my 2002 chev s10 zr5 has about 96000 miles on it. 1 day i turned of the engine with a little over a half tank of gas. an hour later i turned it over and it went straight to full and stayed there for about 3 days. after that i was driving and noticed it went straight to empty where it has been for about a week now. the check fuel light is on and whenever i start it up it does go all the way to full just like it normally does but it goes back down to empty in an instant.
There’s a good diagnosis reference here : http://www.6066gmcguy.org/Electrical-02.htm
This has all of the common stuff and the obvious don’t do this at home stuff.
The fuel sending unit in the tank has quit.
GM dealers have a bottle of cleaner that is supposed to clean the unit and enable it to work again, but in my case, a week after I use it, the gauge quit again.
The first time my fuel gauge went haywire (like yours) I had my tech change the sending unit. 2000 Olds Silhouette.
It’s expensive. The tank has to be dropped to change the unit. The sender itself was $235 (CDN) so I won’t be replacing that again.
I just reset the roof computer and trip mileage counter to keep track of the fuel used.
What was the name of the cleaner? The fuel sending guage on my 2000 Blazer started doing the same thing as the OP’s back in March. The sending unit part of the fuel pump assembly and is $$$ to replace, since a new fuel pump is required. I keep track of my mileage also and try to refuel with 3 or 4 gallons remaining.