How to do your own fuel polishing -- the tools you need and a step-by-step how-to.

DIY Fuel Polishing

After our boat had sat all summer in the heat and humidity of south Florida, we wanted to polish the fuel before putting her back in the water. Although we had left the fuel tanks full, condensation can still occur. And you never know what may have grown in the tanks . . . microbes love water and multiply in it at the bottom of the tank. Biocides can only do so much.

Why and When Polish Your Fuel?

Why worry about water, sludgy microbes and debris? The immediate problem is that your engine will die — it won’t run on water and debris can block the flow of fuel. That’s bad enough if it happens just as you’re entering a tricky place, say a marina or going through a bridge. But the even bigger problem is that both water and debris can cause thousands of dollars of damage to your engine.

IMPORTANT NOTE: Even with a Racor filter on your fuel line, if there is just too much water in the fuel, it can get past the filter on the smaller Racors that most recreational boaters have — large Racor systems sometimes do have a check valve that stops the flow if the water reservoir is full. Always check the sight bowl in your Racor for water at the end of a day; check it several times a day if motoring a long time in order to catch a problem before your engine is damaged. Sludge and debris can clog a Racor in mere hours, again causing your engine to just stop.

Fuel polishing is pumping all the fuel out of your fuel tank(s), running it through a filter to remove water and debris and putting it back in the tank. Some professional services also clean the tank while it’s empty.

When might you need to polish your fuel?

  • When your boat has sat for a while in a humid climate (particularly with day/night temperature swings causing heavy dew or fog, both indicators that condensation is likely in the tank)
  • A new-to-you boat that hasn’t been used recently.
  • Discovering that the fuel cap was loose and water/dirt may have entered the tank.
  • Someone confusing the water and fuel fills and putting water in the fuel. Yes, this happens.
  • Getting a load of “bad fuel.”
  • Discovering an abnormal amount of water or debris in your Racor filter.
  • Before going out in “boisterous” conditions if the fuel hasn’t been polished recently — any water and debris that has settled to the bottom of the tank will be stirred up with the motion of the boat.

In many places, there are commercial fuel polishing services. But it always seems that when we need our fuel cleaned, there are none available, they’re prohibitively expensive, or there’s a long wait for service. Hence doing it ourselves (we know some people who have fitted an electrical fuel pump and filtering loop onto the fuel system in their boat — that’s also an option for doing it yourself, albeit a more expensive one).

Tools Needed to Do It Yourself

It costs about $100 to get the tools needed to do it our way, assuming you already have a drill (or use an electric pump) and have or borrow jerry cans.

To polish your fuel yourself, you need:

  • A way to pump fuel out of your tank(s). If your boat uses diesel, you can use a simple drill pump as diesel is not explosive — that’s what we do. You can also use various electric fuel pumps designed for diesel. DO NOT use a drill pump with gasoline — instead use a pump that’s approved for use with gas.
    • Jabsco Drill Pump on Amazon (same one as we have although we bought ours in an “emergency” situation at West Marine for quite a bit more) — we use ours with a Ryobi Cordless Drill and the lithium battery packs (we have two 18-gallon tanks and can pump one completely out with one fully-charged battery and there’s still juice left in the battery)
  • DIY Fuel Polishing: Whenever we want our fuel polished, it seems, it doesn't work to get it professionally done. So we've figured out how to do it ourselves.
  • A weight on the end of the pickup tube so that you can get all the contents of the fuel tank. Dave had some weird fitting — basically a bolt with a hole through it — but a couple of stainless nuts work too (use something that won’t rust). Make a hole through the tubing and use a piece of seizing wire to keep the weight from slipping off.
  • How to do your own fuel polishing -- the tools you need and a step-by-step how-to.
  • Sufficient containers to hold the contents of one tank. Jerry cans are great and you may be able to borrow some empty ones from friends, too. You can also use 5 gallon buckets lined with clean trash bags. Whatever you use, you have to have sufficient containers to be able to remove all of the fuel from one tank at a time. You can, of course, transfer the fuel from one tank through the filter and into another if you have an empty tank (you may want to clean the empty tank first).
  • A water and particulate filter. Cruisers have used Baja filters for years, but they are no longer made and used ones sell for a minor fortune. We have a Baja filter, but I’ve talked to a number of people who have other brands and the “Mr. Funnel” ones sold on Amazon are considered to work better than anything else. I have no personal experience but several people have told me that the West Marine filters do not nearly as good a job of filtering water out (you can test your filter by pouring water into it and seeing if any comes out the bottom — none should).
    • Mr. Funnel sold on Amazon — this size works well with transferring from jerry cans. If you want a larger filter that can also be used when filling at fuel docks, this size is better — but it’s much larger to store!
  • A way to transfer the fuel from the jerry can through the filter and into the tank. Yeah, you could hand pour it (if so, you’ll have a sore back!). You could use the same pump that you used to get the fuel out of the tank. But the far easier and faster way is to use one of these nifty shaker siphons (read more about them here if you’re not familiar with them and how to use one).
  • Container to put any water caught in the filter into, for transport to an appropriate disposal site.
  • Rags, paper towels or blue shop towels. To catch spills as necessary.

How to Do It

  1. Pump all the fuel from one tank into jerry cans or other containers. You can use the water/sediment filter as you’re filling the containers or you can filter it as it goes back into the tank. (You can filter it on both transfers if you fear serious contamination.)
  2. Wiggle the pickup tube around as much as you can to make sure you’ve gotten as much fuel out as possible. If the boat is in the water, have anyone else on board move so that one part of the tank is lowest and you’ll get the most.
  3. If you filter the fuel coming out and find a lot of water or sediment, it’s a good idea to go ahead and clean the tank before you refill it. How you do this depends on your tank configuration . . . maybe by wiping out the inside through an inspection port (how we did it on our Tayana 37). On our current boat, the only way to do it is to remove the tank then remove the pickup tube from the tank, then go through that hole with a rag on a stick.
  4. If you filtered the fuel coming out of the tank, you can simply use the shaker siphon and put the fuel directly into the tank now. Otherwise, put the funnel filter into the fill hole and use the shaker siphon to put the fuel into the funnel filter and filter the fuel as it goes into the tank. Dave finds it easier to filter at this point, but it means that if he finds lots of water or gunk and decides he needs to clean the tank, he has to pump the fuel back out of the tank again to do so.
  5. Whenever you are filtering the fuel, if the filter really slows down, check it for both sediment and water. Clean the filter if necessary. Pour any water into the container for this purpose and dispose of it properly.
  6. Repeat for any other fuel tanks.
How to do your own fuel polishing -- the tools you need and a step-by-step how-to.

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  • Pamela Dakin Harwood
    Posted at 23 October 2015 Reply

    It seemed that every year we would have a problem with “stuff” in the fuel of my husband’s family’s 28′ lobster boat, so with Katie Mack (our soon to be liveaboard Bridge Deck Cruiser), he has designed and installed a continuous fuel polishing system.

    • The Boat Galley
      Posted at 23 October 2015 Reply

      I know of several people who have done something like that. Great!

    • Pamela Dakin Harwood
      Posted at 23 October 2015 Reply

      Can’t imagine that sailors need anything this complicated, so Dave’s solution is just right, but with only one engine, our powerboat needs to have clean fuel — always!

    • The Boat Galley
      Posted at 23 October 2015 Reply

      Similar . . . but no, not as sophisticated.

  • Georgi Wellington
    Posted at 23 October 2015 Reply

    Dennis Wellington

  • Jonathan
    Posted at 23 October 2015 Reply

    If a tank has a bad case of sludge, the only way to get it out is by hand through an inspection port. That is why so many boats who think they have clean fuel tank run into fuel problems when the tanks get a good shaking. So keep a bunch of filters handy if you are going offshore, and check the sight bowl every hour if you are running the engine in bad conditions. My $.02

  • Jenn
    Posted at 27 October 2015 Reply

    Great article (as always!). I have a total noob question – in step 5, how do I clean the filter? Just wipe it with a rag? Sorry – I’m new to all this but love the idea of being able to do it myself.

    • Carolyn Shearlock
      Posted at 27 October 2015 Reply

      It’ll come with directions on how to clean it, but yes, basically you dump the water out and wipe it off with a rag to get any “gunk” off.

  • Chris in Oregon
    Posted at 29 October 2015 Reply

    Timing is everything… I just received today from Northern Tool a 12v diesel fuel transferring pump. Bought to do two things, #1 being able to refuel boat from jerry cans without having to hoist them on deck from the dingy and #2 adding a filter to allow pump to be a fuel polisher. I looked at DYI and cost wise it was about a break even to just get it from them.

  • Paul Johnson
    Posted at 29 October 2015 Reply

    Hi Boat Galley,
    Have just read your article on fuel polishing with great interest. Here in the UK we have a problem with Diesel Bug, this grows in the interface between fuel and water and causes the black sludge in your fuel tank.
    A recent article in Practical Boat Owner they talked about a product called ‘Fuel Guard’ and their web site shows the product in action, it’s a very effective demonstration, and all you need to clean the filter is clean diesel and a soft paint brush, the filter lasts a lifetime.
    I bought one for my boat and it’s been very impressive, I’m conviced the engine is running better as well!

  • Dawn Read
    Posted at 10 September 2016 Reply

    This is good! Thanks! Lucky, check it!!!

  • Rick Garvin
    Posted at 10 September 2016 Reply

    Great article! Christine, we need to by a Mr. Funnel.

  • Molly Brown
    Posted at 10 September 2016 Reply

    Thanks for the great ideas. We have had dingy motor problems from bad gas and needed a good filter.

  • Eric Meury
    Posted at 10 September 2016 Reply

    I make sure to read all your posts and have you book. I would not consider what you are doing as polishing the fuel, rather re filtering to to get the crude and water out. This method does nothing for the actual chemical make up of the fuel. The link below is the system we have on our boat. This pictures are our boat. We polish the fuel constantly while the engine is running and we polish when ever we fill up. Even in the states we end up removing water and some crud. Oh one other thing. Whenever my main engine isn’t going to be used for a week or more i also do a fresh water flush using the cockpit shower hose. This way no salt water sits in my exchange or exhaust system.

    • The Boat Galley
      Posted at 10 September 2016 Reply

      I’m not sure what “polishing” fuel means to you. I am familiar with it as “fine filtering to remove contaminants and water” — which is exactly what this does. Admittedly it’s not as sophisticated as yours and I’m sure ours doesn’t filter quite as finely or as completely. But we have a much smaller boat — there’s nowhere to put a system such as yours in!

    • Eric Meury
      Posted at 10 September 2016 Reply

      a copy and paste – fuel polishing uses centrifuges, coalescers, and – yes – filters to remove non-combustible particulate matter (sand, dust, cigarette wrappers, lady bugs, and – our personal record – a dead rat) from fuel. In short, everything that wasn’t really supposed to be there in the first place.

      Because water is heavier than fuel, the centrifuges and coalescers do a good job of removing that, too. We then use water blocking filters to pick up the last little bit of suspended – or “entrained” – water available.

      The primary difference between fuel polishing and fuel filtration is that fuel polishing acknowledges that fuel itself can degrade and cause solids. Fuel filters will simply pull the solids out because they plug up filters whereas fuel polishers break down the combustible solids so that the fuel stays within the industry specifications.

  • Obi
    Posted at 15 December 2017 Reply

    my boat takes a ton of fuel. Gonna be a job and a half if I bother. Love folks ideas and comments.. Can only make one wiser.

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