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The recent posts on “Fuel in the Tank” and “Putting Your Boat to Bed” generated a large number of comments including several that requested more information about storing diesel fuel. I asked Columnist and freelance writer Mr. Bob Duthie to review the subject and here is what Mr. Duthie reported.

The good news is that diesel fuel does not deteriorate over time. The bad news is that diesel fuel tanks always contain some water. The water comes naturally from condensation in the tank during storage. Diesel fuel is lighter than water and floats on top of the water.

There is a brand of bacteria that likes to live at the diesel-water interface. Over time this bacteria multiplies and forms dark black particles which eventually form a black sludge. Since the engine pickup tube in the fuel tank is an inch or two above the bottom of the tank, no particles get into the engine until the motion of the boat when underway stirs up everything in the tank. The fuel filters plug up and the engine stops. The best answer to this problem is to have dual filters that allow switching from one to the other. A pressure gage lets you see when one of the filters is plugging up.

The problem with storing diesel fuel can be reduced by filling the tank to the top to minimize the surface area in the tank where water can condense. In addition, always add biocide to the tank that will minimize the growth of sludge. Unfortunately, once the sludge is in the tank biocide does not do anything to reduce the existing sludge.

The only way to eliminate sludge is to have the tanks cleaned. This is a two step process. First the fuel in the tank is pumped though large filters into a storage tank. This removes any particles that have been stirred up and is called “fuel polishing”. Then the tank must be opened up and the sludge on the bottom removed with a wet vacuum. If possible, the bottom and sides should be wiped up. After cleaning, the polished fuel is pumped back into the tank.

Mixing biodiesel with diesel is not the same issue as ethanol in gasoline. Biodiesel is actually better than diesel because it has higher energy content than regular diesel. Biodiesel does clean sludge in the tank, so be prepared to change a lot of filters the first time you fill up with biodiesel. There may be other problems with biodiesel so be sure to check with your engine manufacturer before using it.

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7 Responses to Diesel Fuel In The Tank

  1. Tim says:

    Yes, the water in the diesel fuel tanks is always bad. I prefer to held the tanks cleaned. It is a great article! Thanks!

  2. rick says:

    check out aquasocks.net. this product is available online or at west marine stores. REMOVES WATER FROM FUEL.Easy to use, and effective.

  3. KEN says:

    I have been using BG products inc. {D F C plus} in my diesel tanks for over ten years now , there products do what they say. stabilizes the fuel , reduces smoke, lubricates,and stops growth. And there CF5 in gas tanks works the best i have ever seen . After running the blue stuff you get in the marine stores in my new 90 outboard, the ethanol completely clogged up the carburetor. I put CF5 in the tank ran it for 15 minutes and it was idling like new. saving me the expense of rebuilding 3 carbs.

  4. Larry says:

    is there a similar article for gas tanks on boats and how to treat gasoline stored over long winter months of non-operation. I want to be as proactive as possible to prevent engine problems from ocurring due to water build up, etc.

    • Hi Larry, check out our other United Marine blog post; “Fuel in the Tank” 11/2/2012 and “Putting Your Boat to Bed” 10/17/2012 and “Dangers Lurking in E10 Boat Fuel” 8/13/2012.

      Regards,

  5. I have two 175 gal tanks. In 8 years, I never used an additive, but fill my tanks to the top before the winter lay up. I never had a fuel or filter problem. I change the Racor’s once a year in the spring. The fuel gets polished during use in the summer season. I think the key is that the boat not just sits a lot with tanks not being full.

  6. moe gladu says:

    I have two 125gal tanks. I always use an additive when adding fuel and have never had any filter trouble. Hopefully thats the key. I do not fill up at the end of the season.

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