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  • Diesel Fuel Management

    Diesel fuel is far different from gasoline. With gas, you pretty much put it in the tank and forget it, or you could in the days before E-10. Diesel fuel requires some management or you will suffer from the effects of poor quality fuel. Understanding the major differences goes a long way in helping to avoid fuel quality problems.

    Diesel fuel is a fuel oil that is subject to contamination by both dirt and moisture from poor handling practices, leaks in piping, moisture condensing on the interior walls of tanks at the refiner, the distributor, your marina or retailer and in your boat. Microbial growth occurs in diesel fuel in the form of bacteria and algae that seem to thrive in the presence of water.

    Water or moisture droplets in fuel will cause mechanical problems with pumps and injectors and will cause corrosion in the lines and fuel handling parts (distribution pump and injectors) of the fuel system.

    A lot of moisture is introduced into diesel from the tanks the fuel is stored in. Every tank has this potential, but a boat’s fuel tanks are vented to the atmosphere. As fuel is used, the volume used is replaced by air drawn in through the tanks vent line. The vent is frequently about 3 ft. above the water line, so any air drawn in the vent line is going to be moisture laden. Then as temperatures fall in the evening and rise during the day, the fuel tank will condense moisture on the tank walls in the space above the fuel remaining in the tanks. As droplets form, they will run down and into the fuel. Since water is heavier than the fuel, it accumulates on the bottom of the tank.

    Microbial growth in the diesel fuel is then possible since water is present to feed the growth. The growth seen in diesel is most often long stringy clumps of dark green or black crud that often will adhere to the tank walls and bottom. Sloshing around when the boat is in motion often breaks the microbial clumps off wherever it is adhered so it can then make its way to the filter system. It doesn’t take much to clog up a set of Racors.

    Virtually all boats have dual fuel filtration systems. A primary filter, usually a Racor brand turbine filter, will be located between the tank and the engine. The design if the Racor is such that it will separate water from the fuel and collect it in a bowl at the bottom of the filter housing before it traps any dirt particle larger then the trap size of the filter element. Most Racor primary filters are available in 2 micron, 10 micron and 30 micron trap sizes. Another or secondary filter is located after the Racor, usually on the engine itself and most frequently supplied by the engine maker for his particular engine. Some manufacturers publish the trap size of their filters, some do not, but generally they are 2-5 micron filters.

    Cold weather brings on other problems with fuel since diesel fuel will not flow in extremely cold conditions. It can gel or form wax crystals in the filters and plug them which stops fuel flow. Most fuel refiners formulate their fuels for cold weather use by mixing no.1 diesel (lighter, less viscous) with no. 2 diesel to yield a blend that does not gel in the climate area in which it is to be sold. This is not a major concern to pleasure boaters because we don’t usually go boating when the temperatures are cold enough for fuel to gel.

    Bulk storage tanks and fuel handling between the refinery and the fuel fill on the boat are beyond the control of the individual retail fuel consumer. However, we can insulate ourselves from some of the inherited fuel handling and management problems with a few simple steps.

    1. Try to buy your fuel from the same high volume retailer all the time. A high volume seller will cycle his tanks more frequently and the less time the fuel sits in a tank, the less water will be condensed in it. The less water the fuel contains means less chance of also having a load of sludge (fungus/algae/microbial growth/etc) in it.

    2. Buy from a retailer whose pumps have a filter visible at the nozzle, and look for a date stamp date hand written on the filter body.

    3. Keep your fuel tanks as full as possible to minimize the air space above the fuel in the tank.

    4. Use a biocide in your tanks on a regular or maintenance basis to kill or prevent microbial growth in your tanks.

    5. Use a multi-purpose diesel fuel additive on a regular basis. Look on the label for these attributes:

    i. Contains no alcohol
    ii. Disperses moisture
    iii. Raises cetane levels
    iv. Stabilizes stored fuels
    v. Cleans injectors
    vi. Reduces soot and smoke
    vii. USLD compliant
    viii. Add lubricity
    ix. Protects against harmful effects of LSD and ULSD

    Suggested additives for microbial growth control are:

    1.Power Service Bio Kleen Diesel Fuel Biocide

    2.Racor Diesel Biocide

    3.Hammonds Biobor JF Marine and Jet Fuel Microbiocide

    Suggested Multi-purpose general diesel fuel additives are:

    1.Stanadyne Performance Formula

    2.Power Service Diesel Kleen + Cetane Boost

    There are now 3 different types of diesel fuel in the US. Off-Road high sulfur w/ more than 500 ppm sulfur; low sulfur diesel (LSD) with less than 500 ppm sulfur, and ultra low sulfur diesel (ULSD) containing less than 15 ppm sulfur. While it may be possible to have either off road or LSD at your marina, the truth of the matter is that there is only one normal diesel fuel distribution network and unless your area has a huge demand for off road fuel requiring additional and separate tanks and pumps, all fuel distributed now is most likely ULSD, and we are most likely now receiving ultra low sulfur diesel at almost all distribution points for pleasure boaters.

    The process of removing sulfur also reduces the normal lubricity and reduces the cetane level in diesel fuel. Low cetane leads to hard starting and lower overall economy. Lower lubricity can have serious and expensive consequences on fuel pumps and injectors since the fuel passing through them is what lubricates them. Any diesel engine made prior to about 2003 can develop problems with ULSD. Check with your engine maker to be sure how to handle ULSD.

    A good additive to handle the reduced lubricity in fuel is:

    Stanadyne Lubricity Formula


    1.Learn how diesel systems get contaminated and avoid introducing moisture or trash into your tanks.

    2. Buy your fuel from the same high volume retailer who has and maintains filters on his diesel pumps

    3. Use an anti-microbial additive like those suggested on a regular basis.

    4. Use a general purpose diesel additive with the attributes listed above on a regular basis.

    5.“Regular Basis” means every time you add fuel, you add the appropriate amount of biocide and general purpose additive.

    6.Use a lubricity additive in your fuel.

    I’ve got problems with dirty fuel —Now What Do I Do?

    At some point, if you own a diesel boat long enough, you will experience an engine slowing down, dying or failing to turn up desired RPM due to a clogged primary filter. Always carry several extra fuel filters on your boat and know the proper way to change them. Do what is necessary to get back to your dock or a friendly place to tie up since it is a lot easier to deal with a dirty fuel system when you are secured to a dock than bouncing around in some slop somewhere.

    First, try to clean up the system chemically. Do this by adding 10 oz. of Power Service Bio Kleen Diesel Fuel Biocide for each 200 gal of tank capacity.

    This is a “shock” quantity of biocide and it will kill everything growing in your tanks and will cause the microbial chains to release and break off the tank walls and bottom and become suspended in the fuel. Now add one 80 oz. jug of Power Service Diesel 9-1-1 (red container) for each 200 gallons of tank capacity. Next, go run the boat in some seas that will agitate the fuel in the tanks. The Diesel 9-1-1 has the unique attribute of being able to break up the chains of microbial growth into particles small enough that they will mostly pass thru a 30 micron filter. The secondary filter will catch some and the remaining particles are so small that they will pass through the fuel system and burn in the cylinders and pass on out as exhaust.

    After approximately 5 hours of running time, the final clean up step is to change all the primary and secondary filters again.
    If the above additives don’t clean up the system and you are still clogging filters, your only other alternative is to call a fuel cleaning service who will come to the boat and polish the fuel in the tanks for you.
    Comments 12 Comments
    1. johns botz's Avatar
      johns botz -
      good advice tried and tru so to speak......what i do here in indonesia with the ****ty fuel we get is to use a micfil filter system(out of germany / ) filtration to .5 micron.filter cleaning surface area equal to the size of a football field, racor for example 2 square feet of paper........fuel annalis report 98.5 clean... no water . micfil cannister good to 150psi .racor you would blow the lids off.micfil suck or pressurise , could never get a good life out of my racors cus they sucked ,20 micron already at 8in/mercury at 12in/m i get low pressure readings on bridge (thats using 3 big racors on each engine)put the micfils on the pressure side of lift pumps with a pressure guage,when the pressure gets over 10psi (i'm guessing)i will change (or when i get a low fuel flow indication on bridge)have 230 hrs on filters now no prob yet(supposadly i will get up to 500hrs plus from them) right now i still use the 3 racors as prefilters ,later will install RCI cyclonic fiter (no element) i get 50hrs out of the racors, they always leak and suck air)INERESTINGLY you use the same filters for oil bypass cleaning(cannister too)to half micron ,i have one boat dd8v-92's and one ddec16v-92's the filtration takes out all the 2 stroke carbon by products and quadruples my oil changes (to 600hrs)monitored regularly by oil anaysis
    1. rdarrington's Avatar
      rdarrington -
      Great writeup on diesel fuel. Keeping the tanks full during the season makes good sense. Anyone have advise on the following?....What about long term storage in cold climates where the boat will be stored on land from Nov-April? Is it better to keep the tanks full or empty? Any special types or amount of aditives for winter storage of diesel?

    1. Jackie J V's Avatar
      Jackie J V -
      Very helpful article - thanks. Just wish it was written two months ago - I.e., just before I encountered the power loss conditions noted. It was definitely related to algae in the tanks as the Racors were covered in the black sludge mentioned.

      Other than when away on longer trips, I always fill at my marina - a quality location with a reasonably high sales volume. What I have been doing this year though, is basically only taking the tanks up to half on each fill up. Given that I haven't been taking long trips, I thought I'd be better off not lugging around an extra 1500 lbs in fuel.

      This also coincided with playing around with other fuel conditioners available at major auto supply chain (Canadian Tire). I am now back to the CAT fuel conditioner.

      After changing the primary and secondary filters, I had the remaining fuel polished. I'm still not running completely smooth and am encountering intermittent slight power loss - not quite getting up to maximum RPMs from time to time. I'm guessing that this means that there still some sludge in the tanks breaking off from time-to-time. I'll try the shock process outlined above to see if that completely resolves.

      Thanks again for this valuable write-up.

    1. fwebster's Avatar
      fwebster -
      Quote Originally Posted by rdarrington View Post
      Great writeup on diesel fuel. Keeping the tanks full during the season makes good sense. Anyone have advise on the following?....What about long term storage in cold climates where the boat will be stored on land from Nov-April? Is it better to keep the tanks full or empty? Any special types or amount of aditives for winter storage of diesel?


      Your fuel tanks are vented to the outside of the hull. That means that whatever air is in the space above the fuel in the tanks has the same consistency as the air outside the boat, which is usually moisture laden since boats are stored near water. During temperature changes that occur in winter, moisture in the air above the fuel condenses on the interior walls of the tank and water droplets run down and into the fuel below. Microbial growth occurs in diesel fuel when in the presence of water. The more water you allow to be introduced into the tanks, the greater the risk you have of microbial growth and fuel sludge in the spring or later. Once your conditions allow microbial growth to begin, it doesn't stop until you kill it with additives. Keeping your tanks full drastically reduces the amount of water introduced into your fuel. So, with diesel fuel, it is always best to keep your tanks full.....even in the winter.

      There is a prevailing idea that boat owners in the north don't need to treat their fuel with additives or take the precautions those of us in the South do. I don't believe your location matters, just that it might take longer for sludge to develop in colder climates. It is far easier and cheaper to manage your fuel as though you were in the worst conditions, than it is to be forced to clean up sludge filled tanks and filters.
    1. cod's Avatar
      cod -

      I agree with you but just want to expand on the condensation issue. Correct me if I'm wrong, but I don't believe condensation develops to much during the winter months up north during temperature changes. The relative humidity of colder air is low (not much moisture in the air) and I don't think you get enough temperature change to create condensation. I think it would be most likely to occur during the fall when you have large temperature swings with warm sunny days and cold nights. If the warm air inside the tank does not cool down as quick as the air outside the tank, condensation will occur. The only reason I would think the air inside the tank would not cool down as fast as the air outside the tank is because of the fuel temperature not changing as quickly and keeping the air warm inside the tank. Condensation occurs when warm moist air comes in contact with a cool surface. (The drippping margerita glass or cold beer on a hot day). Additionally, I doubt it would ever happen in the spring as the cold fuel would keep the air in the tank colder then the air in the engine room. Just my 2 cents.......

      With that said, I agree that the best defense against condensation is to keep the tank filled during lay-up.

    1. fwebster's Avatar
      fwebster -
      Every geography is different.......even the the boat's location in a boat yard is a variable. Maybe northern conditions are less likely to cause condensation and maybe the ambient temps are cold enough to retard microbial formation and growth. I've had to clean up more than one fuel tank full of sludge and it is a royal PITA to deal with, not to mention the expense and inconvenience (try changing Racors in a 5-6 ft swell 100 miles offshore). I will always lean towards removing the variables and causes for dirty fuel and if I lived in the North, I would do exactly the same fuel treatment as I do now in Florida on the boat and Tennessee on my other equipmemt.

      Look at post #4 in this thread and ask Paul what he thinks.................
    1. Jackie J V's Avatar
      Jackie J V -
      My boat is certainly north - Georgian Bay - and I certaiinly encountered the problem this year. What I can't say is definitively is what happens over the winter.

      I've had diesel boats now for 7 years. The current boat (in my third year) is stored at the marina in inside heated storage for the winter - with the temperature kept at 7C. I would expect that'd be a reasonably stable environment. The previous boat was stored outside (four years) and I never encountered a problem. I've always put the boats to bed for the winter with ~7/8th of a tank of fuel. For a gasser, I know you have to leave some room for fuel expansion - maybe this is less of, or a non-issue for diesel.

      Anyway, I've definitely encountered the problem this year and I think I'm still dealing with it. We had a really crappy, damp start to the season this year, and then suddenly on Canada Day (July 1) the season turned hot and glorious and stayed that way through to Labour Day. Maybe these conditions contributed to the problem. I use fuel stabilizer at every fill.

    1. Alex F's Avatar
      Alex F -
      I agree with Frank's theory 100% and see no point to take risks. I always store my boats with full tanks.

      Other than Biocide, Power Service Diesel Klean and STANADYNE LUBRICITY FORMULA is there any other Fuel Stabilizer should I use for the winter storage?

      In my gas boats I always used Sta-Bil additive, would you guys recommend using Sta-Bil 22254 Diesel Formula Fuel Stabilizer and Performance Improver?
    1. Jackie J V's Avatar
      Jackie J V -
      An update to my experience. First off, the stuff I had been using all summer as an additive (bought at Canadian Tire) was Power Diesel Service Kleen + Cetane boost. I'm not blaming it, but contrary to my first post, I guess I wasn't using a cheap substitute. I'm sure the cause of the problem was my "half-tanked" philosophy as per above. Anyway, I was still encountering problems, even after fuel polishing.

      I first added the Racor Biocide at shock levels to the quarter-filled tanks, together with the Diesel 9-1-1 and then added another quarter tank of diesel. Went out on the water and shook things up pretty good. Waited a week, and then took it out for a longer, harder ride on October 30th for the last run of the season. I'm happy to report that the boat ran at full operating parameters - reached 2350 RPMs and 36.7 mph, for the first time in months. Went back to the fuel dock and topped it up to 7/8ths and tucked it away in the shed for the winter, after adding CAT Diesel conditioner.Lesson learned. Will be keeping the tanks more full next season! Process above is great advice.

    1. markrinker's Avatar
      markrinker -
      Great article - consistant with my experiences keeping onroad trucks and diesel powered equipment 'happy' here in Minnesota where we have a biodiesel content mandate, cold winters, and hot summers.

      Have never 'shocked' the tanks with biocide - but have been careful to buy only pre-treated marina fuels at high volume suppliers. Have been restricting 10M RACORs to where 2800rpm WOT is limited within hours, and 2200rpm cruise will be effected <30hrs run time.

      This season I have been running Howes products - my preferred family of onroad fuel additives.

      In >120hrs of total run time, i have gone through four pairs of RACOR 10M primaries, and one pair of CAT 2M secondary filters. My indicator for changing all has been the classic 'running slow' where I cannot maintain 2800 target RPM at WOT, usually characterized by problems maintaining engine synchronization.

      Current cruise plan, based on the suggestions of this article:

      1. Shock the tanks, agitate and leave the tanks topped off for one week prior to our late August vacation cruise
      2. Switch from RACOR 10M to 30M primaries
      3. Stock up on CAT secondary filters
      4. Add 9-1-1 on first tank prior to cruise, which will give an opportunity to burn and filter out dead microbial chains
      5. Continue to use cetane/lubricity additives and ongoing algae treatments at a maintenance level throughout the cruise

      Thanks, Frank for a great article.
    1. gerryb's Avatar
      gerryb -
      Never had a fuel problem with our 40 as I have followed the advice of using a biocide and additives as suggested above. However, I think the Diesel 911 may not be the right product. Looking at the manufacturer's site,
      it looks like the BIOKLEEN is the better option as the 911 is for winter related issues.
    1. fwebster's Avatar
      fwebster -
      Power Service now has a product called Clear-Diesel:

      It is being markets as a "fuel polishing formula" and is supposed to contain a biocide, a moisture dispersant, and is supposed to disperse contaminants.

      Diesel 9-1-1 disperses moisture and will break up microbial growth so it can pass thru filters. I believe that wording was removed from the 9-1-1 packaging because the 2 products read as though they do the same thing. The main difference to me is that Clear Diesel is only good if you use it all the time and 9-1-1 can be used only when you need it (assuming you have been using Diesel Kleen and a biocide) and is extremely valuable to help clean up a dirty fuel system.

      Also, this is a personal preference, but I prefer to use separate biocide, general fuel additives and lubricity enhancers because there are time when you need to shock the fuel tanks to eliminate a problem and because you can buy biocide and Diesel Kleen in concentrated versions which reduces the hassle when fueling the is just easier to add 20z of something than it is to mess with 1.5 16 oz bottles or 70% of a 32 oz jug.