced the tank and transferred the old sending unit, the new tank might have a different sending unit. The replacement process should have specified whether the sending unit was transferred. If a new tank came with a new sending unit, make sure it's properly installed and functioning correctly.
2. Check the Wiring: Carefully inspect the wiring harness from the sending unit to the instrument panel. Look for any signs of damage, corrosion, or loose connections.
3. Check for Ground: Ensure a good ground connection exists for the fuel gauge circuit.
4. Test the Sending Unit (Advanced): This requires some electrical testing. You'll need a multimeter to check the resistance of the sending unit at different fuel levels. This is a bit more involved and requires some automotive electrical knowledge.
5. Consult a Repair Manual: A repair manual specific to your 1991 Buick Park Avenue will provide wiring diagrams and troubleshooting steps for the fuel gauge system.
If you're not comfortable performing electrical diagnostics, it's best to take your car to a qualified mechanic. They'll have the tools and expertise to diagnose the problem accurately and efficiently. The problem is likely with the sending unit or wiring related to it, however.
