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question on bleeding lines

Discussion in 'Ask All Sailors' started by John R, Mar 11, 2018. Add this thread to a FAQ

  1. John R

    John R

    Joined Oct 9, 2012
    107 posts, 11 likes
    Catalina 36
    US Emeryville
    A month or so ago, my boat engine wouldn't start, and it turned out that the cause was air in the fuel line. That, in turn, was due to my having run the engine when it was low on fuel. After bleeding the line, everything was fine. So my question: Why is it that if you have air in a line (fuel or - I think - also coolant) that the pump won't just push the air through the line? Why does the air stop the liquid from circulating?
     


  2. Rich Stidger

    Rich Stidger

    Joined Feb 10, 2004
    2,724 posts, 202 likes
    Hunter 40.5
    US 1997 h40.5 Bristol, RI
    There are typically two pumps in the fuel system. There is a low pressure pump that pulls fuel from the tank and thru the filters and delivers it to the high pressure injection pump. Then the high pressure injection pump forces the fuel at 2000-3000psi to the fuel injectors with the correct timing to be burnt in the cylinders. One would think that with all that pressure any air would be forced completely through the system.
    But the injectors in cylinders are a spring-loaded valve that don't open until the fuel reaches a "pop-off" pressure. That pressure is less than the maximum pressure of the injection pump, but is well over 1000psi. These two devices- the injection pump and the injectors are calibrated and timed to allow fuel into the cylinders at the right time and deliver the right amount. As long as the fuel lines are completely filled with fuel, everything works as designed. The key point is that the diesel fuel cannot compress so the injection pump can push the fuel through the injectors. However if there is air in the lines, when the injection pump pushes the diesel, the air between the fuel and the injector simply compresses because the air is a gas. It is just like a air-shock absorber. So the fuel never gets to the injector and the injector never opens because it never sees the full injector pump pressure. Bottom line is that the fuel never gets to the cylinder, nor does the air get expelled from the fuel lines.
    So when you crack open an injection line at the injector, you allow the air in the lines to be pushed out leaving only diesel fuel in the line. Now the injection pump can push the diesel through the injector and the engine receives fuel.
     


    Mickstr, Whatfiero1, NYSail and 5 others like this.
  3. John R

    John R

    Joined Oct 9, 2012
    107 posts, 11 likes
    Catalina 36
    US Emeryville
    Thanks for the clear explanation.
     


  4. Sumner

    Sumner

    Joined Jan 31, 2009
    5,098 posts, 180 likes
    Macgregor 26S/Endeavour 37 .
    US Utah's Canyon Country


  5. Capt jgw

    Capt jgw

    Joined Feb 8, 2014
    987 posts, 54 likes
    Columbia 36
    US Muskegon
    Yes, great explanation.
    As for the coolant lines, if it's a centrifugal pump, those won't pump air so once the bubble hits the pump it's all done. A rubber impeller pump usually will push the air out.
     


  6. heritage

    heritage

    Joined Apr 22, 2011
    441 posts, 48 likes
    Hunter 27
    US Cherry Point MCAS
    You have explained the air in the system so well that even I can visualize it. Thanks, Rich.
     


  7. Rich Stidger

    Rich Stidger

    Joined Feb 10, 2004
    2,724 posts, 202 likes
    Hunter 40.5
    US 1997 h40.5 Bristol, RI
    I'm glad that my explanation has helped and I appreciate your comments. But I can't take all the credit. My source is the United States Power Squadron course on "Engine Maintenance". This is just one of the many courses that are offered to promote safe boating and to help us boaters to be capable of dealing with the variety of issues that arise from time to time.
    Disclosure: I am a 32 year USPS member, a squadron Past Commander, and have completed every course offered. I highly recommend membership and courses to everyone who goes to sea.
     


    SailormanDan and jssailem like this.
  8. MitchM

    MitchM

    Joined Jan 20, 2005
    592 posts, 73 likes
    Nauticat 321 pilothouse 32
    US Erie PA
    hey rich nice explanation ! i too am a longtime USPS member, but just an EO . i'm currently struggling with junior navigation and sextant sights right now (in case i ever get ship w recked without a gps and need to know where i am ...) unfortunately my sextant sights are putting me somewhere near tanzania. sigh...
     


  9. Rich Stidger

    Rich Stidger

    Joined Feb 10, 2004
    2,724 posts, 202 likes
    Hunter 40.5
    US 1997 h40.5 Bristol, RI
    MitchM - Nothing wrong with EO! I am EO right now and I've also done AO for a few years too. As for JN, the sad truth is that it will all become clear after you complete Navigation! For your sextant sights, just make sure you have the correct time and your known time correction. Have someone else record your time with your sights to eliminate errors in your time. A few seconds of error can put you out of your needed accuracy. But, rejoice, because you no longer need to do the 3-body fix!
     



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