The fuel gauge in my 04 has been doing something strange the last few times I filled up. After filling up the tank the gauge always use to read full right away after starting the truck up, but the last 3 or 4 times I have fueled up the gauge has been taking roughly 20 mins to move up to full. Everything else is fine, just the gauge has been "acting strange".

So what is the problem? Bad sending unit or something else??

 

If you leave the key on when fueling, stop doing that.

 

Nope the key has been off and out of the ignition, either sitting in the cup holder or on the console lid.

 

Get a bottle of TECHRON, run your tank down to a quarter tank, throw the techron in the tank and fill it up. You might have to do this twice. If your using a high sulfer gas, the sulfer settles on the sending unit causing your issue. It happened to me and it took care of my issue.