Nissan Repair: 2001 Frontier Gas Tank Issues, gas mileage, fuel sending unit

Question
In the past few months, I've noticed that my gas tank has not been functioning properly.  I would fill up the tank and by the next day, after only driving 60 miles, I'd be hovering around half-tank, maybe slightly more.  I used to get great gas mileage on a full tank (wouldn't get to 3/4 tank until after around 75 miles) so this confused me.  I thought I just needed an oil change so I got one with the fuel system cleaner and engine treatment.  That was almost two weeks ago (7/23).

Now, it won't allow me to fill it up.  I can only fill it to about 3/4 and it keeps fluctuating throughout the day (I last tried to fill it on Sunday 7/31).  If it's been sitting for a long while, the indicator may shoot straight up to Full but, as I continue to drive, it drifts back down to between 1/2 and 3/4 tank.

I thought that doing the oil system cleaner, fuel system cleaner and the engine treatment would resolve the problem, but it appears to be worse now.  This is really bothering me, so any help you can provide will be greatly appreciated.

Answer
Monica,

I bet your fuel sensing unit is defective.  This is like the float in the back of the toilet in your house.  If the sensor that is hooked up to the sender float is defective you will be reading a different level most of the time.  Here is how to check it.  Fill the gas tank and clear the trip odometer.  Now drive a hundred miles and refill the tank.  Divide the miles by the gallons used and you should see that the mileage you are getting is normal.  This will tell you that the sender is indeed defective.  The bad news is that the fuel sending unit will have to be changed and to do that you will have to drop the gas tank.