* Faulty Fuel Level Sending Unit: This is the most common culprit. The sending unit is a float inside the fuel tank that moves with the fuel level and sends a signal to the gauge. If the float is stuck, damaged, or the sending unit's electrical components are malfunctioning, the gauge will give inaccurate and fluctuating readings. This often involves corrosion or debris interfering with the float's movement.
* Wiring Issues: Worn, corroded, or damaged wires between the sending unit and the gauge can cause intermittent signals and fluctuations. This could be anywhere along the wiring path. Movement of the wiring harness can also cause intermittent contact.
* Faulty Fuel Gauge Cluster: Less common, but possible, the gauge itself within the instrument cluster could be malfunctioning. This is usually diagnosed by testing the gauge's response to a known good signal from the sending unit.
* Low Fuel Level: Sometimes, especially when nearing empty, the float's sensitivity and the slight sloshing of fuel can make the gauge seem more erratic.
* Debris in the Fuel Tank: Rust, sediment, or other debris can interfere with the float's movement in the sending unit.
How to Diagnose:
1. Check the Fuel Level: Try filling the tank completely. If the fluctuation continues after a full tank, it's less likely to be simply low fuel.
2. Inspect the Wiring: If you're comfortable working with car wiring (and disconnect the negative battery terminal first!), visually inspect the wiring harness leading from the fuel tank to the instrument cluster. Look for any obvious damage, corrosion, or loose connections.
3. Test the Fuel Sending Unit (Advanced): This often requires dropping the fuel tank, which is a more involved process. You'd need to test the resistance of the sending unit at different fuel levels to see if it's providing a proper signal.
4. Professional Diagnosis: The most reliable method is to have a qualified mechanic diagnose the issue. They have the tools and expertise to test the sending unit, wiring, and gauge accurately. They can also rule out other less likely causes.
In short, while the fuel sending unit is the most probable cause of a fluctuating fuel gauge in a 1999 Chrysler Cirrus, a thorough investigation is necessary to pinpoint the exact problem. Don't attempt advanced repairs unless you have the experience and proper safety precautions in place.
