Cleaning diesel fuel tank

[sanibel sailor]

I replaced my rusty old steel diesel tank with a new aluminum one about 12 years ago. It is only 32 gallons. I have been scrupulously keeping water out- keep tank full, buy fuel from reliable high volume sources, BioNor fuel additive, fresh o-ring on the fill periodically, check all fuel for water, high loop on vent hose, etc.
Due to extenuating circumstances, my boat had little use for three years. I didn’t think diesel went bad if dry, but apparently it does. Despite my efforts, my tank has become dirty.
I pumped all the fuel out of it and pulled the tank. It has a generous access hole in the top and only one baffle so it is all accessible.

The inside of the top is more of a soft cream colored stuff that brushes off easily, but the lower part in contact with fuel has a hard shiny black deposit on it. It is quite adherent. This stuff never would’ve been removed with “fuel polishing”.

It may not cause any problems but I am reluctant to just leave it especially since the tank is out. Any ideas about an easy way to clean it? I cannot get my hand in there to scrape it easily.

[Lepke]

Over time the fuel additives that kill the organisms dissipate. The hard deposits are the organisms waste. A good fuel conditioner will slowly dissolve them for the filter to catch. The filter media will turn black long before it's plugged. You should have a vacuum gauge to indicate condition. A good conditioner also helps a Racor type filter remove the water.

I use a fuel conditioner every fueling. In the old days when fuel was refined for performance we didn't need conditioner unless the fuel was going to set a long time.

[Rucksta]

MEKP will turn the hard encrusted black stuff to slime that will fall off the sides in sheets and flush as a greasy residue.

No elbow grease required just application.

Note the product warnings and use in a well ventilated area.

[Tsuru]

I agree that a biocide and stabilizer will extend the shelf life of diesel, but 3 years is a long time. Modern ULSD diesel and especially bio diesel are not as stable as the older straight distilled and blended diesel. I’m not sure I agree that older diesel was refined for performance, but there has always been a specification.
If it was me I’d fill or partially fill the tank with hot water and a small amount citrus detergent (industrial degreaser if you can get it) and let it set over night. Extra points for circulating the mix every couple hours. 1 or 2 treatments and your tank will be clean.
Good luck

[CMH]

Of the tank is out of the boat, how about pressure wash the interior?

[NorthCoastJoe]

Throw some rough gravel in it and shake well. Then rinse and do the MEKP route.

[Tin Tin]

I used acetone and sloshed it around for a while, then water pressure blasted with a right angle jet, wet and dry vac, then reverse vacuum to dry it out.

[Leadfree]

You are right re: fuel polishing, which I think is snake oil.

My experience was that I had to cut access ports (tanks had none), reach in and wipe.

After that Excursion Into Hell (tanks in-situ, in August, in Mississippi) I then installed a couple Filter Boss systems (have two engines) and I always try to filter fuel going in the tanks.

[sanibel sailor]

Took it to the car wash and washed the inside. Engine cleaner setting followed by high pressure rinse. Helped slightly.

Put a bit of gasoline in it and scrubbed with a stainless scrub pad on a stick. Decided to leave the “varnish” on the tank sides. Not perfect but close enough for the next decade.