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Old 02-16-2021, 03:29 PM   #1
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Diesel tank leak

I have a strange situation with a diesel tank leaking.
Diesel in the sump. Oh no. Cleaned it up. Seems to be at bottom of 200 gal steel tank. Emptied, washed, emptied. Air test. 4 lb 2 days. Hmmm no leaking. Added 2 8 inch inspection holes cleaned again. Air test. No leaking. Added fuel. No leak. Came back 2 days later gallon of fuel on the floor. Emptied tank washed, air test
No leaking! HELP!
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Old 02-16-2021, 03:57 PM   #2
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Could a fuel hose be weeping? Put some paper towels around where you suspect a leak and they will show the leaks. Put them all around and see which ones get wet.
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Old 02-16-2021, 04:23 PM   #3
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I've had leaks where a fuel line or hose wore a hole where it went thru a clamp hanger. Engine vibration over time wore a small hole. Since the hole was caused by the hanger, the hole was against it and didn't always leak. Eventually I reasoned that it leaked at night when everything was cooled down. The fuel line contracted slightly and opened the hole.
Best way to fine it is with air pressure in the line and listen for the leak.
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Old 02-16-2021, 05:37 PM   #4
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If there is sludge in the bottom, it may pass a pressure test but not a leak test
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Old 02-16-2021, 05:47 PM   #5
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Sounds like a syphon going on with a fuel line.

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Old 02-16-2021, 06:25 PM   #6
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Try capping all of the fuel tank outlets, and then pressure test again. If that works okay try pressure testing each of the fuel lines and filters. Thake it one step at a time until you find the leak.
Dave is correct WHITE paper towels laced out everywhere will help spot the leak.
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Old 02-16-2021, 10:27 PM   #7
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I wish it was as simple as that. All fuel lines are closed. Air vent closed, fill cap closed. One day it leaks, the next day it doesnít. Paper towels are good today, tomorrow I may need a bucket!
I left 5 # air pressure on it today. Iíll check it again tomorrow. It has baffled me. The leak comes from the bottom of the tank. Or the back and then runs to the bottom front. Next will be to fill it with water and see if that leaks out. Thank you all for your input. Please donít stop sending ideas...Click image for larger version

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Old 02-16-2021, 11:08 PM   #8
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You said you cleaned the tank out several times so I don’t think it is a layer of crap on the bottom of the tank plugging up some pinholes and they leak intermittently, but that would be my guess with the latest info you have provided. If the tank leaks when all the fuel hoses turned off then you have a leaking tank. The solution is to either replace the tank or try one of the tank sealing products and see if you can find one that works. Who knows, I have read posts that said the sealing products work and others that say they don’t. The sure fix is a new tank, but expensive. How old is the tank? Good luck.
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Old 02-17-2021, 04:35 PM   #9
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Can the tank be removed without tearing the boat apart?
The pics look like the tank maybe coated with something.
If they are S.S. then no coating is needed.
If they are coated it may be hiding further problems.
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Old 02-17-2021, 05:58 PM   #10
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Pressure testing an empty tank with air is problematic. The air under pressure is partially compressed, and there is a lot of volume, it would have to be a big leak to show up as a reduction of pressure. Also, pressure reading will vary a bit with temperature changes.

When we pressure tested with air, we would hold the pressure and "snoop test" (dilute soap solution) to look for bubbles. Not practical in a coated, installed tank.

Since you have access panels, I'd look into sealing the inside. Unfortunately I am no help on the best method.
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Old 02-17-2021, 06:20 PM   #11
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Fiberglass fuel tanks are built of numerous layers of fiber and resin. A leak in any one area does little to indicate where the leak starts. If the glass is delaminating the fuel can trickle a long was before it escapes the tank. Although your tanks look sound I suspect they are delaminating. Please tell me the dark grey and light blue colors are a trick of lighting. If not, they are saturated and ready for replacement

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Old 02-17-2021, 07:09 PM   #12
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No, itís take out the engines or through the side. Air test again today
9:30 am 5 pounds 3 pm 5 pounds.
Tomorrow Iíll try a vacuum test.
Colors are light shading.
After vacuum test, Iíll try colored water test. At my wits end!
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Old 02-17-2021, 07:14 PM   #13
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Carful with a vacuum test . You may make a portable tank out of it.
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Old 02-17-2021, 07:17 PM   #14
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Quote:
Originally Posted by Arc View Post
Carful with a vacuum test . You may make a portable tank out of it.


Yeah, my vessel engineer said 1 pound should be enough.Click image for larger version

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I lowered the pressure this afternoon to two pounds in hopes too much pressure is sealing the leak up
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Old 02-19-2021, 09:32 AM   #15
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When you air test it is there fuel in the tank or are you hoping to push air out of the hole? Just wondering thanks and good luck
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Old 02-19-2021, 05:56 PM   #16
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Air test was just air. Today I put in 80 gallons of fluorescent water and 4 pounds of air. Tomorrow we will see.
I had an old welder tell me that he used to work in vessels that x rays could not find a leak in, but diesel could! Sooner or later Iím going to figure this out. Through friends daughter who is a coatings specialist for the navy, I hopefully figure out what to coat the inside of the tank with......time will tell
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Old 02-19-2021, 07:07 PM   #17
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Quote:
Originally Posted by fgarriso View Post
Dave is correct WHITE paper towels laced out everywhere will help spot the leak.
I actually find that the brown paper towels, the cheap gas station kind, show leaks better. They change color from light-tan to dark brown with a drop.

Wet white somehow seems to look like dry white. At least to me.
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Old 02-19-2021, 08:07 PM   #18
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I have found the blue Scot or similar blue towels show even a small bit of wetness very distinctly. The white not so much.
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Old 02-20-2021, 02:13 PM   #19
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If removing the tanks is out of the question then your only solution is coating the inside. That will require removing all lines, vents, and valves. Then install brass plugs everywhere . You will need the brass plugs installed so the sealer coating won't seal up the ports.

https://www.kbs-coatings.com/tank-sealers.html
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Old 02-20-2021, 02:33 PM   #20
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After looking at the pics again the tanks appear to be fiberglass. If that is correct do to delamination issues with fiberglass the leaks could be coming from anywhere. The only solutions is cutting the hull sides out to remove the tanks, and then reglassing the hull. Or, sealing the the tanks from the inside. Pressure testing a fiberglass tank may not work do to the pressure sealing up the laminations
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