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Diesel in Bilge?

Discussion in 'Technical Discussion' started by av8tor69r, Nov 22, 2021.

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  1. av8tor69r

    av8tor69r New Member

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    The bilge on my Marquis 59 is usually clean with just a little clean water in there. For weight and performance considerations, I rarely top off my fuel tanks, usually running at half to three-quarters tanks or so. The other day in prep for a longer trip, I topped off the tanks. To my surprise and consternation, after topping off, I found diesel in the bilge. I bailed it out, put it into a container, and found about 2 gallons of diesel and water mixture. What in the world? Has anyone had something like this? Is this some sort of "environmental protection" strategy so overzealous fuelers don't spill fuel...? Any ideas appreciated.
  2. motoryachtlover

    motoryachtlover Senior Member

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    Don’t know what engines you have but on my V block diesels I have had 2 fuel leaks that built up in the “valley” and then ran out when the boat went up on plane. Of course do the obvious and check your fuel filters for leaks. Good luck.
  3. av8tor69r

    av8tor69r New Member

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    Twin MTU 825 HP Must be some sort of leak at the top end of the tanks...
  4. Capt Ralph

    Capt Ralph Senior Member

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    Check vent, fill and sender connections to tank top.
    Err, Ahh, Ohh, Can you get to the tank tops to inspect these items?
    I remember one issue that a anti-vent-serge device was not secured well on a vent line in one DIY customers boat. Would you have any of these devices installed?
  5. av8tor69r

    av8tor69r New Member

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    Thanks Capt. Will check!
  6. Capt Ralph

    Capt Ralph Senior Member

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    Don't forget to lay some oil-sorb sheets around the bilge pump to capture any last fuel that may head that way.
    I'm sure you know already, soapy bilge water is not how to clean fuel oil from your bilge.
  7. av8tor69r

    av8tor69r New Member

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    Thanks for the reminder. I did exactly that with the pads. Still collecting up residue. Was thankful it was diesel and not gasoline I was dealing with!!
  8. Kafue

    Kafue Senior Member

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    Did you discover the fuel in the bilge immediately after filling up or after running the boat with full tanks? If the latter, then it could be bad plumbing from the tank/s to the fuel overflow & therefore not a leakage. If you check one of my post from years ago in regards to a new UNIESSE 48 overflowing diesel whilst throttling up on to the plane, then it might explain my rationale. I would hazard a guess that it is more to do with bad plumbing from the top of your fuel tank/s given that you only found fuel in the bilge after filling the tanks.
  9. Pascal

    Pascal Senior Member

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    How many tanks and where are they? Try tracing which tank the fuel was doubt closest to. Where was the fuel? Center bilge? Under one engine? Close to one of the fillers ?
    As Ralph suggested start with the top of the tank, check vent connections, etc. also the gauge fitting
  10. bayoubud

    bayoubud Senior Member

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    Mycelx Technologies has a Bilge Kleen filter system for use with bilge pumps when cleaning up diesel and oil leaks without putting a $heen on the water.
  11. av8tor69r

    av8tor69r New Member

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    Kafue, Thank you for this trouble shooting logic. I read your post on your UNIESSE 48. Yes, the diesel was in the bilge right after topping off. I cleaned it all up and put a clean white absorbant pad in the deepest part of the bilge and went out for a little cruise this morning. Checked it once back in slip -- Pad is still as white as can be!! Nothing!!! Bad plumbing from fill hole to one/both tank(s)...
    Kafue likes this.
  12. Capt Ralph

    Capt Ralph Senior Member

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    Tell us how you filled up;
    Pump nozzle clicked off, any up spray/surge? Kept filling for a few more clicks?
    Any fuel out the vents?
    Have you checked the tank tops yet?
  13. av8tor69r

    av8tor69r New Member

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    Not terribly sophisticated, but the fueler running the fuel pump filled until he just started to hear gurgling indicating it was almost full and he stopped. No fuel came out of the vents (I've had other boats where this was the case, but not with this boat.) I've checked the tops of both tanks. All clean and no full leaks.
  14. Capt Ralph

    Capt Ralph Senior Member

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    The fuel fills are not above the tanks, still in the engine room?
    Tops of tanks have dry dust, I'm guessing loose hose connect at the fill.
    You did not mention if you have one of those vent whistlers or vent stoppers. If no leak there, I'm still thinking loose fill hose.
    I know of no bad history with the tanks them selves. BUT, if this continues un-resolved, You may want a real tech on board to check deeper.
  15. DOCKMASTER

    DOCKMASTER Senior Member

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    Have you checked your vent hoses? That fuel had to come from somewhere. If not a bad vent hose, is it possible the hose came off where it clamps to the thru-hull vent fitting?
    cleanslate likes this.
  16. cleanslate

    cleanslate Senior Member

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    Something is going on with that vent hose . There is usually some sort of check valve in-line on the vent /over fill hose going overboard. Start at the over flow/vent discharge hole and work your way back to the tank. Check valve on my boat is bronze mounted right on top of the tank. I had to remove my copper vent hose from the check valve, spray it with PB blaster penetrating oil and got a stout screw driver into it to free up the check valve. I was not able to remove it with out major work, let alone fully get to it.

    It gets stuck from sea water spray and bugs/spiders etc.

    Then check the entire vent hose and clamps for the leak. It may not show up with out filling the tank and creating air pressure to see it leak.
    Anyway a stuck check valve will really create high pressure with in the tank. The air has to go somewhere , in place of the fuel.

    As others have said, you have to check ALL the hoses and clamps from start to finish to find the leak, again which might only for now, show up under pressure while filling up.