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Old 11-06-2024, 08:18   #1
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How clean should a diesel tank be?

I tried to clean the 90 liter diesel tank from the inspection hatch on my Najad 331. I did it with a manual squeeze pump, trying to suck the dirt I could see at the bottom. On the pictures, you'll see the bottom of the tank after cleaning and the plastic bottle with the dirty diesel that came out.

I've never opened a diesel tank before and I wanted to ask you:

- Do you think what came out is a lot, a little, or just about normal for a 23 years old boat? (I don't know if the tank has ever been cleaned, for sure not in the last 8 years)
- Is the residual dirt on the bottom acceptable or should it be removed completely to avoid potential blockages or other issues? The dirt that came out is slimy in consistency.
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Old 11-06-2024, 09:03   #2
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Re: How clean should a diesel tank be?

A diesel tank should be spotless. NO water, NO gunk, NO suspended solids, NO foreign matter. That tank is filthy and your engine would likely stall in rough seas as that crap got mixed to the point it reached the intake line and clogged filters, or worse, injectors.

Seriously, your diesel tank should be cleaner than your potable water tank.

Have that tank professionally polished before you run the motor.
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Old 11-06-2024, 09:06   #3
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Re: How clean should a diesel tank be?

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Have that tank professionally polished before you run the motor.
Or unprofessionally polished if you prefer to use the funds for building a polishing loop.
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Old 11-06-2024, 09:11   #4
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Re: How clean should a diesel tank be?

A lot. Should be more or less spotless. I would treat with biocide, fill, change fuel filter, and keep an eye on the fuel filter. If you have a Racor or similar it will catch a good deal of that without clogging. Just be alert for possible problems and replace it at the first sign of problems at full power. Be prepared to do this on the water, have a spare and any tools needed and awareness of the procedure. If you're not good with that fuel polishing is an alternative.
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Old 11-06-2024, 09:14   #5
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Re: How clean should a diesel tank be?

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Or unprofessionally polished if you prefer to use the funds for building a polishing loop.
Very few “unprofessional” systems actually circulate fuel fast enough to stir up crud off the bottom of the tank, and even fewer have the capability of cleaning the various sections of a baffled tank.

The systems I have seen on most sailboats might help keep a clean tank clean, but for cleaning up a mess like this they are inadequate.
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Old 11-06-2024, 09:31   #6
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Re: How clean should a diesel tank be?

A fact to consider is that there are many areas where fuel polishing services are not available. The OP does not state a location.
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Old 11-06-2024, 09:36   #7
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Re: How clean should a diesel tank be?

Quote:
Originally Posted by efanost View Post
I tried to clean the 90 liter diesel tank from the inspection hatch on my Najad 331. I did it with a manual squeeze pump, trying to suck the dirt I could see at the bottom. On the pictures, you'll see the bottom of the tank after cleaning and the plastic bottle with the dirty diesel that came out.

I've never opened a diesel tank before and I wanted to ask you:

- Do you think what came out is a lot, a little, or just about normal for a 23 years old boat? (I don't know if the tank has ever been cleaned, for sure not in the last 8 years)
- Is the residual dirt on the bottom acceptable or should it be removed completely to avoid potential blockages or other issues? The dirt that came out is slimy in consistency.
I'd say that is not unusual and pretty clean really. Mine were often opaque and not even clear like that due to growth in the fuel. Diesel getting old is an issue if you are not full time cruising. Have good filters and a way to switch to another filter is nice also if one gets clogged. Treating with biocide may kill the growth but doesn't make it disappear! It's still floating in the fuel.

A low pressure home polishing setup is nice if you start with a clean tank, otherwise you need aggressive flow to clean well and often even that cant get the stuff behind the baffle.
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Old 11-06-2024, 10:07   #8
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Re: How clean should a diesel tank be?

Do yourself a huge favor. Assembly your own polishing system. Cheapo Racor-like fuel filter, hoses, 7-10 PSI fuel pump, mesh filter, vacuum gauge to keep eye how clogged your filter is from Amazon should not set you back more than $100. Install intake pipe from bottom of your tank. As soon as I hit waves sailing it is on and polishing the fuel. If your tank has gunk on bottom "vacuum" it first with mesh filter only. If your fuel is really dirty make your own filter using dippers or cloth to get rid of biggest particles and save your "Racor" filters.
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Old 11-06-2024, 11:44   #9
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Re: How clean should a diesel tank be?

My tanks are cleaner than most people's cooking vessels. With inspection ports that I can climb through and really get inside. I would keep cleaning those until spotless.
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Old 11-06-2024, 12:29   #10
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Re: How clean should a diesel tank be?

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Originally Posted by Cheechako View Post
I'd say that is not unusual and pretty clean really.
My thoughts too for a 23 year old tank. Below are the before and after pictures of when I opened up my 35 yr old tank just on a whim to see what they were like

Quote:
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A low pressure home polishing setup is nice if you start with a clean tank, otherwise you need aggressive flow to clean well and often even that cant get the stuff behind the baffle.
I did try this 3 or 4 times a year for several years, but in fact it did nothing. The diesel bug and asphaltenes required a 2" wall paper scraper to remove the crud from the walls of the tank.

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Old 11-06-2024, 12:33   #11
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Re: How clean should a diesel tank be?

Since you have access to the tank, I would think that a simple homemade polishing system would be fine, since you can manually agitate the tank contents so the polishing system can capture any remaining gunk. May take a while, but you should do fine with frequent agitation amongst your other boat jobs.
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Old 11-06-2024, 13:08   #12
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Re: How clean should a diesel tank be?

The fuel systems for both Cat engines have Racor vacuum gauges. Both engines ran great until one wouldn't. Fortunately, we were close to a marina. One tank (218 gallons each) was crystal clear to the spotless bottom. The tank with the non-running engine had fuel that looked the color of Hershey's syrup. The vacuum gauge was pegged in the red while the other was normal but an hour before the engine quit, both vacuum gauges read comfortably in the green. The contaminated tank was pumped, rinsed by spraying diesel under pressure, pumped, repeated twice more. then refilled with clean fuel and a double dose of biocide. Not only was the filter plugged but the pick-up was also plugged. The other filter looked new. Both tanks were filled at the same place at the same time for many years but one was clean while the other was nasty.
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Old 11-06-2024, 15:49   #13
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Re: How clean should a diesel tank be?

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I'd say that is not unusual and pretty clean really. Mine were often opaque and not even clear like that due to growth in the fuel.
Come on guys...

What you are talking about should not be what you consider to be "normal" for a badly neglected 20+ year old tank, but what is required for reliable operation of a diesel engine. A diesel engine lives and dies by the cleanliness of its fuel supply. One single speck of dirt holding an injector open will disable most engines.

If there is visible contamination of the tank or the fuel it it that should be considered UNACCEPTABLE. The kind of crap that the OP shows in his photos will not be removed by a simple racor fuel circulating system.

If there is no fuel polishing service available to the OP locally, then the only answer is to completely empty the tank and clean it manually. Yes, it is a lot of work. So is a complete rebuild of an injection pump because it was contaminated, or replacing the injectors because the nozzels were blown apart by water droplets.

Fuel filters are there to solve unusual events, not to clean dirty fuel as a routine. If you keep the tank appropriately clean, you will never clog a fuel filter. My boat accumulated 8000 engine hours in two circumnavigations with no primary filter at all, just a water separator. I have since installed a single Racor primary. I change it every 400 hours just because it seems like the thing to do. It has never accumulated a drop of water, or shown any increase in pressure drop.
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Old 11-06-2024, 17:02   #14
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Re: How clean should a diesel tank be?

Interesting thread.

As above, on a whim, I opened a diesel tank on my boat, maybe 10-12 years old, and the inside looked much like the pics Pete7 posted.
It's a good thing the pickup tube was positioned some 2" above the bottom of the tank, otherwise I'd be sucking that crud into the engine.

I had two options, try to clean it up or get a new tank made.
I opted for a new tank.

I questioned how the tank got to be in such bad shape. and was informed I should think back on the diesel supply I got. Diesel in far flung places does not necessarily get pumped from a nice clean pump. It can arrive on the back of a truck and get pumped from every description of a container.
While a multitude of a filters are available these days to catch detritus before it enters your tank this was not always the case for me and I'm sure others.

Hard to believe anything could grow and even survive in such places like a diesel tank, but apparently that is the case I'm reliably informed.

Once bad stuff enters the tank by whatever means, it can multiply.
Though I'm familiar with biocides I have no background in these to validate their effectiveness.

To anyone contemplating buying a used boat, some fuel polishing should be considered as a " must have " option.
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Old 11-06-2024, 18:38   #15
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Re: How clean should a diesel tank be?

To the best of my understanding, water in the tank is the issue if the fuel came from a clean source. In both my boats the fuel fill is protected from the outside world. If by any chance my fill plug"O" fails, water still cannot reach the tank unless the boat is sinking.

On the vent side I run a Desiccant Dryer, to prevent moisture coming in from the vent.
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