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Thread: Finding an air leak in the fuel system.

  1. #1
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    Default Finding an air leak in the fuel system.

    Hi There.

    I have an old Yanmar SB12 single cylinder diesel that dies on me roughly 3-5 running hours after I bleed the fuel system. This has happened a few times, and i've been lucky to be able to sail to safety... i don't want it to happen again. Each time it dies, all that needs to be done is to bleed the primar, then secondary filter (never bled the high pressure lines) and she starts right up again for another few hours of run time.

    I am almost positive there is an air leak somewhere in the system. The fuel system is as follows: Standpipe -> primary filter (racore R12S) -> mechanical pump with priming bulb -> secondary filter -> high pressure injector pump -> injector -> return line to secondary filter.

    There was a note on the service manual cover from the previous owner that said "replace metal lines." That was the year the pulled the boat and it wasn't used again till I bought it last year. The metal lines have not been replaced, but there is no evidence fuel spraying from these lines during operation - why else would those lines need to be replaced??

    How can one find the location of an air leak?

    How do you test fuel filters for air leaks?

    many thanks!

  2. #2
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    Default Re: Finding an air leak in the fuel system.

    TYPO corrected:

    "...all that needs to be done is to bleed the primary, then secondary filter (never bled the high pressure lines) and she..."

  3. #3
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    Default Re: Finding an air leak in the fuel system.

    Inam, Likely your PO wanted to change ( or his surveyor) out the metal lines to hose as ABYC recommendations no longer support the metal lines. Remember that the fuel system from the tank to the pump is a suction system, it wont leak externally and tell you where the trouble is, you've got to do your due diligence and rework or check the entire system so you know it is working flawlessly...Tank PU tube on through to the fuel pump. Cheers, BT

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    Default Re: Finding an air leak in the fuel system.

    Go over the system and clean it thoroughly. Then run it for an hour and go over it again looking for any small weeps of fuel. When you find a small drop of fuel, it will help you to find your leak. I would look at some of the banjo fittings around the fuel pump. Last year I had a similar problem, it turned out to be the hand pump on the on-engine fuel filter bracket. The rubber diaphragm in the hand pump had a small crack. That let enough air in to shut the engine down although very little fuel leaked out. I found it by looking for the leak.

  5. #5
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    Default Re: Finding an air leak in the fuel system.

    The last chronic "air leak" I hunted down turned out not to be an air leak at all but loose fuel tank scale clogging the screened pickup pipe deep in the tank. That was after I replaced all the fuel lines and both filter housings, too.

    You have to go over the entire fuel system from tank pickup to injectors.
    Last edited by Bob Smalser; 06-14-2011 at 08:28 PM.

  6. #6
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    Default Re: Finding an air leak in the fuel system.

    Sure it's not just the opposite problem- starving for air? Could be creating a vacuum in the tank after a couple of hours and stopping the engine- I've never had it happen with a diesel but have twice with petrol engines. Easy to check- listen as you open the fuel tank after it's been running for a while- you'll hear air rush in. That or blockage in a filter- possibly in the tank. A friend of mine had an air leak that used to stop his ex army truck- always miles from home. He nearly went mad trying to find it. Turned out to be a tiny hole rubbed through a fuel line where it touched the chassis and couldn't be seen. It wasn't a problem until the tank was half empty and the engine was relying on the pump with no help from gravity. JayInOz

  7. #7
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    Default Re: Finding an air leak in the fuel system.

    So where the #$@^%$ were all of you guys on the other thread???

    http://forum.woodenboat.com/showthre...tarting-Issues

    Down around #19???

    You left me on my own!!!

    I was wingin' it!

    No help from anybody!!!!

    I thought this was supposed to be a community.

    I felt so alone...deserted...isolated...invisible...


  8. #8
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    Default Re: Finding an air leak in the fuel system.

    Are there bubbles in the fuel in the Racor. It's not normal practice to return the fuel to the secondary filter

  9. #9
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    Default Re: Finding an air leak in the fuel system.

    Hi Everyone,

    Thanks for the hints. I had posted a couple times on another thread and had been following that (for some reason they didn't send me email notifications about this one).

    As for the suggestions here: Hwyl - Its hard to tell if there are bubbles in the Racor, as there is not really a bleed screw on it, and when I bleed the whole system I pull fuel through the Racor first, lots comes out and its hard to determine if there were any bubbles. The return line to the secondary filter is how the engine was built, I have seen more diagrams bringing the fuel back to the tank, but... the return to the secondary filter seems to work for the early Yanmars. Other than mine apparently.

    JayInOz - Thats an interesting thought, and I checked for what you suggested but found no real evidence for air starvation - how would that produce bubbles from the bleed screws on the secondary filter though?

    Bob - I actually took the tank out last winter and cleaned it; we suspected bad fuel. I don't think there is anything clogging the standpipe but its pretty well solidly screwed in there, so will probably wait till i have tried replacing the fuel lines before I take the tank out again to get to the standpipe.

    I did find some evidence of leaking fuel around the secondary filter, so I am assuming there is a leak either in the hoses, from the banjo screws or from the secondary filter housing itself. I ordered a bunch of new copper washers to fit the screws and some new rubber lines. I am going to start there for now and let you know how it goes. As it is now I can still bleed the system and get in and out of the harbor, so i'am not too unhappy.

    thanks again.

  10. #10
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    Default Re: Finding an air leak in the fuel system.

    You can anneal the copper washers and reuse them.
    I suspect the Racor, they are not really designed to be on the suction side, too many parts. It is a tiny leak that takes that long to fail. Have you tried just pumping the squeeze bulb when it starts to falter?

  11. #11
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    Default Re: Finding an air leak in the fuel system.

    You may want try pressurizing the fuel tank, then you will get fuel weeping on the suction side if there is a leak.

  12. #12
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    Default Re: Finding an air leak in the fuel system.

    It is usually running fine, then goes within seconds. No time to get to the primer on the fuel pump.

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    Default Re: Finding an air leak in the fuel system.

    That is a clue, and it sounds like debris in the tank. It could be as simple as a piece of tape or something drifting around in there that obstructs the pick-up tube.
    When it is obstructed it will leak air into the system from any number of places. That air leak you will never find...
    As soon as the engine stops the debris usually drifts away from the pick-up (Suction is gone) and after a quick air bleed all is well until the next time.
    A simple fix/test for this is to remove the pick-up tube and insert into the business end a small coil spring or a bent piece of wire or paperclip leaving it sticking out a bit. That will prevent something from lying flat against the tube end and sealing it shut. A simple screen wrapped around the end will also work.

  14. #14
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    Default Re: Finding an air leak in the fuel system.

    Agreed, it's not a slow air leak

  15. #15
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    Default Re: Finding an air leak in the fuel system.

    huh.. Thanks guys. I guess I will take that damn thing out and attach a screen. How about the theory of sucking air while heeling over with a 1/4 tank of fuel (i.e. all fuel is at the far side of the tank)? It would need to cause a delayed reaction since it died after we were level again. Perhaps it takes a while for the air to move through the lines and choke the engine?

  16. #16
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    Default Re: Finding an air leak in the fuel system.

    Canoeyawl know a thousand times as much as me. But I have run out of fuel and I have had air in the system, there has always been a gradual slow down or a sputter, never a dead stop.


    The conditions you describe would produce both symptoms

  17. #17
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    Default Re: Finding an air leak in the fuel system.

    Quote Originally Posted by Inamist View Post
    huh.. Thanks guys. I guess I will take that damn thing out and attach a screen. How about the theory of sucking air while heeling over with a 1/4 tank of fuel (i.e. all fuel is at the far side of the tank)? It would need to cause a delayed reaction since it died after we were level again. Perhaps it takes a while for the air to move through the lines and choke the engine?
    Now, there is another clue.
    It would take as long to fail as it took the air bubble to reach the high pressure fuel pump, then it would be just like turning off the key. That could be several minutes on a little single cyl Yanmar with two filters and a squeeze bulb. That little thing can't use more than five gallons in a year!
    A good and proper marine diesel fuel tank will have a small "sump" with the pick-up submerged in that. That eliminates any possibility of sloshing causing an air problem. Ideally the tank would be shaped like a funnel, but that is impractical. I have fabricated a few tanks and always weld on a small sump to prevent sloshing air into the line. (about 1 cup sump for the pick-up tube on a small tank and about 1 quart for large tanks with a drain so to both trap and drain water)
    Last edited by Canoeyawl; 06-23-2011 at 08:07 PM. Reason: typo

  18. #18
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    Default Re: Finding an air leak in the fuel system.

    I traced an air leak to a 2-way fuel valve.....set up so you could have an aux tank to switch to. I had taped over that other aperture just to keep the dust out......turns out that as the tape dried and failed, it let air into the system.....the valve was set so that that side was supposedly closed. Moeller brand brass valve.

  19. #19
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    Default Re: Finding an air leak in the fuel system.

    Quote Originally Posted by Canoeyawl View Post
    Now, there is another clue.
    It would take as long to fail as it took the air bubble to reach the high pressure fuel pump, then it would be just like turning off the key. That could be several minutes on a little single cyl Yanmar with two filters and a squeeze bulb. That little thing can't use more than five gallons in a year!
    A good and proper marine diesel fuel tank will have a small "sump" with the pick-up submerged in that. That eliminates any possibility of sloshing causing an air problem. Ideally the tank would be shaped like a funnel, but that is impractical. I have fabricated a few tanks and always weld on a small sump to prevent sloshing air into the line. (about 1 cup sump for the pick-up tube on a small tank and about 1 quart for large tanks with a drain so to both trap and drain water)
    Ya, my tank has no sump. Just checked. It's a bit of a custom build and its essentially a long rectangle much wider than taller. The pick-up tube is off to one side. Which would add to the problems of fuel sloshing over to one side with any heel at all - or even going over a wake/swell while motoring.

  20. #20
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    Default Re: Finding an air leak in the fuel system.

    Quote Originally Posted by Inamist View Post
    Ya, my tank has no sump. Just checked. It's a bit of a custom build and its essentially a long rectangle much wider than taller. The pick-up tube is off to one side. Which would add to the problems of fuel sloshing over to one side with any heel at all - or even going over a wake/swell while motoring.
    This type of tank is fine for a gasoline engine (except for the drain problem) and is common. But with a diesel it is bad news.

  21. #21
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    Default Re: Finding an air leak in the fuel system.

    The space for my sailboat fuel tank required it to be 12" high by 24" wide by 20" deep (fore n aft). This plastic tank's suction line came down from the top and ended about 1/2" above the bottom of the tank. When I first installed it I could see, even at the dock, that sloshing was going to be a problem. And in the case that I might want to use the engine to motor-sail upwind off a lee shore in seas, in a pinch, the engine would not be getting fuel at a critical time.

    So, I installed an external sump below the existing tank. Now I can run the tank virtually dry, with a heal angle up to 45-degrees, before sucking air. The sump also has a drain at it's low point for purging water. It is just an aluminum cube with inlet and outlet threaded holes. The suction line comes down from the top and stops a couple inches from the bottom so that sludge and water can collect there. From the sump the fuel goes up to a racor with water bowl. Here's what I came up with:

    [the tank is held down and contained by devices not shown here, but which allow it to be removed relatively easily]




  22. #22
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    Default Re: Finding an air leak in the fuel system.

    That is an elegant solution...

  23. #23
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    Default Re: Finding an air leak in the fuel system.

    Quote Originally Posted by KAIROS View Post
    The space for my sailboat fuel tank required it to be 12" high by 24" wide by 20" deep (fore n aft). This plastic tank's suction line came down from the top and ended about 1/2" above the bottom of the tank. When I first installed it I could see, even at the dock, that sloshing was going to be a problem. And in the case that I might want to use the engine to motor-sail upwind off a lee shore in seas, in a pinch, the engine would not be getting fuel at a critical time.

    So, I installed an external sump below the existing tank. Now I can run the tank virtually dry, with a heal angle up to 45-degrees, before sucking air. The sump also has a drain at it's low point for purging water. It is just an aluminum cube with inlet and outlet threaded holes. The suction line comes down from the top and stops a couple inches from the bottom so that sludge and water can collect there. From the sump the fuel goes up to a racor with water bowl. Here's what I came up with:

    [the tank is held down and contained by devices not shown here, but which allow it to be removed relatively easily]
    Apologies for resurrecting a rather old thread, but I have been having an air-in-fuel-line problem that is driving me up the wall. The engine is new, professionally installed about 2 years ago; a new secondary filter/separator was installed at the same time so everything from there to the engine is new. The old tank was a pain in lots of ways and I put a new plastic one in around March this year and seem to have spent the rest of the time since then trying to solve the air leak problem. I now have a single hose from the tank connector to the secondary pump (no taps, suction bulbs or whatever to break the line). I've fitted semi-transparent hose to try to keep an eye on what is going on. The tank has a dip tube that goes to about half an inch from the bottom of the tank. Whether running the engine on not, a little air bubble forms at the top of the hose - that is, where it connects to the tank, which is the highest point, and then gets bigger as time passes.

    I cannot find any weeping anywhere to suggest a leak. My suspicion is that the leak is occurring at the point where the hose connects to the tank: the take off from the tank is only plastic and I over tightened the hose clip which wasn't perfectly round and deformed the tank connector a little. I've now found a better clip but the damage is done. The take off seems to be molded into the inspection hatch of the tank so I'm not sure that I can replace it, I'd have to drill a hole through the cover and try to get another dip tube (and I can't find anyone in UK selling them). As the leak (if it is where I suspect) means there is a bubble of air always present at the point of air entry, it never weeps. The bubble doesn't prevent fuel flowing but as it gets bigger it gets more worrying, and I disconnect the tube at the filter and bleed it out regularly. It's all very irritating.

    The old tank had the take off at the bottom and worked ok for 40 years! I only replaced it because it was small, awkward to fill, and very difficult to work out how much fuel it had in it.

    Looking at your arrangement, I assume the fuel comes from the tank from the 2 take off points at the bottom, one on either side: done like this to compensate for sloshing? If that is so, did you fit these take offs yourself? I am considering just fitting a take off like this and leading it straight to the filter as per the old arrangement. The tank is above the engine so the whole syphoning thing doesn't add much in the way of safety benefits that I can see, as unless any break in the line was high up it would drain the tank anyway.

    Thanks in anticipation.

  24. #24
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    Default Re: Finding an air leak in the fuel system.

    I'm facing this just now. It should be easy as the Racor was going dry. My mechanic fixed things with a new valve on the tank and it ran great for a bit then died, dry racor again. Before having him come out, my mechanic suggest that I temporarily put some clear plastic hose from tank to Racor and then from Racor to the fuel pump to see if any bubbles.

    That will be Sunday's fun.

  25. #25
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    Default Re: Finding an air leak in the fuel system.

    The transition from vacume to pressure is the fuel lift pump.

    It the lift pump leaks , cant hold the vacume.....diesel will fall back down the intake line into the tank. Service the lift pump ....normally a new diaphram or O rings depending on the pump.

    Obviously anything before the lift pump...filters, valves should be checked. The scientific way is a vacume gauge ...but visual inspection normally spots trouble . Its almost always a poorly fit fuel filter or worn lift pump.

    It there is a leak after the lift pump..on the pressure side ...you will smell the diesel leak.

  26. #26
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    Default Re: Finding an air leak in the fuel system.

    I did have a suction/air leak in my fuel shutoff valve a couple of years ago. It was a three-way valve - off, main tank, spare tank. The spare tank hose was a 3" stub with a piece of tape over the end to keep dirt out (never used a spare tank). In the 'main tank' position, air leaked into the fuel line via the taped hose end, causing the engine to starve. So the valve was faulty. Replaced it.

    You might try emptying your fuel lines, putting soapy water on hoses and fittings, and forcing some air into the line (may damage filters). Look for bubbles.

    Jack
    Last edited by KAIROS; 09-12-2014 at 11:42 PM.

  27. #27
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    Default Re: Finding an air leak in the fuel system.

    What's causing the problem is air in the fuel. Air is compressible, unlike a liquid like fuel. Which makes liquid a very direct transmission system for force. Go "Bang" on one end of a pipe full of liquid, and the same "bang!" will be trasmitted to the other end. If a bubble of air is introduced into the line, it acts as a spring. Go "Bang!" on one end, and the air compresses, absorbing the force, greatly modulating the "bang!" at the other end.
    Unfortunately, diesel injectors have powerful spring sets that allow them only to inject only when that stout "Bang!" is transmitted to the injector. Introduce a bubble of air (a compressible spring) into the system, and that "Bang!" gets modulated to a "bip..." not enough pressure to cause the injector to open, and spray fuel mist into the combustion chamber.
    This condition is called "air lock" in polite terminology.
    Where's the air coming from?
    Most likely from plumbing on the suction side leaking.
    In my experience (and I've had a lot), the best way to ferreting this out is to put moderate (5psi) positive pressure on the suction side, and work the hoses, tubes, and connections with soapy water which is the best way of detecting outward leaks.
    Is there maybe a transient block on the suction pickup that's causing the problem. Yeah, I guess. Kind of like it is POSSIBLE to get pregnant from sitting on a toilet seat. (But very uncomfortable!)
    Both vacuum cleaners (on the blow, not suck side) and inflatable boat air pumps are good ways of introducing high volume, low pressure air into a system, without danger of exploding the whole damn thing...
    Did I mention? When you soap the outsides of the hoses and fittings, air leaks show up a bubbles. My own personal favorite soap is "Pustefix" soap bubble-blowing liquid. Lots of film strength!

  28. #28
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    Default Re: Finding an air leak in the fuel system.

    Quote Originally Posted by Canoeyawl View Post
    That is a clue, and it sounds like debris in the tank. It could be as simple as a piece of tape or something drifting around in there that obstructs the pick-up tube.
    When it is obstructed it will leak air into the system from any number of places. That air leak you will never find...
    As soon as the engine stops the debris usually drifts away from the pick-up (Suction is gone) and after a quick air bleed all is well until the next time.
    A simple fix/test for this is to remove the pick-up tube and insert into the business end a small coil spring or a bent piece of wire or paperclip leaving it sticking out a bit. That will prevent something from lying flat against the tube end and sealing it shut. A simple screen wrapped around the end will also work.
    I had just that problem and found a small bit of plastic label in the tank ??? I removed that of course but also drill a couple of other holes in the pick up line where it goes parallel to the bottom of the tank.
    Try to work out what the marketing guy wants you to do then do precisely the opposite.

  29. #29
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    Default Re: Finding an air leak in the fuel system.

    Quote Originally Posted by seo View Post
    What's causing the problem is air in the fuel. Air is compressible, unlike a liquid like fuel. Which makes liquid a very direct transmission system for force. Go "Bang" on one end of a pipe full of liquid, and the same "bang!" will be trasmitted to the other end. If a bubble of air is introduced into the line, it acts as a spring. Go "Bang!" on one end, and the air compresses, absorbing the force, greatly modulating the "bang!" at the other end.
    Unfortunately, diesel injectors have powerful spring sets that allow them only to inject only when that stout "Bang!" is transmitted to the injector. Introduce a bubble of air (a compressible spring) into the system, and that "Bang!" gets modulated to a "bip..." not enough pressure to cause the injector to open, and spray fuel mist into the combustion chamber.
    This condition is called "air lock" in polite terminology.
    Where's the air coming from?
    Most likely from plumbing on the suction side leaking.
    In my experience (and I've had a lot), the best way to ferreting this out is to put moderate (5psi) positive pressure on the suction side, and work the hoses, tubes, and connections with soapy water which is the best way of detecting outward leaks.
    Is there maybe a transient block on the suction pickup that's causing the problem. Yeah, I guess. Kind of like it is POSSIBLE to get pregnant from sitting on a toilet seat. (But very uncomfortable!)
    Both vacuum cleaners (on the blow, not suck side) and inflatable boat air pumps are good ways of introducing high volume, low pressure air into a system, without danger of exploding the whole damn thing...
    Did I mention? When you soap the outsides of the hoses and fittings, air leaks show up a bubbles. My own personal favorite soap is "Pustefix" soap bubble-blowing liquid. Lots of film strength!
    Food colouring too, a bit of bright green is easy to see.
    Try to work out what the marketing guy wants you to do then do precisely the opposite.

  30. #30
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    Default Re: Finding an air leak in the fuel system.

    lots of bubbles..... lots of troubles

  31. #31
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    Default Re: Finding an air leak in the fuel system.

    If you are using a racor for your secondary filter I have seen the check foul. On the inlet to racor housings they use a small ball check, if this sticks it acts like a plugged filter. It is pretty easy to check if you have a gravity fed day tank, just isolate the filter pull the top then crack the inlet, no fuel most likely stuck check good flow it is something else.
    Schooner Sassafrass Rebuild Blog Web Album

  32. #32
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    Default Re: Finding an air leak in the fuel system.

    Flexible Fuel hoses are always constructed with a liner, and sometimes the liner will separate from the outer cover and collapse internally on the suction side. If this happens, air will find its way in from somewhere. When the engine quits, the hose relaxes and a pressure test will not reveal any problem. Be certain to use a vacuum rated hose, and replace flexible fuel lines at regular service intervals, like maybe every five or seven years. Along with belts and coolant hoses, and that hateful little "hose" at the stuffing box. (I like five because it's easy to remember, 2005, 2010, etc)

  33. #33
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    Default Re: Finding an air leak in the fuel system.

    Thanks all for the suggestions. NB that everything I have is max 2 years old: the tank is about 5 months old (and the pick up tube does not have a filter on the end, and I vacuum cleaned the tank before installing so there are no random bits of plastic floating around in there); the fuel line to the 2ary filter is literally a few weeks old and is the transparent bit, the actual filter in the 2ary filter/strainer was replaced a few weeks ago, and everything else was new when the engine was installed. The fuel pump has only done about 50 hours from new: I know that doesn't absolutely preclude something like a broken diaphragm, but it really shouldn't have such a thing.

    How do people feel about changing to an electric pump? If placed between tank and 2ary filter, it limits the vacuum part to one run of fuel tube and everything else is pressurised. According to some things I have read, it would also mean that I would not need to get a mouthful of diesel every time I bleed the system.

    If I did go electric, would it be best to then bypass the mechanical lift pump and take the electric pump output straight to the fine filter?

  34. #34
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    Default Re: Finding an air leak in the fuel system.

    With only fifty hours your fuel pump is brand new. You never know , but its not likely to be the problem.

    Is you fuel tank air vent line clear ? The whole tank could be undr vacume if its plugged or closed.

    is your fuel return line clear ? A closed return can cause Back pressure in the injector pump and starve it.

    i dont like electric fuel lift pumps. They work but add complexity to a simple system .

  35. #35
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    Default Re: Finding an air leak in the fuel system.

    First, to correct what I wrote in #27. The pressure test is done with the fuel lines empty of fuel. You then pressurize the lines with air, and then soap the fittings and the hoses and pipes, looking for air leaking out. A simple was to pressurize is to take the tank vent line off its overboard fitting, and put air in there. As stated before DON"T use an air compressor. 3 psi is plenty.
    Adding an electric pump is not going to eliminate the air leak which is causing the problem. Given how new all the components are, it's possible that there's a defective component in there somewhere. I've never seen the problem described in #32, but it is possible. If the pressure test of the system doesn't discover the leak, Then I suppose I'd take my handy Mitey Vac pump and put some suction on the line, say 2 psi of suction. If the gauge slowly droops down to zero, you've got a leak.
    It would be very surprising if the problem is something like a blocked check valve or blocked pick-up tube, because you've already stated the problem, which is that air is getting into the lines. If the system is properly built, if there's a block (such as you forgot to open the fuel shut-off valve on the tank...) the engine will stop from fuel starvation, but there won't be any air in the lines, because the system doesn't leak.
    A small note on filter placement. With a fuel tank with a top suction, which is required these days, the best place for the filter is with the top of the filter even with the level of the tank's bottom, if that's possible. That way, as long as the fuel feed tube from the tank to the filter is flooded, you have a siphon, and positive pressure in the system. To change the filter element, turn off the fuel shutoff, open the drain on the filter bottom, then open the top and change the filter. Don't put the top cover on. Shut the drain, and open the fuel shut off. Fuel will siphon in from the tank, and flood the filter. Close the shut-off when the filter element is immersed, and let it soak for a minute, so any bubbles have a chance to escape. Then put on the filter cover, but loose enough to leak. Have a catch pan under the filter. Open the shutoff valve, and when fuel starts leaking out the top, tighten the cover. Voila! Fully flooded filter, no air.
    If your filter must be mounted higher than the tank bottom, then the level of fuel in the tank has to be higher than the top of the filter for you to get siphon pressure. The higher the "Head" the more pressure, so you're less likely to have trouble if you change filters with the tank nearly full than nearly empty.
    If you have to drain your fuel system for some reason (like changing a hose) you will lose siphon pressure. The easiest way to flood the system is to take off the tank vent hose from the overboard fitting, and blow into it. You only need 1 psi of pressure to push fuel up 27".
    A SIDE NOTE

    I'm assuming that your fuel filter is below the level of the fuel in the tank, and so you have siphon pressure at the filter, and also that you have a top feed system, with a dip tube going from the top of the tank down to near the bottom. The interesting thing is that when the lines are flooded you will have positive pressure on all parts of the system that are BELOW the fuel level, and negative pressure on all parts ABOVE the fuel level. So, the place to look for that air leak is on the parts of the system that are above the fuel level.

    This is why fuel tanks in the bilge, below the engine, are such an abomination. You have no siphon pressure to flood filters, etc, and so must have a priming pump to create pressure mechanically.

    SIDE NOTE #2

    Fuel tanks are supposed to be filled with...FUEL!! Not water. But somehow, water does get into fuel tanks. Because water is heavier than fuel, it settles to the bottom of the tank. Because your fuel suction dip tube is a little bit clear of the bottom of the tank, the water can accumulate down there before it gets sucked up into the filters. Or, you get into a rough sea with a half-full tank, and the fuel sloshing around stirs up any junk on the bottom, and the water, and it gets sucked up into the filters and clogs them, the engine shuts down just as everybody is scared and feeling seasick. What FUN!
    A further problem is that bacterial slime lives at the fuel/water interface in a tank. Slime clogs filters, and is quite corrosive AND abrasive. Just what you don't want in your diesel fuel injection system, with its incredibly close tolerances. There are two ways of preventing and getting rid of bacterial slime: a) poison the little buggers. The generally accepted additive that does this is called Biobor, which is essentially borax, which does kill slime very effectively. b) Get rid of the water at the bottom of your tanks. In the good old days of fuel tanks with bottom feeds, this was pretty easy to do, because any water that got into the tank would go out into feed line, and (hopefully) get filtered out in the separator. With a top-feed system with a dip tube, the dip tube is always held up a little bit off the bottom, sometimes as much as an inch.
    The only way to get rid of this water is having a system for sucking the water off the bottom of the tank, commonly known as "stripping water bottoms." To do this you need to be able to get at the top of your tank. The way I do this on my boat is to remove the fuel return fitting on the tank, which almost all diesel systems have. Then I use a very small (1/8") copper tube which is a loose fit in the tank fitting, and is soft so that I can bend it around and steer it into the starboard forward corner of the tank, which is the low point in the tank because I move some weight to the starboard side to make her list that way, and let her sit for a few hours on a calm day, so all the water can settle to the low point. Then I connect a PAR Handi-boy pump to the copper line, and suck out whatever is in the low corner, into a glass jar so I can see what I've got. As long as water is coming out, I keep pumping. Any black slimy stuff I keep pumping. When I hit clear fuel, the job is done. The tube is stored on top of the tank, so it's always available. The watery, slimy, fuel gets mixed with sawdust and used for lighting brush fires in my woodlot.
    How often? At least once a year, although I once sailed as engineer on a tug that had eight fuel tanks, for a total capacity of 28,000 gallons, and burned about 1,000 gallons a day. Fuel would be moved from bunker tanks (which got stripped every third day) to the engine room deep tanks, which got stripped every day. From there the fuel went into a 400 gallon day tank. Bigger tugs have centrifuges to get rid of water, which is great, but not cheap.
    If you plan ahead, you can stay ahead of the slimy little buggers.
    Last edited by seo; 09-16-2014 at 08:57 PM.

  36. #36
    Join Date
    Jul 2013
    Posts
    6

    Default Re: Finding an air leak in the fuel system.

    Thanks again to everyone for continuing responses. The boat is out of the water at the moment so a good opportunity to fix this - hopefully - once and for all.

    > I'm assuming that your fuel filter is below the level of the fuel in the tank

    Yes, it is located almost directly under the tank, so is lower than the tank even if it's fairly empty (though it's about three quarters full)

    > also that you have a top feed system, with a dip tube going from the top of the tank down to near the bottom

    Yes I have

    > when the lines are flooded you will have positive pressure on all parts of the system that are BELOW the fuel level

    that is pretty much the whole system, as the engine is lower than the filter

    > and negative pressure on all parts ABOVE the fuel level

    which is only the top of the fuel line and the top of the dip tube. This reinforces my opinion that it is the join between the fuel line and the dip tube that is leaking. There is no fuel tap at all: I did fit one but it did seem to weep fuel slightly so I took it away as a source of possible air ingress.

    A propos the water, I don't think there is any in the tank - yet! Sounds like good advice to do the regular removal of rubbish though and I will do that; sounds like using the slime killing additive might be a good idea too as a preventative measure. I don't use much fuel so even if it's expensive for what it is it won't cost all that much.

    I'm off down there next week and will try as much of the above advice out as I can. I suspect my best long term bet will be to replace the plastic dip tube and fittings with a brass one, though I haven't seen these sold anywhere in UK.

  37. #37
    Join Date
    Sep 2007
    Location
    Bass Harbor, ME
    Posts
    1,779

    Default Re: Finding an air leak in the fuel system.

    Your problem sounds very much like the problem I had last year. My engine would start, run for maybe 15 seconds then quit. Vigorous pumping of the hand priming pump and then cranking with the throttle wide open would get it started and running fine. The problem only occurred when the engine had been shut down for more than a couple of hours. I finally decided to replace the fuel pickup tube in the tank thinking that the 26 year old tube might be cracked. As part of that job, I had to remove the hose barb that was screwed into the top of the pickup tube. When I did that I discovered my air leak, The thread sealer on the hose barbs 1/4" pipe threads had dried out and was letting air in. When my tank wasn't completely full, the air leak let the fuel level in the pickup tube gradually drop down to the level of fuel in the tank leaving an air bubble in the top of the pickup tube. When that air bubble got to the injector pump, the engine stopped. A small bit of diesel rated pipe dope on the threads of the hose barb eliminated the problem. Incidentally, the old pickup tube was fine, so now I have a spare.

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