Boat Maintenance Throwback Thursday

How to Polish Diesel Fuel

Thursday, May 19, 2016S.V. CAMBRIA

This blog post was first published on Three Sheets Northwest, on online boating magazine local to the Pacific Northwest, in February of 2015. The original article can be read here

We were motoring up Fitz Hugh Sound along the Central Coast making our way to Ocean Falls on June when the engine started hunting, fluctuating 150 RPM from where it was set. It’s always a concern when there’s something going on with the motor, but the symptoms were consistent with a dirty fuel filter – something David was planning to change out once we got there – so he eased the throttle back a bit, which seemed to help. The next day he replaced the old fuel filter; and when we left Ocean Falls, the engine was running smoothly again.

Fast-forward to October and issues with our diesel-powered hydronic heating system: We had started getting flame-outs about 12 months beforehand, which coincided perfectly with a dying house battery bank. Ever the optimist in search of the easiest solution, I thought the problem resulted from low voltage and would resolve itself once we replaced the batteries. David (the mechanical engineer and practical one) wasn’t convinced, so he changed out the fuel pump along with the filters and we were back in business . . . for a while. But there we were, months later, plagued with flame-outs; it was starting to look like we’d picked up some bad fuel along the way.

To check, David disconnected the fuel lines (supply and return) to the heater’s boiler and installed temporary lines that he ran from a jerry can that contained fresh, clean diesel. The system tested well: The hydronic loop heated up quickly and the cabin fans went online in less than 10 minutes (a big improvement over recent start-up times).  Similarly, the domestic water Aquastat was quickly satisfied – all with no flame-outs. 

Our questions had finally been answered: We had “The Bug”.

Getting Rid of the Gunk

Our fuel system is set up so that diesel can be drawn from all three tanks (starboard side, port side and forward) simultaneously. The fuel is then run through the engine’s filtering system before it flows to the engine and the excess is fed back to the port side tank. Because the fuel in this tank had already been technically “polished”, we were confident it was clean . . . or at least the cleanest. The forward tank sits higher than the other two and feeds directly into the starboard side. This tank was already empty, so we knew the only fuel we had to remove was on the starboard side.

First, David built a pump system to empty the remaining fuel from the starboard tank.  From Cambria’s log:

The fuel transfer rig consisted of a piece of 3/8-inch OD Type copper tubing about 6-feet long to be able to reach all the way to the tank bottom; two pieces of matching-sized clear hose, one connected to the pump intake, and the other as discharge into the buckets; and a 12-volt Facet Cube fuel pump (a spare for our Hurricane Heater).

When I set up the fuel transfer/polishing "rig" I was in two minds about installing a bank of filters (10-5-2 micron) ahead of the pump, and finally decided that would have cost more than the value of the fuel I was trying to clean. Instead I elected to pump the fuel directly out into buckets in order to really see what was living in the tank. Then, depending on its condition, either return it to the tank via Baja filter, attempt to "polish" it chemically and return it filtered to the tank, or discard it entirely.

With the transfer rig in place, we started to remove the diesel from the starboard tank and placed it in clean white buckets. Initially, the fuel coming through the lines was clean with no obvious signs of contamination – approximately eight gallons or so. Once we started to see particles in the fuel, we stopped pumping. David then treated the “clean” fuel with a shock dose of Bio-Kleen biocide and transferred it to the port side tank, filtering it through the Baja filter as he did.  

We continued to remove the rest of the fuel from the tank:

The remaining seven gallons of diesel were obviously contaminated – black particles, held in suspension ranging in density and size from negligible to significant, trace “smudges” to small pieces, yet still large enough to clog the fuel pump periodically during the transfer operation. The amount of contaminant drawn from the tank also varied as the suction tube was moved around within the tank. As the fuel passed through the pump, the algae/growth particles were broken down and blended into the fuel.

Very surprisingly, I chemically treated the "molasses" fuel and allowed it to stand overnight. By mid-next day, there was a heavy black sediment in the bottom of the buckets and the fuel was perfectly clear and ready to be filtered back into the tank. From a total of 12 -15 gallons drawn from the contaminated tank, the quantity of fuel unable to be re-used amounted to less than a half gallon.

With as much of the sludge removed from the tank as we could manage, we then filled all three tanks to 100% with fresh diesel and treated them with the following products from Power Services:

  • Bio-Kleen Diesel Treatment (kills microbes in diesel fuel): a shock dosage for all three tanks.      

  • Clear Diesel Treatment (removes water and contaminates from diesel fuel): a maintenance dosage for the port and starboard tanks.

  • Diesel Kleen With Cetane Boost (injector cleaner): a shock dosage to all three tanks.

How Much Did We Save?

Out of curiosity, David contacted an all-service fuel company in Everett to see how much they would charge to polish our tanks. For a two-man team to come out to the boat, the hourly rate is $215 plus travel time. Assuming the job took two hours, the cost to us would have been in the neighborhood of $700.

Here’s what we actually spent:

Copper Pipe:                      $10
Clear Hoses and Fittings:     $10
Diesel Treatment:               $70
Fuel Pump:                         $0 (we used a spare; a new one costs $40)

Total:                                $90 (and an afternoon’s work)

It’s been over a year since we polished the fuel and, so far, the heater has been performing very well. We did start to experience occasional flame-outs again after a couple of weeks but the problem was resolved by cleaning the deposits from the burner nozzle on the boiler. The engine is also running well, but there’s likely some debris sitting at the bottom of the tanks that we weren’t able to reach. The biocide, however, would have killed the algae and kept it from growing further.

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  1. Very useful article. Something we may need to do to our diesel. Although, I'm not sure if we can do it ourselves at our marina or if we're required to use an external contracts.

    1. Honestly, I was surprised at how easy the process was and how well it went. I give the fact that we were down to our last 15 gallons (give or take) s lot of the credit.


    2. We knew we wanted to polish our diesel before leaving the docks, but I was scared at how long it would take and how much of a pain it would be. We actually did it after work one evening - it probably took about an hour and a half for two tanks (two engines) - and we could leave the docks knowing our engines wouldn't be silly on account of contaminated fuel. -Lucy

  2. So wonderful to have a handyman onboard. We lucked out on Irie and never had to clean our diesel tank. From the moment we owned our boat, we always and ever used a Baja filter before the diesel reached the tank. That patience and persistence might have paid off. :-)

    1. The silly thing is we have a filter aboard as well but haven't used it much because the diesel here is so safe but we were in a somewhat remote area where they probably didn't sell enough fuel to keep their tanks fresh. It's not a mistake we'll make again! But at least we were able to take care of the problem ourselves.

  3. Absolutely fascinating! I never knew diesel could be cleaned or that it needed to be :)

    Mars xx
    @TrollbeadBlog from
    Curling Stones for Lego People

    1. We've been really lucky and have only caught the "bug" once in 15 years so it was our first time having to clean it. Now that we know how easy it is, we'll never pay for the service!

  4. The picture of the three Power Services products could have been taken on my boat. Great products. I started using them a few years ago at the suggestion of an old timer who serviced my injectors. Have not had an issue and suggest these to other folks as well.
    My aluminum tanks are coming out this year. One has a leak and I want to steam clean and add clean outs to both. Just in case.
    Have fun!