I'm guessing you mean when the tank is full. It is probably the fuel sensor.
An automobile temperature gauge can fluctuate when the thermostat is opening and closing erratically. The temperature gauge can also fluctuate when the ground wire is loose.
The fuse for the petro gauge can be found in the fuse box. The fuse will be labeled as the instrument panel fuse.
A short in the turn signal wiring can cause the voltage gauge to fluctuate. A loose ground wire can also cause the gauge to fluctuate.
usually under the the drivers side rear passenger seat, lift off a large black plastic cover and underneath you shall have your fuel gauge sender unit + pump if it's a petrol.
Temperature.
Fuel gauge problems are usually the fault of the fuel gauge sending unit in the fuel tank, just saying.
Could be low on coolant or a sticky thermostat.
A fuel gauge.
It is hard to gauge exactly how long the meeting will take.A petrol gauge is an instrument to show how much petrol or diesel is in the vehicle's fuel tank, which helps to avoid running out of fuel.
Drive and see how long it lasts.
low coolant
When you turn a corner the guage fluctuates.