old fuel?

none whatsoever. provided it was kept clean and moisture free but proper water separating filtration takes care of that. diesel is not like petrol(gasoline) and doesn't deteriorate particularly if stored properly and as you add a fuel conditioner it will also help keep diesel bug away which only grows due to dirt and moisture contamination.

now petrol on the other hand is stuffed within 3 months unless a stabiliser is used as it readily absorbs moisture and goes rotten. strangely 2 stroke mix lasts a bit better must be the extra oil.

i would simply dose it with more stabiliser and sell it or take it with you.

you can blend it with new fuel also. have you considered selling it?   at a heavy discount you could probably make money????

If it has that black sludge growing in it, it could really clog things up. I would pump it through a big filter first.

If it has that black sludge growing in it, it could really clog things up. I would pump it through a big filter first.

About 15 years ago I "inherited" an overhead diesel tank with fuel that was at least 15 years old. #2 It appeared to be clean but I do not know if it had any additives added prior to when I got it. I put a good filter on the tank that I think was about 25 microns or so and added Power Service fuel treatment and Power Service Biocide or something like that to the overhead tank. When I used the fuel in the D4 I ran the fuel from the overhead tank into the fuel tank on the Pickup that has a 5 micron filter to remove water and any algae etc. to fill the D4. Bottom line is I noticed no difference than using the fuel right from the tank wagon delivery. Probably used some of the fuel in the 112 as I was building terraces at the time.

The double filtering assured me the fuel was clean and I noticed no difference in performance. Like Trainz said gasoline is a different matter. One of the overheads had regular gasoline in it and it would hardy burn when trying to start a soaked rag for weed burning. It would have never worked in an engine.

I had 30+ year old diesel fuel in my basement. I think it was there from 1990 when my grandfather both it for agricultural use. It was around 11 gallons of it. All sealed inside a canister. I was debating on what to use the fuel for. In the old day's before break cleaner was available people used to clean engine parts with Gasoline or Diesel. But since that is today a very obsolete method. I decided to see how good it will burn in my crawler(the Soviet crawler T-74). I added some additives and made sure there was no sludge anywhere so filtering out it was a task on it's own. From 30+ years of sitting it did have a different smell and the lighter parts had probably all evaporated over time. Because the fuel was so old I made a mixture of 1 part of the old and 1 part of the new. It all burned up in the engine and nothing clogged in the fuel supply system.
In a way it was a dangerous experiment but one thing i can say. The old fuel was much better in terms preserving overtime then the fuel you can get today.
Not sure if it's all the additives or bio components added to the modern fuel or there is a general scam going on. But I shall save that to another topic.

My D6 had been sitting in the shed with no engine in it since it was pulled down in the late 70's. Although the diesel in it had changed color and smelled stale it was clean and the engine ran fine on it when I started it in 2019. I did not add anything to the fuel and the D4600 ran fine. I wouldn't use it in a modern diesel though they are a bit too sensitive.

You need water in the fuel for the growth to occur.
From another website;
"Diesel fuel algae is really a misnomer that’s been passed down from generation to generation of diesel mechanics. Algae are what grow in the yacht owner’s aquarium in the main salon, but not in his fuel tanks. Algae require sunlight to grow, and of course, that’s impossible within a fuel tank. Diesel fuel algae received its name because of its appearance, not because of what it truly is.

Some engineers also call this algae “diesel bug.” And that label is incorrect as well, since it’s neither an insect nor a disease. Just what is it? It is actually a bloom of microbial growth within the fuel tank. This slimy bloom of organic contaminants can be composed of bacteria, mold, yeast or other types of fungi. There is one simple rule when it comes to microbial growth in a diesel fuel tank: It cannot exist without the presence of water."


If the fuel has been stored so no water can get in then it should be fine to use but maybe pour some into a bucket and inspect it. My brother had issues with his excavator and all it did was block his fuel filters up.