I have a 2004 monte carlo ss that the fuel gauge is not reading right. It seems that when I first start the car it will read right but when I drive it for a bit then stop, it will start to move a lot up and down until I start to drive again then it seems like it is reading correct again until I stop again then it will do the same thing again.

 

My first guess would be the fuel level sending unit it wore out. It has little metal contacts the rub to give fuel reading and eventually they wear off and can't get a good reading anymore. The fuel level sending unit is on the actual sending unit and pump in the tank and there is an access door in the trunk just behind rear seat under the carpet.

 

Mine just started doing the same thing. It is also causing the wrong reading on the heads up.

 

The other problem you may have (and it is very common) is that you gauge motor has gone bad. This problem happened in almost every GM vehicle from the early to later-mid 2000's. In my 2004 Monte Carlo SS, my fuel gauge, speedometer, tachometer, and oil level gauges are all bad and give wild readings. They still sometimes come back to normal, but always go out again. I have the newer, less problematic replacement motors, but I haven't had the chance to take apart the dash and install them.

 

I have an 02 Monte that reads full all the time. I also have an 00 GMC Sierra that none of the gauges work. I was told it is a faulty ground to the instrument cluster.