This is the plan I have for later this afternoon, as a way to clean my fuel tank.

The fuel tank has not been cleaned for a while. Hence there is this sludge or slime that sit on the bottom. It sometime break loose and would enter the fuel line as a mass of thin sludge that is combined into a big lump.

This would sit on top of my CAV filter, and block the filter top surface area. Reducing the filter flow capacity by a large margin. Which will in turn stall the engine.

After happening twice, where I had to replace the filter element before the engine would go again, I think it is time for me to do something about it.

The plan is to use the oil changing vacuum pump and its inlet hose to suck the bottom of the tank. This hopefully will get rid of all the sludge, without cleaning the tank properly. As the access hole is too small to do anything meaningful. The opening is only about 2" in diamter. It is where the VDO fuel level sender is bolted onto the tank.

I will let you know the result in the future.


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I finally pulled my finger out and did this clean out.

The result was fantrastic.

Before the clean, using a torch, I can see the water to fuel interphase and see that the water was black. Most likely from the algae etc that grows in the water.

After the clean they were all removed. And the oil vacuum pump collected some fuel and most of the water and gunk.

Since the reason for my engine to conk out was when a layer of this algae get sucked in and blanket my CAV filter, I believe that largely the problem has been eliminated.

In my view it is not critical to leave some particles in the tank (if there are any), as they will get filtered out by the fuel filter normally. Not the Algae, which tend to cover a large area of the filter and limit the maximum flowrate going through the filter.

So, this method is proven to be a success. If you have engine problems and it tend to conk out, and it is due to the Algae covering the fuel filter, please give this method a try.
your procedure sounds essentially like polishing the fuel, which is a great practice to remove water and algae..if you use the right filters.

What isn't getting done is cleaning the layer of sludge that forms at the bottom of the tank. In normal boating the sludge will stay there on the bottom. But when you are in rough waters (offshore) the tanks start sloshing and the sludge mixes in with the clean fuel,. This eventually clogs your fuel filters.

My boat is a 1985 and has spent a lot of it's life in the tropics where algae is more of an issue. And on the first offshore trip I saw the fuel in the Racor bowl looking very brown. Every time we stopped on that trip I changed the filter.

The only viable means of cleaning the sludge is to introduce a high pressure stream of fuel (i.e. pumped) returning from the polishing filters and keep recirculating until the fuel runs clean.

The pro outfits have setups to do all this but you can make your own with a pump, filter, tubing(metal) for a wand to stick in the tank. You will likely need to disconnect the fuel fill hose connection to get the wand in and maneuver it around.

I had a service come out to CT from RI and do this for me. It cost around $400 to do my two tanks (100 and 50 gallons). No mess. Great job right at the marina dock bulkhead.

The new fuels are more of an issue than in the past. They break down and separate faster. So if your fuel has been sitting in the tank for a long time you may develop problems. Fuel and tank cleaning is a growth business.

BTW in older boats you might find a similar issue with sediment that settles out of the water in your water tank over the years. The same procedure applies to that issue.

Yes it is a diesel tank.


Thank you for your advise.

The commercial fuel polishing and its associated cost is exactly what I was trying to avoid.

FYI, my engine conk out twice earlier, with the filter covered in algae sludge. I believe the algae sludge is the thin layer that formed in the tank. I also suspect that this layer is only formed in the tank part that has water permanently covering the bottom.

Luckily for me, my tank is sloping all the way to a corner. In this corner the water sit. The level can not be very high, or the water will be sucked in by the fuel line.

Between the 2 occasion of the filter blockages and the final clean up using the engine oil vacuum pump, I saw that the bottom of the tank are now all clean. I saw a few particle or dark spots on the bottom of the tank. Pointing the vacuum hose to these area does not seemed to clean it up.

Based on this I think I have eliminated the sludge of algae problem.

Additives is a possibility. But I plan to do this clean up every year or two anyway, since it is so simple to do, and it gets rid of any water in the tank.
On a stifled response, but now i had a beer or more, i'll ask,,,i thought simply pressurizing would do it but i know ye said there was "slime" in the ice machine". will pressurizing not get it out,,,i reckon not if it has really solidified. maybe some type acidtone? if thats the right word? later,, my beer is transferrinf petro/gas from metal cans is bad too, filrtes fail to filter smaller crap...I have some home remodel questions no doubt,,,
if you could see the tank bottom and it looks clean it sounds like you have cleaned it well. The cleaning service highly recommended additives. No need to get the "marine" type you can get them in an auto supply house. Look at the labels some are anti-algae (important in tropical areas), some remove water, some boost cetane and some are combinations.

Algae can clog filters, clogs injectors and can starve the engine of power or eventually stop it.

Water in the fuel can blow the tip of the injector off when the water flashes to vapor when it hits the high engine combustion temperature

Anyone want to offer an opinion on the dark spots? My tanks are stainless and after cleaning they were all shiny. If I saw dark spots in my SS tanks I would suspect rust.

What are your tanks made of?
Tank is made out of SS. Some higher grade SS. I forget to note the numbers.

The darker spots are small. But it was there. May be my vacuum hose end did not go over it directly, so they do not get sucked out. It is a little hit and mis (mostly mis) on pointing the bent plastic hose and try to aim it to the right place.

I will look for the additive. Everyone recommended it.
if the spots really concern you find a borescope or fiberscope and get a closer look, or just be happy with clean tanks
Great trick to keep water out of your fuel tank is drop in a small zinc every couple of years.


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