1997 Nissan 180SX • 4 cylinder FWD • 120,000 miles

I have a 1997 Altima with about 120k on it. I bought it used in 2000 with about 17k on it. According to the manual, it is supposed to have a 16.5 gal. Fuel tank. When I ran out of gas early on, I noticed that it took only 13.5 gal. Ever since then, the amount it takes to fill it when I intentionally let it run out of gas has gradually decreased. Not it takes about 8.5 gal.

The fuel gauge never registers more than about 3/4 tank, but then it seems to go down correctly.

My mechanic checked the sender unit and the tank (to make sure it wasn't collapsing) and couldn't find any problem.

I even brought it to him after filling it up, and he visually checked and said it was full. When I brought it to him empty, he confirmed that it was empty.

I'm getting normal gas mileage, with no leakage.

My guess is that something is causing a sensor to detect that the tank is empty when it's not, but my mechanic's visual inspection of the empty tank seems to contradict that.

Any ideas?
January 17, 2011.

The fuel gauge is operated by a rheostat. It is nothing more than a resister that sends different amounts of power to the gauge based on where the float gauge says the float is located. Since it never shows full, I think that is the problem.

Would a malfunctioning rheostat explain why not only does the gas gauge read Empty but I actually run out of gas after using only 8 1/2 gallons of gas?

Jan 25, 2011.
It could