Does anybody have an idea of what the "shelf life" is of diesel
fuel? I have a Mercedes that has set in the garage for 3 years,
and I just put a battery in it last week and got it going again.
It has about 1/2 tank of fuel, which has set, but it seems to run
just fine...

 

The owner's manual for a diesel generator we used to have said that 5 years was the max for untreated #2... any longer and the generator might be hard to start. Although I did read a story about a tank of 32 year old diesel that worked fine after being treated with a reconditioner.
I think if it sat that long I would forget where I parked it....