Diesel fuel, how old it too old?

Question

When diesel fuel goes bad what happens to it? I know water can be seperated easily and algae can grow in it, can algae be pre-filtered out then what is the fuel ok then? References would be nice not just opinion if possible.

Answer 1

I have experience with various kinds of marine diesels. For what I know normal diesel for cars is not that much pray to algae as marine diesel, because diesel for cars is delivered clean in your car, while seagoing ships mainly have to filter out the last dirt and water themselves to make the fuel cheaper on delivery.
But it is true there can be algae of whatever kind of bacterial growth in diesel. Unfortunately i don't know if there are any rules for how long it will take for them to develop, but be careful with diesel older than a few month, especially when there can be condensation of water in the fuel and there are higher temperatures involved.
To prevent them from starting to grow you should drain the fuel/storage tank from water regularly (every day) and you'll have no problem. Sometimes this is just not possible, then you should check the diesel first before use if there are no traces of water or emulsion inside. Emulsion is oil or fuel mixed with water and looks white or grey and is hardly transparent.
The problem with these algae is that they clog and block any normal maze filter with very (dark) slimy stuff and engines don't like em and tend to stop on them if they don't stop on a low fuel pressure because of the clogged filter.
As far as i know the only way to get rid of them is draining the tank on the lowest point until water and emulsion are gone and there is only nice and clean transparent diesel flowing out, or feeding the diesel through a purifier (centrifugal ' filter" that can remove water too besides solid particles). But still algae do no good for the quality of the diesel either..
I encountered this annoyance once myself, though it was not severe. What I remember is that this diesel was inside this tank for over a year and I've been draining and recirculating this tank over a purifier until there was only clear oil left. But for these kinds of reasons, for any tank of fuel, doesn't matter what, i always try to use the oldest stuff first and never order too much and always empty tanks as much as possible before refilling.

Answer 2

I have ran diesel fuel in farm tractors that has sat in bulk storage for about a year. It worked fine, however I don't think I would try it with highway diesel. If you are familiar with diesel you can notice a change in odor after it sits for a long time. I know gasoline goes bad after just a few months, which is why you are not supposed to use your lawnmower gas that has been stored over winter but I'm not completely sure about the life of diesel.