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  1. #1
    Join Date
    Oct 2004
    Location
    Ireland, (Crosshaven)
    Posts
    947

    Default fuel system woes

    I lost this season due to fuel system air leakage.

    I did all of the normal things, that I have done on this boat many times previously and in addition to doing the same thing on dozens of pieces of machinery over the years but yet was unable to get it right.. my final thought was that it was a cracked pickup tube in the fuel tank allowing air and fluid to be sucked up, depending on the movement of the pickup pipe...

    Having removed the pickup pipe it seems fine, so I am stumpted. There are two different threads on the banjo bolts used in the system and it may well be that some of these are mixed up and thereby damaging threads/seals.

    I would like to rebuild the sytem with as few weaknesses as possible and add some gauges to show fuel pressure at the pump and vacuum to show if filters are getting blocked or if air is being drawn in ( I presume there is a happy medium which can be established where just fuel is being drawn through a clean filter)

    I would also like to add a secondary supply, I have not really worked this out but I was thinking of a Valved tee in the suppply to the injection pump and feed this with a day-tank mounted above the engine which would be kept fed from a 12v inline pump, with a filter setup from a mitsubishi l200, simple spin on filter with built in bleeder pump.

    Questions.
    Has anyone used a replacement lift pump on an md21b, orbitrade make one for about €200.

    There is a non return valve on the secondary filter housing to which is linked the return line from the highpressure pump.... what effect would removing this connection have ?, it seems to me to be a weakness...

    attached is a pdf of the fuel system..any advice appreciated
    Attached Files Attached Files
    Last edited by pandos; 16-11-19 at 22:45.
    Finally found out how to change my signature...

  2. #2
    Join Date
    May 2007
    Posts
    39,131

    Default Re: fuel system woes

    How do you really know it was air leakage?

    Personally I would consider directing the return flow back to the tank or a swirl pot to extract any air from it.
    Test the llift pump by measuring its output pressure.
    Maybe add a clear filter after it so you can see any air and keep it out of the injector pump.

  3. #3
    Join Date
    May 2001
    Location
    UK East Coast
    Posts
    37,221

    Default Re: fuel system woes

    Quote Originally Posted by pandos View Post

    I did all of the normal things, that I have done on this boat many times previously and in addition to doing the same thing on dozens of pieces of machinery over the years but yet was unable to get it right.. my final thought was that it was a cracked pickup tube in the fuel tank allowing air and fluid to be sucked up, depending on the movement of the pickup pipe...

    Having removed the pickup pipe it seems fine, so I am stumpted. There are two different threads on the banjo bolts used in the system and it may well be that some of these are mixed up and thereby damaging threads/seals.
    I can understand your frustration. The most bewildering air leak I've ever had defied diagnosis for a long time. Like you, I tracked logically through the fuel system, including the pick-up pipe. The second time I did it, when replacing the bolt in the banjo fitting to the tank, the bolt didn't feel as if it was tightening properly. I applied more pressure, and the bolt broke off. It had been cracked for some time, allowing the banjo connection to relax and allow air to leak in. Keep looking, you'll find the problem eventually!

    A vacuum gauge is a good addition; it can show you when the fuel filter is starting to get blocked. I fitted one to my last boat after suffering a diesel bug infestation.

  4. #4
    Join Date
    Oct 2004
    Location
    Ireland, (Crosshaven)
    Posts
    947

    Default Re: fuel system woes

    Quote Originally Posted by lw395 View Post
    How do you really know it was air leakage?

    Personally I would consider directing the return flow back to the tank or a swirl pot to extract any air from it.
    Test the llift pump by measuring its output pressure.
    Maybe add a clear filter after it so you can see any air and keep it out of the injector pump.
    Thanks for the replies,,

    I saw air bubbles dancing in the filter on the engine,

    Not sure how seeing air in the system can can help in keeping it out of the injector pump (except by turning the engine off)..

    The pump produces plenty of pressure,

    yes my inclination is to delete the connection from the filter to the return line and the one-way valve,. not sure what a swirl pot is, but if the engine is operating correctly the return fuel ought not be aerated,?

    My problem with the pump is that I overtightened the connections, trying to eliminate the bubbles, but they were already tight...

    I need to assess the viability of repairing the threads on the pump or fitting inserts to accept new banjo bolts, I ,may be able to solder internally threaded inserts.. (I'd rather keep the pump original than fit an after market one.)
    Finally found out how to change my signature...

  5. #5
    Join Date
    Oct 2002
    Location
    Chichester
    Posts
    839

    Default Re: fuel system woes

    I also had trouble with the fuel lift pump on my Bukh DV20. I had several years trying to track down fuel leaking out rather than air leaking in - my problem was over tightening of fastenings also. In my case - the body fastenings of the lift pump. I had bought a refurb kit for it and in my zeal to make it secure I distorted the flanges of the pump around the diaphragm. The fuel leak, of course, was near one of the banjos so It was difficult to tell where it was coming from. I managed to repair the flanges by grinding them carefully. Is it possible air is getting into your pump that way? It might be if the pump has to lift fuel from below the engine.

  6. #6
    Join Date
    Oct 2004
    Location
    Ireland, (Crosshaven)
    Posts
    947

    Default Re: fuel system woes

    Quote Originally Posted by neil_s View Post
    I also had trouble with the fuel lift pump on my Bukh DV20. I had several years trying to track down fuel leaking out rather than air leaking in - my problem was over tightening of fastenings also. In my case - the body fastenings of the lift pump. I had bought a refurb kit for it and in my zeal to make it secure I distorted the flanges of the pump around the diaphragm. The fuel leak, of course, was near one of the banjos so It was difficult to tell where it was coming from. I managed to repair the flanges by grinding them carefully. Is it possible air is getting into your pump that way? It might be if the pump has to lift fuel from below the engine.
    Yes I suspect it is a banjo connection it seems I have at least two if not 3 different threads on the bolts between the fuel pumps and filter housings, I suspect I mixed up at least one of these thereby upsetting two connections,

    I have found a replacement pump from a tractor source for relatively small money and also a source for fuel lines with pre made connections so when I get a chance to consider things I might replace the system I will try to find a pump or drill and tap one to accept the fittings from the cav filter housing/ the pump if these are identical...... they seem to be 1/2 UNF 20... whilst the lift pumps all seem to take m12 1.0 or even m10 1.0
    Finally found out how to change my signature...

  7. #7
    Join Date
    Jul 2018
    Location
    Bewdley, Worcs
    Posts
    602

    Default Re: fuel system woes

    Connect source of air pressure to the fuel supply pipe, block the return pipe, apply 5psi pressure ... hunt for the leaks/bubbles ... a leak going in the "inwards" direction can be hard to find .. but a leak in the "outwards" direction is much easier to locate.

  8. #8
    Join Date
    Oct 2004
    Location
    Ireland, (Crosshaven)
    Posts
    947

    Default Re: fuel system woes

    Quote Originally Posted by rszemeti View Post
    Connect source of air pressure to the fuel supply pipe, block the return pipe, apply 5psi pressure ... hunt for the leaks/bubbles ... a leak going in the "inwards" direction can be hard to find .. but a leak in the "outwards" direction is much easier to locate.
    the entire system is in bits now so hopefully once overhauled /rebuilt/replaced it will be perfect and will not require further diagnosis... good idea and Ill keep it in mind....
    Finally found out how to change my signature...

  9. #9
    Join Date
    Nov 2001
    Location
    scotland
    Posts
    3,459

    Default Re: fuel system woes

    You may find replacing the copper and alloy washers may make a big difference they work harden and or squash

  10. #10
    Join Date
    Jul 2006
    Location
    Emsworth Hants
    Posts
    12,695

    Default Re: fuel system woes

    > lost this season due to fuel system air leakage.

    Air doesn't normally leak from a fuel system but it can be sucked in through a microscopic hole, diesel leaks out and at that point the engine cuts out. To find the leaks I used kitchen roll wrapped arouud fllters, the fuel pump and and all connections, turn the engine over and you will find the leaks. If the fuel pipes are copper they are renowned for sucking in air, we had them and I changed to nomal fuel pipes.

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