Why Should You Polish Your Fuel?
Fuel polishing is a way to get water, microbes and any other contamination out of fuel before it harms or stalls an engine.
Hot, humid weather will allow condensation to form in a fuel tank, particularly if it’s only partially full. Condensation in a fuel tank allows water to get into the fuel. Various microbes love that water and will multiply in it at the bottom of the tank. While a biocide can help, it can only do so much. And you’ll still have sludge at the bottom of the tank from all the now-dead microbes.
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 Is Fuel Polishing Needed?
Your fuel will likely need polishing:
- 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)
- If you buy a used boat that hasn’t been used recently.
- Should you discover that the fuel cap was loose and water/dirt may have entered the tank.
- If someone confuses the water and fuel fills and putting water in the fuel. Yes, this happens.
- If you get a load of “bad fuel.”
- Any time you discover 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 for DIY Fuel Polishing
It costs about $100 to get the tools needed to polish fuel our way, assuming you already have a drill (or use an electric pump) and have or borrow sufficient 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. For diesel, get either:
- 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)
- 12 volt Diesel Transfer Pump with Alligator Clips on Amazon — we do not have this but friends do and are happy with it. The big thing is having power available (you can replace the alligator clips with a cigarette lighter style plug if you have a handy outlet, too).
- 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)
- 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 the end of the pickup tube.
- Sufficient containers to hold the contents of one fuel 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 Polish Fuel Yourself
- 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.)
- 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.
- 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.
- 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.
- 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.
- Repeat for any other fuel tanks.
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Pamela Dakin Harwood says
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 says
I know of several people who have done something like that. Great!
Pamela Dakin Harwood says
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 says
Similar . . . but no, not as sophisticated.
Georgi Wellington says
Dennis Wellington
Jonathan says
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 says
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 says
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 says
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 says
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 http://www.fuel-guard.co.uk 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!
Carolyn Shearlock says
Thanks for the info!
Dawn Read says
This is good! Thanks! Lucky, check it!!!
Rick Garvin says
Great article! Christine, we need to by a Mr. Funnel.
Molly Brown says
Thanks for the great ideas. We have had dingy motor problems from bad gas and needed a good filter.
The Boat Galley says
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 says
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 says
G’day,
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.
Steve Hardison says
“Large Racor systems sometimes do have a check valve that stops the flow if the water reservoir is full.”
— Sorry,not true. The ball floats on fuel or water. It is pushed out of the way when fuel is flowing. Its purpose is to check the fuel from flowing back to the tank when flow stops. If the Racor housing fills with water, water will be forced through the media. The best defense against water is a new filter element. As filters clog, they don’t stop water as well as they did when clean. It’s cheap insurance to just change the filter when vacuum reaches 4 to 6 inches Hg above the starting vacuum. Going up to 15 inches Hg vacuum risks poor water removal performance.
Jan Hansen says
One question, I have made a similar set up, using a pump with metal impellers, rather than rubber.
I have a Racor 500 w clear bowl, I am waiting for the manometer, and the drain cock rather than the unscrewable nut.
I have no idea, how bad my tanks are ( 2nd hand vessel from the 70s ) I would have to destroy a fair bit of the interior, to be able to access anything but the base of the tanks , as they are square, and are ” On End ” rather than ” Lying Down ”
My plan, was to run both hoses, into the same tank ? One hose sucking up, and the other agitating the bottom, with clean returning fuel ? I would rather spend a few dollars on filter elements, than have to have a carpenter cut away my interior ?
Jan Hansen says
Sorry, bases are square, the tanks themselves are oblongs, stood on end
Carolyn Shearlock says
If I understand correctly, you just want to use your Racor as a fuel polisher. Sure, some people do that. It works but if you have any serious contamination you’ll go through a bunch of filters.
Jan Hansen says
Yes, I do not have anything, to empty the tanks into, but most responses seem to state exactly that.
As said, the way the boat is built, trying to access the tanks, to cut inspection holes, is beyond the cost of destruction & re build of the interior.
On the other hand, I would have to buy a 100 gal tank, to empty the tanks into, one at a time, then try & re sell the tank, since there is no room to keep it on board.
So, my idea ,as said, is to run the pump & filter, with both hoses in the same tank.
Yes, I do understand , it will probably use a lot of filters, but Im assuming it will be better than the alternative.
Ok, Ill get on , with the job, Im still waiting for the manometer and tap, but, once i get started, I will post the results here.
Of course , I do wish the tanks had drainable sludge pots, in the base, but ,sadly no.
Cheers
Allen says
I’ve used basically the same technique as Jan suggests… just recirculating as I stirred up the muck in the bottom. I used cheap filters until the fuel looked better (went through about 10), then switched to a Racor 30, then 10 micron. I did have access to the tank via the fuel sender port, which is where I inserted the suction tube. I also have one of those cheap scope cameras that I used for inspection… pretty handy. Like Jan says, rig something on the end of the pickup tube that will weight it down and stir up the bottom. I used a 10 gpm diesel transfer pump, which stirred the fuel up pretty well as it re-entered the tank… just have to keep on it till it’s clean!
jan Hansen says
Well, so far, so good, Ive used 4 filters so far and picked up loads of sludge & bits, just bought another 10 filters today, and will carry on.
So far Ive only worked on 1 tank, with a metal bar whipped to the fuel hose & small polythene funnel, cut down and ground slots into, so it sits on the bottom with holes enough to allow crud to be sucked up.
also, the returning fuel, agitates the bottom so more crud crud is stirred up.
So far, no sign of water , in the tank Im working on, but lifting the suction hose, and bouncing it off the bottom, certainly fills the clear bowl with a black cloud of bits.
I guess the tank spigot, is raised above the bottom, because getting that suction hose to the bottom, jeez the filth that the filters get filled with is awesome.
So far, no sign of any water, in the bottom, just black sludge.
So Far.. So Good.
JAN HANSEN says
Well, between the 2 x 100 Imp Gall tanks, it took 11 filters, to clean it up and get water out.
Having the manometer on top, is worth the extra cost, because it removes all guesswork.
Once it gets well into the red, its time to change filters.
I was going to post a piccy of the system I built on a cheap shopping trolley frame, but cannot see how.
Cheers.
Nasty Chicken says
Nice work Jan, I trust the clean tanks made a huge difference!
Jan says
I have not done any rolly polly passages, as yet, but, there is certainly a load less sludge & bits, in the tanks.
So if I do get into rough stuff, im assuming my fuel filters , will be ok.
Thanks !
Art says
I have an IO with at 16 gal. tank. It has been setting for 25 or so years. There is a thick sludge in the bottom of the
tank. How can I get that sludge out of the tank to clean it. Will fresh gas dilute the sludge so that I can pour it out? or
What else can I use to clean out the sledge? I’m restoring the boat and want a clean tank.
Art from Virginia – fresh water boater.
Carolyn Shearlock says
You will probably need to cut an inspection port into the top of the tank so that you can get your arm in there to really clean it out. Fresh fuel is a bit of a solvent, but you’re going to have to do some work if it’s really sludgy.
Robert Hogward says
So, what about draining gas from Boat fuel tank? I concerned about some residue buildup in my fuel tank. And can draining gas from my fuel tank be hazardous? Many thanks!