* Faulty Fuel Level Sending Unit: This is the most common cause. The sending unit is located inside the fuel tank and floats on the fuel. It sends a signal to the gauge indicating the fuel level. If it's malfunctioning (worn out, corroded, or damaged), it'll give inaccurate readings, often showing empty even when the tank is full. This needs replacement.
* Faulty Fuel Gauge Cluster: The problem might not be in the tank, but in the instrument cluster itself. The gauge itself could be broken or the circuitry connected to it could be faulty. This requires either repair or replacement of the instrument cluster.
* Wiring Issues: Check the wiring between the sending unit and the instrument cluster for breaks, corrosion, or loose connections. A damaged wire can interrupt the signal, leading to an inaccurate reading.
* Float Arm in Fuel Sending Unit: The float inside the sending unit could be stuck, broken, or not moving freely. This would prevent it from accurately measuring the fuel level.
* Rare Possibilities (Less Likely):
* Computer issue (ECM): While less likely, a problem with the car's Engine Control Module (ECM) *could* affect the gauge's readings, but this is usually accompanied by other symptoms.
* Ground Fault: A poor electrical ground connection somewhere in the circuit could disrupt the signal.
Troubleshooting Steps:
1. Check for obvious leaks: Before jumping to conclusions, visually inspect the fuel lines and tank for any visible leaks.
2. Wiggle the fuel gauge needle: Gently tap or wiggle the fuel gauge needle. If it moves erratically, it suggests a problem with the gauge itself.
3. Check the wiring (advanced): This requires some mechanical aptitude. You'll need to access the fuel sending unit (usually involves dropping the fuel tank) and the instrument cluster to inspect the wiring for damage. Caution: Gasoline is highly flammable; take appropriate safety precautions.
4. Diagnostic Scan: If you have access to an OBD-II scanner, it might pick up codes related to the fuel system. This might point towards a more serious issue.
Recommendation:
Since the gauge immediately shows empty after filling, the most probable culprit is the fuel level sending unit. However, you need to consider the potential for the wiring harness to have a break. Replacing the sending unit is usually the most straightforward and cost-effective solution. Attempting wiring repairs yourself is advisable only if you have experience with automotive electrical systems. Otherwise, take your Alero to a qualified mechanic for diagnosis and repair. Replacing the whole fuel gauge cluster should only be considered as a last resort after all other possibilities have been ruled out.
