I was sitting at an intersection today in my '92 Camry and literally watched the fuel gauge drop from a half tank down to the empty line within 10 seconds. It had been sitting outside at work all day in warm weather (30 Celcius), which it has done for the past few weeks. By night it is parked indoors in an underground parking garage, shaded, and relatively cool. There were no leaks that I could see in the parking lot or at the intersection.

After driving it on the highway for approx 2 minutes, and then exiting, the fuel gauge slowly climbed back up to half a tank. This morning when I left for work it showed 3/4 tank full, which is where I expected to see it this evening (work is not far from home). Any ideas on what the problem could be?

Of note, I did have the oil sending unit replaced last October at the recommendation of the Toyota dealership garage. Could this be related?

your fuel guage sending unit may be shorting out, usually when it goes all the way to empty that means something is grounding out, you could check for connections. oil sending unit is not related

It could also be related to a pressure sensor in the fuel system. (Error code P420)

My gauge jumps around like that because a pressure sensor went out on my car. It's not serious enough to replace and doesn't always activate the check engine light. Although the gauge usually is often near correct (like when it righted itself overnight), I just have to guestimate fuel levels based on miles traveled.