My daughter has a 97 Honda Civic EX. The fuel gauge reads right when she starts out, butafter about 15 - 20 minutes the fuel gauge goes all the way to full. After she cuts it off and it cools down, the gauge reads right again.I have replaced the unit that has the float and the low fuel sensor. Has anyone else had this problem? If so, how did you fix it? Thanks

 

Some kind wire between gauge and fuel tank need to be cleaned, I had similar problem, but didn’t had a time to sort it out. I sold a car

 

Probably the gauge itself in the cluster. Try with the sending unit disconnected; I think it should read empty constantly. There's an off chance the wire is shorting to ground somewhere, but since it is so consistent I'd really suspect the gauge. If you were to lose ground to the cluster the speedo and tach are also going to do weird things.