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Go Back   SailNet Community > On Board > Gear & Maintenance > Engines > Diesel
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Diesel This is a new forum dedicated to diesel engines and their applicable accessories.


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Old 10-30-2009
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Pressure testing a fuel tank / system

I think I may have a small fuel leak. When manually pumping the bilge, I get a bit of a sheen of diesel fuel. Granted that it could be from a previous owner's spill as a tablespoon of diesel will create a large rainbow but I would like to make sure that my aluminum tank that sits in the bilge is not leaking. I understand that a method of testing for leaks is to pump 3 psi into the tank and let sit for about 4 hours to see if there is any loss of pressure. I would like some advice upon the best tools to buy for this job. Or should I turn this job over to my diesel mechanic? Which will cost more - the tool or the mechanic? I have a shop air compressor. What else is needed?

Tod
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Old 10-30-2009
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The key to doing a pressure test is in how you seal the tank and how you fill it. I have not pressure tested one in a boat but have done many automotive tanks at this point. I always disconnect all of the lines and plug them(you can usually do that by sticking the correct size bolt in and using a hose clamp. I have found that you can usually put one fuel line around the end of the air valve and clamp it there. I have seen people who have much slicker setups where they have a valve, gauge and hose barb/pipe fitting all in one.

If you fuel tank is accessible, once you get it up to pressure, put some dish soap in a squirt bottle and hit all of the possible leaky areas. Any bubbles that you see indicate a leak.
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Old 10-30-2009
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I've done a bunch of aluminum tanks in the shop, as we have to certify them before installing 'em. I've got a test rig with a ball valve and a pressure guage plumed into a tee. Then it's just a matter of bushing it to the fitting you're using. Cap the rest with pipe plugs and you're all set (assuming you can get a wrench to them of course!!). For us, they have to hold pressure for 24 hours and the soapy water trick is the best I've found for pinholes too. Don't worry about the pressure in your shop compressor - the volume of the tank is so high that it'll come up to a few psi (we use 5) slowly enough to watch and close the valve.

From what I've seen from refits, leaks are usually from fittings or rotten/chafed hose most of the time, not usually from welds. When the tanks is filled right to the gills, a bit leaks out till the fluid level is down the tank a little - that might explain why there's so little but, like you say, it doesn't take much.

Stainless water tanks will get pinholes but I personally haven't seen it with diesel (for whatever that's worth). I've never had to replace an aluminum tank for anything.
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Last edited by Deadeye; 10-30-2009 at 06:34 PM.
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Old 11-02-2009
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Thanks for the responses. I have 4 different hoses entering the top: 1 large fill hose, 2 hoses that are the same size and 1 slightly larger. From what I gather, it seems to make sense to buy a small amount of these hose sizes to attach to the barbs so I can plug them with bolts or plugs of solid rubber, secured with hose clamps. 1 of the hoses should go to a "T" with a guage attached and then to a valve band then to my air compressor. I should pressurize to 3 to 5 psi and let sit for a day or 2 to see if there's a drop in pressure. Since I can't see all around the tank, I won't be able to see if there are any leaks. However, if the pressure goes down, I can be assured that there is a leak somewhere and I should pull the tank. Will I be risking catestrophically enlarging the hole and losing 45 gallons of diesel in the bilge with 5 psi? I gather that it will take a bit of time to get up to 5 psi. Are my assumptions correct? Tod
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Old 11-02-2009
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DO NOT pressure test for leaks when the tank is full of diesel. You are asking for a real mess. If you need to find out right away drain or pump the tank dry before you put pressure on it. Air will find the leak more quickly, and much more neatly, than diesel will.

I would clean the bilge and engine space floor to eliminate any existing fuel; then see if the sheen comes back. If so, from where? the engine fuel connections and lines are more likely than a leaking tank. If it is the tank it will stop leaking when the fuel level drops below the hole in the tank. that should help you find the leak when the tank is empty and you can pressure test it.
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Old 11-02-2009
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Sorry, I should have mentioned that first time around:
Test the tank empty.
Before deciding it's the tank, be sure to check the fittings on top, including the temporary plugs, with soapy water.

It takes only a couple minutes in our shop to pressurize a 100 gallon tank from a 125 psi system, but our system is big enough that the compressor doesn't have to cycle either.

As for getting at the leak - don't cross that bridge until you need to, lol.
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Old 11-02-2009
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You have to REALLY careful with the pressure on square tanks AS when they say 3 PSI on the tag they mean IT


I test pressure vessels that have steam jackets and there always filled with water and bought to test pressure using a hand pump
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Old 11-03-2009
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Ok, THANKS! I guess you guys saved me from creating a real mess and an expensive one!

As far as any other likely sources; the pan under the engine is clean and dry, no mess on engine; no obvious leaks at Racor.

The bilge is very deep at the back. I have 2 electric bilge pumps. Their pickup is under the fuel tank or slightly forward of it, not the lowest point. The manual bilge pump's pickup IS at the lowest point. It is only from this pump that I get the sheen. The fuel tank sits in the bilge on timbers made out of ?? to keep it up out of the water. Process of elimination seems to point to there being a pinhole leak in the tank or possibly in the fill hose, hose to Racor, vent line (doubtful), or resulting possibly from inadvertant spillage when changing filters. Or maybe around since before I owned boat since it seems hard to readicate with throwing cleaners down the bilge.
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Old 11-03-2009
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how old is the tank? I would not use any pressure. try to lift the tank and see if the wood supports are soaked with fuel. (bet they are) you have pin holes in the tank. Most if not all boat tanks do not have any fittings at the bottom. no one ever wants to replace a tank, I didn't either, but when mine went we changed it out in in few hours. I can only hope yours is removable. good luck!
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Old 11-04-2009
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The tank is original to the boat - built in 1987. I don't expect fittings at the bottom but fear pinhole leak somewhere. The tank is probably removeable without too much destructive activity. One would have to remove the holding tank first though. Just a major hassle. You are right, not something I would like to do...
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