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When is diesel fuel too old?

shank

.
Apr 20, 2016
83
Catalina 27 Lake Champlain
I have a small sailboat with a Universal 5411 that uses only about 1/4 tank per summer. I fill the tank and treat it with fuel stabilizer each winter but I'm mixing new fuel with old fuel and I have no idea how old the old stuff is. Can I just keep treating it indefinitely or should I pump out the old stuff and replace with fresh fuel? I'm not having any problems yet. I refill it every Fall only because I read somewhere that the tank should be full before it's laid up for the winter to prevent condensation in the tank otherwise I would just burn down what's in the tank.
 
Sep 25, 2008
6,315
Alden 50 Sarasota, Florida
Actually, diesel fuel does have a finite ‘lifetime’. According to API, the recommended storage time should not exceed 6 months. I can’t find the citation now for lack of time for anyone skeptical but anyone searching should easily find it if interested.

The stabilizers serve to minimize water absorption but do nothing to maintain other characteristics including cetane rating.
 
Feb 21, 2013
3,791
Hunter 46 Point Richmond, CA
..........Can I just keep treating it indefinitely or should I pump out the old stuff and replace with fresh fuel?...........I refill it every Fall only because I read somewhere that the tank should be full before it's laid up for the winter to prevent condensation...........
Great question!!. Yes. Ditto dlochner...........subject comes up frequently on boat forums. As a chemical engineer, I designed many diesel production facilities, including ultra low sulfur diesel hydotreaters and diesel cetane and lubricity additive systems, and can attest it lasts a very long time since it is very stable, i.e., does not have gum producing compounds like gasoline and dry, i.e., no free water.

I will add .........my diesel truck and sailboat were idled for 3 years diesel and when started up burned the old diesel just fine. I have a similar problem as you turning over my 100 gallon sailboat and 530 gallon powerboat diesel fuel tanks since I only consume 10 and 100 gallons per year, respectively and have never had an issue. My best practice in the San Francisco Bay is to keep the fuel (gasoline and diesel) tanks topped off to mitigate condensation, as you noted, and add a stabilizer (with biocide for diesel).
 
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jviss

.
Feb 5, 2004
4,627
Tartan 3800 Westport, MA
Great question!!. Yes. Ditto dlochner...........subject comes up frequently on boat forums. As a chemical engineer, I designed many diesel production facilities, including ultra low sulfur diesel hydotreaters and diesel cetane and lubricity additive systems, and can attest it lasts a very long time since it is very stable, i.e., does not have gum producing compounds like gasoline and dry, i.e., no water.

I will add .........my diesel truck and sailboat were idled for 3 years diesel and when started up burned the old diesel just fine. I have a similar problem as you turning over my 100 gallon sailboat and 530 gallon powerboat diesel fuel tanks since I only consume 10 and 100 gallons per year, respectively and have never had an issue. My best practice in the San Francisco Bay is to keep the fuel (gasoline and diesel) tanks topped off to mitigate condensation, as you noted, and add a stabilizer (with biocide for diesel).
Wow, excellent post, thank you. I will have to keep you in my rolodex as the diesel expert!
 

jviss

.
Feb 5, 2004
4,627
Tartan 3800 Westport, MA
I have a small sailboat with a Universal 5411 that uses only about 1/4 tank per summer. I fill the tank and treat it with fuel stabilizer each winter but I'm mixing new fuel with old fuel and I have no idea how old the old stuff is. Can I just keep treating it indefinitely or should I pump out the old stuff and replace with fresh fuel? I'm not having any problems yet. I refill it every Fall only because I read somewhere that the tank should be full before it's laid up for the winter to prevent condensation in the tank otherwise I would just burn down what's in the tank.
This is a funny topic for the obsessive-compulsive engineers among us. For example: in your car, do you always run it to empty before filling it? If not, then you are gradually diluting the original gas with new gas, but never entirely getting rid of the original gas - you're asymptotically approaching that point, but never reaching it. :).

So, the exercise for the interested student is to determine how much original diesel remains in one's tank if you replace 1/4 of the tank with new fuel annually.
 
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Likes: shank
May 1, 2011
2,321
Pearson 37 Lusby MD
So, the exercise for the interested student is to determine how much original diesel remains in one's tank if you replace 1/4 of the tank with new fuel annually.
I have no desire to go back that far with my very rusty math skills! :beer:
 

shank

.
Apr 20, 2016
83
Catalina 27 Lake Champlain
Yes, I've noticed that sailing definitely attracts people of a certain mindset. That's a compliment.

I reached out to the previous owner of my boat who had her for 30 years. His annual regimen was fill the tank 90-95% full then add a dose of Star Tron and a dose of BioBar JF. Change the filter. Says he didn't any problems with this approach so I'll stick with it for now.

Thanks for the great advice
 
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Likes: sail sfbay

JRacer

.
Aug 9, 2011
1,239
Beneteau 310 Cheney KS (Wichita)
Good discussion, I too, top off periodically and each fall on haul out add the winterizer stuff with cetane. But, since 2012 have never drained the tank empty.
 
May 17, 2004
3,483
Beneteau Oceanis 37 LE Havre de Grace
So, the exercise for the interested student is to determine how much original diesel remains in one's tank if you replace 1/4 of the tank with new fuel annually.
Fun thought experiment. If you replace 25% annually then 75% remains in year 1. In year 2 you’ll have 75% of the 75%, which is 56%. It continues to 42%, 31%, 24%, 18%, 13%, etc. the general formula is (3/4)^n.
 
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jviss

.
Feb 5, 2004
4,627
Tartan 3800 Westport, MA
As long as you can keep the algae out, or whatever it is that grows in diesel tanks, and drain the water off before it gets to the injectors, you are fine.
 
Jun 7, 2016
312
Catalina C30 Warwick, RI
not to start a side argument, but I am in the camp of pumping my fuel tanks dry every fall and leaving the tanks empty until spring. If there is no fuel than there is no medium to collect water. Also when I pump them dry I am removing any old contaminants and moisture. In the spring when I look at the tanks they are clean and have now water or moisture in them. Then I fill them up and enjoy the season.

( as a side note I put the fuel I pump out in my home heating oil tank)
 
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Likes: Charles Erwin
Jan 30, 2012
1,093
Nor'Sea 27 "Kiwanda" Portland/ Anacortes
ontherocks approach is accurate. I have super clean tank following his views. But if you have 600 gallons a different approach is warranted. Consider this:
 

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Jun 7, 2016
312
Catalina C30 Warwick, RI
ontherocks approach is accurate. I have super clean tank following his views. But if you have 600 gallons a different approach is warranted. Consider this:
Yes, thank you / good point. My approach is a bit biased as I only have a 17 gallon tank and is easy to pump dry, however I still stand behind my method if feasible.
 
Oct 26, 2010
1,432
Hunter 40.5 Beaufort, SC
Jun 7, 2016
312
Catalina C30 Warwick, RI
I'm not saying I am a Maine Sail fan boy but he has not let me down yet. A lot of my refit was based on his advice and I couldn't be happier.

To add a more educated response;) The previous owner of my boat had always left the tank full, and "treated" it with additives each winter. When I looked inside the tank it was ugly. I replaced the tank and used this method. When I pump it "dry" I get it as low as possible. There is maybe a few ounces of fuel left in the lowest part of the tank that I can't get out but it is a nominal amount. In the spring when I check (and feel the top and sides of the tank) there is no moisture and there is the same amount of "liquid" at the bottom of the tank and it has not changed color or developed growth. I've attached a picture of the old tank that used the "keep it full method"

fuel tank.jpg
 
Oct 26, 2010
1,432
Hunter 40.5 Beaufort, SC
Pretty sure Dow is right. However, since you disagree, please explain the results of your experiments concerning water accumulation in empty tanks. Can you tell us what happened? If there was any bacteria/fungus accumulation in empty tanks as you contend then are you willing to tell us which of these organisms multiplied in the contaminated water (condensation) that occurred in the empty tank?

Looking forward to your results - most particularly those results that conflict with the Dow study. Moreover tell us how your experience yielded different results than those MainSail conducted.

Charles
@Charles Erwin , please note that I deleted my post after reading the Maine Sail website discussion.
 
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Likes: ontherocks83
Jun 7, 2016
312
Catalina C30 Warwick, RI
@Charles Erwin , please note that I deleted my post after reading the Maine Sail website discussion.
Aww man, I was all prepared to have some snarky remarks and everything;):biggrin:. Honestly though I am not a scientist and I didn't sleep in a holiday inn express last night but my understanding of it is, fuel absorbs/attracts moisture therefor if there is no fuel there is nothing to attract the moisture to it. (yes I know this is over simplified) Part of my reasoning for also doing this is that, the previous owner (and other boats I have seen) have always done the "top it off method" and had the same results of nasty tanks. I figured if this method kept producing the same results than why not try something different. What's the definition of insanity again, something about doing the same thing over and over and expecting different results.