Results 1 to 22 of 22

Thread: Finding an air leak in the fuel system.

  1. #1
    Join Date
    Apr 2011
    Location
    Woods Hole, MA
    Posts
    16

    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
    Join Date
    Apr 2011
    Location
    Woods Hole, MA
    Posts
    16

    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
    Join Date
    Dec 2008
    Location
    Bay Area, Northern California
    Posts
    618

    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

  4. #4
    Join Date
    Sep 2007
    Location
    Bass Harbor, ME
    Posts
    1,690

    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
    Join Date
    Jun 2003
    Location
    Seabeck, WA
    Posts
    11,012

    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 07:28 PM.

  6. #6
    Join Date
    Nov 2009
    Location
    Gulgong. Central west N.S.W. Australia
    Posts
    1,006

    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
    Join Date
    Mar 2008
    Location
    Long Beach, CA
    Posts
    2,586

    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
    Join Date
    Jan 2003
    Location
    Southern Maine
    Posts
    16,714

    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
    Join Date
    Apr 2011
    Location
    Woods Hole, MA
    Posts
    16

    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
    Join Date
    Jun 2003
    Location
    Central Coast, Ca
    Posts
    9,116

    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
    Join Date
    Nov 2009
    Location
    Coastal NC
    Posts
    87

    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
    Join Date
    Apr 2011
    Location
    Woods Hole, MA
    Posts
    16

    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.

  13. #13
    Join Date
    Jun 2003
    Location
    Central Coast, Ca
    Posts
    9,116

    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
    Join Date
    Jan 2003
    Location
    Southern Maine
    Posts
    16,714

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

    Agreed, it's not a slow air leak

  15. #15
    Join Date
    Apr 2011
    Location
    Woods Hole, MA
    Posts
    16

    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
    Join Date
    Jan 2003
    Location
    Southern Maine
    Posts
    16,714

    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
    Join Date
    Jun 2003
    Location
    Central Coast, Ca
    Posts
    9,116

    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 07:07 PM. Reason: typo

  18. #18
    Join Date
    Jun 2006
    Location
    Puget Sound, WA, USA
    Posts
    844

    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
    Join Date
    Apr 2011
    Location
    Woods Hole, MA
    Posts
    16

    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
    Join Date
    Jun 2003
    Location
    Central Coast, Ca
    Posts
    9,116

    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
    Join Date
    Jun 2006
    Location
    Puget Sound, WA, USA
    Posts
    844

    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
    Join Date
    Jun 2003
    Location
    Central Coast, Ca
    Posts
    9,116

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

    That is an elegant solution...

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •