This unit, located in the fuel tank, measures the fuel level and sends that information to the gauge. When the car is stationary (and you first start it), the unit might be making contact correctly, providing an accurate (though possibly still slightly off) reading. However, the movement and sloshing of the fuel during driving disrupts the connection or the float within the sending unit. This leads to an inaccurate, usually full, reading.
Other less likely possibilities include:
* Wiring issues: A break or short in the wiring between the sending unit and the gauge could cause intermittent problems. Movement could aggravate this.
* Gauge cluster problem: While less probable given the starting behavior, a failing gauge itself could be misinterpreting the signal.
* Ground connection issue: A poor ground connection somewhere in the circuit can cause erratic readings.
How to troubleshoot:
1. Check the fuel level: Does the gauge still show full even after you've driven significantly and used a substantial amount of fuel? This strongly points to a sending unit problem.
2. Inspect the wiring: If possible, carefully inspect the wiring harness leading from the fuel tank to the instrument cluster for any damage or corrosion. This is difficult to do without specialized tools.
3. Have a professional diagnose the issue: A mechanic can use a scan tool to check for any fuel level sensor codes and perform a more thorough diagnosis. They can also test the gauge itself, confirm the wiring, and replace the sending unit if necessary. This is generally the most efficient way to resolve the issue.
Replacing the fuel sending unit is often the solution, as it's the most common culprit in situations like this. It's usually not a particularly difficult repair for a mechanic.
