Fuel System Modification: Thoughts?

Discussion in 'Diesel Engines' started by jmwoodring, Jan 7, 2016.

  1. jmwoodring
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    jmwoodring Junior Member

    Hi
    I have an older Yanmar 3QM30. A friend with many years as a marine engine mechanic under his belt suggested a modification to the fuel system to allow it to "self-bleed". While I respect his opinion, I am hesitant to modify the engine because I am not sure it will work with my system.

    Basically, he suggests installing an electric fuel pump in addition to the manual one and adding a return line from the top point of the fuel system back to the fuel tank, which supposedly would allow air bubbles to find their way out of the system. I am concerned that:

    1. Air bubbles will dead end and become trapped at either the injection pumps or injectors

    2. The banjo fitting he suggests modifying isn't actually at the top of the system

    Here are some pictures of:

    1. A fuel flow diagram of the engine
    2. The front of the engine
    3. The modified banjo fitting

    I would be interested to know if you have ever installed or come in contact with similar modifications and what your opinion is of them.

    Thank you!
     
  2. baeckmo
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    baeckmo Hydrodynamics

    The natural question is: have you seen any problems with the existing setup?

    Please note that the fuel "loop" after the primary pump is closed, and under a slight overpressure, meaning that there is no possibility for any air to enter the system, ergo there is no need for any air bleeding.

    ONLY if there is a pressure below atmospheric in the fuel line from tank to primary pump AND a leak in that line, there is a risk for air in the fuel. The injection pump is originally designed so that air cannot enter the individual pump cylinders during the suction stroke, so no air that way.

    These engines are simple and robust in standard condition. As long as the fuel piping up to the primary pump is professionally built (...no leaks), there is no need for a return line. In fact, I have seen a few modifications along the line your fellow is proposing, and they have been troublesome (that's why I was involved). If you take a return from the top of the filter directly back to the tank, it MUST have a pressure regulating valve, otherwise the closed loop part of the system will be depressurized, which means there will be trouble.

    Now if you have problems with this engine's fuel system, first search for leaks in the fuel piping from tank to primary pump. AND DON'T SPOIL THE INHERENT ROBUSTNESS OF A MECHANICAL INJECTION BY ADDING ANY ELECTRICAL DEVICES!
     
  3. FAST FRED
    Joined: Oct 2002
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    FAST FRED Senior Member

    Do not run out of fuel and you should NEVER need to bleed the system.

    Install can type fuel filters that can be filled with diesel before installing .
     
  4. gonzo
    Joined: Aug 2002
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    gonzo Senior Member

    Actually, you need to bleed the system each time the filters get changed. I use an electric fuel pump to bleed diesels. It saves a lot of time. On my boats I have installed them permanently with a switch to run them while bleeding. For costumer boats I have a portable pump with alligator clips.
     
  5. kapnD
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    kapnD Senior Member

    Prefilling new fuel filters is risky.
    You can install a simple outboard motor squeeze bulb in the supply line to fill it without any chance of contamination on the "wrong" side of the filter.
     
  6. kapnD
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    kapnD Senior Member

    If you are getting lots of air in your injectors, it is likely to be due to a problem in the supply side of the fuel sysytem.
    Go through it thoroughly, looking for leaks and clogs, don't forget the tank vent.
     
  7. jmwoodring
    Joined: Apr 2012
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    jmwoodring Junior Member

    In the world of perfect Platonic forms, my engine runs beautifully. In fact, she runs well, despite her age, but some issues with the fuel system were plaguing us on our cruise last year. Several times during operation we experienced loss of power and engine shutdown, resulting in panicked scrambles to change filters, bleed the engine, pull our hair out...

    So, last winter, we took apart and cleaned the entire fuel system, sans the tanks themselves. We took apart and cleaned our multiple shut off and routing valves, as well as the filter housings themselves.

    We also removed an electric fuel pump that was installed in line to assist in bleeding (mechanical robustness is great, but thumbing that little diaphragm pump manually gets old). We removed the electric pump because we realized that it's design incorporated a small, inaccessible choke point into the system. Even though it was installed downhill of our Racor primary filter, we were concerned that crud might be lodged inside. Since we did not have a bypass line around this pump, any flow restriction might have been causing the overall problem. This was supported by the fact that turning on the electric pump sometimes alleviated the issue after the engine would shut down.

    Unfortunately, the cleaning our main tanks is impractical for several reasons. Though I am sure it would be a good idea, I don't have any reason to think that they are terribly dirty or full of growth. I have always kept them full of diesel and our filters rarely have any water or crud in them, even when crashing around offshore. The engine seemed to cut off only when we were motoring for long stretches on calm, flat water and rough rocking never seemed to make any difference.

    Anyway, I hope that has given you some relevant back story to this question.

    Again, if my setup existed in Platonic perfection, I'm sure no air would ever enter the system. When changing primary and secondary filters, however, bleeding becomes a necessity. We have had a lot of practice, but unfortunately we don't always get it done on the first, or even second try (air bubbles can be stubborn). The involves squatting over a hot engine with poor access, and trying not to drop tools, round bolts or strip threads, not to mention get diesel everywhere while still needing to torque down hard enough to get a proper seal. Ideally, we would replace the crush washers each and ever time but we almost always re use them (Gasp!). If this were limited to every 250 hours or so, I guess we would just deal with it, but a simple (?) modification that would eliminate the need is alluring.

    What kind of pressure are we talking here? The line in question is the one coming out of the last fuel injector, back to the secondary filter. My mechanic friend did mention the fact that the aperture sizing in his custom fitting was important to ensure that too much fuel didn't flow out of the system. I guess you would be looking at losing power if that were the case. Still, as long as the pressure into the injectors is being supplied by the injector pump, does back pressure from that part of the system really play a big role? There would still be a head of fuel flowing from that point back up to the tank. Also, the electric fuel pump would be supplying a steady stream of fuel into the system, hopefully in excess of what was in the return line.

    A pressure regulating valve is an interesting idea. Do you think a check valve would work? What about a simple shut off valve that would allow bleeding when necessary, but that could be closed during normal engine operation. That would make for another run of fuel line with a dead end in the system, but might work ok.

    I appreciate your insight here, because these are the kinds of questions I'm not qualified to answer myself.

    I am laughing inside, because I should know better than to come to a forum populated by designers and engineers to ask about shade tree modifications ;)

    Thanks everyone for your input. Since cleaning the fuel system out thoroughly, we have not any any issues in operation this year. Still, this modification appeals to me because it would (hopefully) make the fuel system "self-bleeding". Even though best practice would be to have a perfectly clean tank, lines, system, etc, the truth is that I am operating with a fixed budget, time, and within practical constraints. We have done the best we can to maintain everything as it should be, but I think this modification has practical value to us. I am just trying to educate myself of pitfalls and ensure that in trying to "improve" on the design, we don't make things worse.
     
  8. baeckmo
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    baeckmo Hydrodynamics


    The injection pump inlet must have a certain overpressure "by design", in this case determined by the feed pump pressure setting. If you make a return line without a pressure control valve, there is no overpressure left.

    And a simple, spring loaded check valve is just what I was figuring. I'd prefer having it installed in the tank end of the return pipe, letting the pipe volume function as a buffer. There should be a figure on the pressure setting of the feed pump in the manual. Adjust the check valve spring to a slightly lower pressure.
     
  9. gonzo
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    gonzo Senior Member

    The feed pressure is not critical. It usually hovers at around 5-7PSI. However, if it went up to 20 it would not change the injection pump delivery pressure of about 1800PSI
     
  10. baeckmo
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    baeckmo Hydrodynamics

    Yes, that's ok, but a lower manifold pressure will cause an increase of the free gas volume during injection pump inlet stroke. This because the typical injection pump is, in fact a "supercavitating piston pump", since it has no inlet valve. During the initial part of the suction stroke, the cylinder pressure is reduced below the vapourization/gassing pressure until the inlet ports open.

    If the manifold pressure is reduced, the gassing volume will increase, which primarily will cause an ignition delay, as the free gas has to be compressed before the injection (ie pressure valve opening) can start.

    I am using the term "gassing" here, because diesel fuel is holding comparably high volumes of free gas in colloidal suspension. It is therefore not a classical cavitating situation.

    So, in conclusion: the injection pump can work with reduced inlet pressure, but with consequenses for the performance.
     
  11. gonzo
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    gonzo Senior Member

    I agree. However, an inline electrical fuel pump will increase, not decrease, the fuel delivery pressure. Also, they are free-flow pumps, if they are not running, they will not decrease the pressure considerably; less than a filter.
     
  12. baeckmo
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    baeckmo Hydrodynamics

    ONLY if there is a pressure regulating valve in the return line. If you add a return directly from the filter top without a prv (as suggested by the "mystery mechanic"), the pressure is "floating", no matter the number or type of auxiliary pumps installed.
     
  13. Barry
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    Barry Senior Member

    It appears that you have not been able to clean the tank so there are a couple of other fuel delivery choke points to be aware of.
    1) There might be a filter on the draw tube in the tank. Over a long run, crud can accumulate on the filter screen and choke off the fuel flow. When the engine is stopped, the crud falls off and redistributes itself on the bottom of the tank. Normally this filter screen is above the bottom of the tank but not always

    2) Most fuel tanks have an anti siphon check valve at the top of the tank on the fuel draw line. Several years ago we experienced problems of reduced fuel flow and engine rpm drop. On our inspection, we found these valves, and (they were about 20 years old) there was a build up of debris behind the spring which limited the check valve from opening.

    3) I think that ABYC requires that any electric pump on a fuel delivery system has to be wired to the ignition switch so that it cannot be turned on without the ignition switch being on.

    4)You originally said "fuel tank" singular. If you have two tanks you have to ensure that your switch gear either 1) switches both draw and return lines on each tank at the same time and they must be fail safe, ie manual or 2) the return line could possibly go back into one tank IF there is a common bottom leveling line between the tanks. ( and preferably without a valve to isolate the tanks. Otherwise you would have the risk of the return fuel line going to a full tank, and the excess going out the fuel vent

    IE Two ISOLATED tanks, A and B, your return line feeds A but you draw fuel off B, A will overfill
     
  14. philSweet
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    philSweet Senior Member

    Can anyone provide a reference. My lift pump is wired to the start battery's disconnect switch and runs continuously if the engine is enabled. I can switch the bilge blower off with a toggle, but not the lift pump. Its quiet ticking reminds me to isolate the engine. Since I have a separate disconnect switch for the house batteries, I don't see this as a problem beyond some unnecessary wear and tear. If I'm in a sketchy anchorage, it may get left on for a week.
     

  15. Barry
    Joined: Mar 2002
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    Barry Senior Member


    Hi Phil
    July 2008-2009 ABYC Standards
    H-33 33.14.4
    Electrically operated fuel pump(s) shall be connected to be energized only when the engine ignition switch is on AND THE ENGINE IS RUNNING. A momentary type override (five second) is acceptable for starting

    I had forgot about the "and the engine is running" part.
    This would make any remote power supplied manually switchable electrical pump that is part of the fuel line system to help bleed the system non-conforming. No one would want to be running fuel through an atmosphere exposed bleed fitting in an engine compartment while it is running.

    While ABYC does not post a reason for this rule, I suspect that in the event of a fire in an engine compartment and you turned off the engine, if the electric pump were continue to run, it could feed fuel into the fire.

    If a person added in an electric bleed pump that did not shut off with the ignition switch and loss could be attributed back to the insured, could an insurance company deny a claim???
     
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