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  1. #1
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    Replace high-pressure fuel pump in VST and now fuel pouring out at the ….

    Engine 1998 250 merc EFI (pair of them).

    >>>>Background. I needed to replace the high-pressure pumps in both motors (applied voltage to pumps and nothing). Boat sat for awhile after hurricane Ian.

    I replaced port motor high pressure/VST pump (to quantum) and now runs on trailer/muffs -perfect-.
    Replaced starboard high press pump in VST same procedure as above on port.

    >>>>>Pictures: "Zoom out" pic show area of motor of interest. The close ups are just past throttle linkage behind VST. Port pic show wet/fuel at arrow. Starbord show dry area at same location as port.

    >>>>>PROBLEM: After installation of quantum pump on starboard motor and put everything/VST back together as port motor, I squeezed up the primer bulb to get some fuel in the vst. After a handful of squeezes of primer bulb I noticed strong fuel smell. It turns out fuel is pushing out at the location shown in the photo. Everytime I squeeze the primer bulb I get a tablespoon or so of fuel at the red arrow port pic. So much fuel poured out it somewhat filled the mid fairing of the engine with fuel before i noticed (I removed the fuel in the fairing obviously).

    >>>Observation: Typically, I can just squeeze the primer bulb till it gets firm/hard and I stop. However, this time the primer bulb never got fully hard but bulb did get firm but again fuel would pour out everytime I squeezed the primer bulb. For refernce I took a pic of the starboard side of the same area as the port showing totally dry (no fuel).

    ++++Question 1: Did I install the pump or reassemble the VST incorrectly to create this problem?

    ++++Question 2: What are next steps? Take VST apart and look for something…look for what? I was successful with the port and did the exact same procedure but perhaps I missed something.

    Apologies for the somewhat wordy message but I am trying to be super clear.

    Thanks thanks thanks for any help!! My mechanic wanted over a $1000 each motor and 3 weeks to replace high press pumps. So I appreciate any help more than words...
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    Last edited by jkscarab; 04-10-2023 at 12:05 PM.

  2. Member
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    #2
    remove both small hoses on top of vst float is either stuck ,bad or inlet has debri causing it to flood do not start this engine until problem is found you need to drain air plenum of fuel with engine tilted full up , also you may have pumped enough fuel in engine to flood oil res in boat with fuel diluting oil in res ,and hydraulic locking #5 and #6 cyls , be very careful dangerous situation there
    .................................................. ...the scariest thing in life is the unknown ...................................

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    #3
    Thanks JOE54!! Appreciate your detailed reply. Just a little more info would be greatly appreciated.


    Perhaps, the VST float got stuck in the down position (float will not float) when I was replacing the vst pump. I will take apart VST to look at float.
    Question/statement: You wrote: “ remove both small hoses on VST to allow fuel out of the top of the vst”. In the interest of learning what will removing these small hoses do in the context of my problem.

    you need to drain air plenum of fuel with engine tilted full up.
    Question/statement: I have the engine manual. Is there anything in the manual that would describe how to “drain air plenum”? Engine trim works fine so i can tilt it up as your state.

    also you may have pumped enough fuel in engine to flood oil res in boat with fuel diluting oil in res
    Question/statement: I will check resevoir for fuel.

    and hydraulic locking #5 and #6 cyls , be very careful dangerous situation there.
    Question/statement: Would it be sound to remove plugs in cyls #5 and #6 see if they are wet with fuel ?

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    #4
    removing small hoses when you pump bulb will verify vst flooding #2 no remove one of the bottom allem head screws or just loosen and fuel should drain out #3 yes #4 rotate flywheel with kill switch pulled key off with socket on flywheel there is alot of fuel built up in that engine , NO SPARK
    .................................................. ...the scariest thing in life is the unknown ...................................

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    #5
    You wrote: #4 rotate flywheel with kill switch pulled key off with socket on flywheel there is alot of fuel built up in that engine , NO SPARK

    Question/statement: The previous owner removed the kill switch.

    1. I assume I can accomplish the same thing by disconnecting the battery(s) and socket on flywell to turn engine over?
    2. NO SPARK meaning “disconnect battery(s)” but leave all the spark plugs in? If Hydralock is a major concern once I get the VST to shut off properly/ resolved problem, turning the engine over with no spark plugs would remove the risk of a hydrolock situation? You inferred removing plugs was unnecessary?


    Sorry to keep asking you follow up questions but Im almost there/know how to proceed. So thanks again!!!

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    #6
    if there is a large amount of fuel in in crank case and or ports if you spin it whit starter it will blow fuel out plug holes , roll it by hand with plugs out first ,,, if you dont ask you wont know ,,,just dont get hurt or tare up anthing
    .................................................. ...the scariest thing in life is the unknown ...................................

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    #7
    Thanks for your time to help in the "if you dont ask you wont know". Appreciate your patience. This is an old motor but it runs like a beast and so Id like to not do something stupid.


    I believe I have misunderstood your sentence below (copy/paste from earlier).


    #2 no remove one of the bottom allem head screws or just loosen and fuel should drain out


    I thought initally you meant to remove the gas in the VST remove the drain screw in the bottom of the vst but that is a flat head screw I just saw.

    What component are you referring to to "REMOVE ONE OF THE BOTTOM ALLEN HEAD SCREWS OR JUST LOOSEN AND FUEL SHOULD DRAIN OUT" ?? You dont mean the VST as there are no Allen head screws on the VST.

    Much appreciated!!

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    #8
    on the front of the air plenum long bolts that hold it to reed plate either one on bottom , takes allen wrench
    .................................................. ...the scariest thing in life is the unknown ...................................

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    #9
    Thanks again for leading a "horse to water"...From the pic I assume you mean one of the two (allan head) bottom "A" bolts => loosen to let any fuel drain out?

    Thanks again
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  10. Mercury 3L/4 Stroke/Verado Moderator EuropeanAM's Avatar
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    #10
    JK: If you would, please take a couple of minutes to read through the ANNOUNCEMENTS at the top of the Forum, and be sure to provide the Required Info (serial number) when posting. Thanks!

    The engine that is leaking fuel across the throttle butterflies has an over-flowing VST due to a float or needle problem. Either there's debris in the needle, float bent, or some other abnormality there. That's the underlying CAUSE on that engine, but make sure you verify what JOE has recommended as well.

    Above all: Please remember that leaking fuel carries with it a risk of FIRE or EXPLOSION. Don't attempt to run the engine again until this has been addressed/corrected!


    Dual Mercury Master Technician- for Mercury Outboards, Mercruiser and Mercury Racing at European Marine in Greenville, SC.
    Still consider myself a "Marine Apprentice" after 47 years (learn something new every day).
    Mercury Parts, Mercury Outboards, Smartcraft & Accessories, Injector Service, TDR Reeds- BBC Sponsor

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    #11
    EuropeanAM Thanks for the additional info!! I didnt have the serial number handy and wanted to get some info on next steps (ya know how a problem just can get under your skin). I will do as you suggest going forward (serial #). I now understand the fuel across the butterfiles as a float/needle issue. I am going to take the VST apart to investigate to find out why the needle isn't closing properly. And I will carry out the additional steps JOE outlined. Fire/explosion risk noted and understood. Thanks


    One of the corrective actions Joe mentioned was the below copy/paste from above:air plenum bolts.jpg
    1.you need to drain air plenum of fuel with engine tilted full up.
    2.remove one of the bottom allem head screws or just loosen and fuel should drain
    3.on the front of the air plenum long bolts that hold it to reed plate either one on bottom , takes allen wrench

    CONFIRMING>>> Just confirming the allan bolts referred to in the above are the “A” bolts as shown in my pic correct?
    Thanks in advance

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    #12
    yes sir
    .................................................. ...the scariest thing in life is the unknown ...................................

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    #13
    oh fyi your map sensor is broke
    .................................................. ...the scariest thing in life is the unknown ...................................

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    #14
    The zoomed out pic of the motor is not my motor. I just wanted to be clear where my close up pics were taken from . HOWEVER, you are def a wizard to find from the pic. Thanks so much for all your help here. I wish they had a venmo tip option here to throw $10 into your Starbucks fund.
    Thanks again for your time/help

    P.S. On a general note. I am still running the original oil injection pump (motors 350 hours/Year 1997). I am fearful I am going to lose oil to the motor (injection pump failure) and by the time the alarm goes off too late. I added a digital temp gauge to bolt on the motor as a back up to the merc idiot light alarm.

    What are you thoughts on:
    A) replace the merc oil injection pump?
    B) dont mess with it and remove oil injection and run premix ?
    C) go as is ...keep a close eye on temps
    Thanks

  15. Mercury 3L/4 Stroke/Verado Moderator EuropeanAM's Avatar
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    #15
    A) or D) (None of the above- maintain the system by inspecting hoses and replacing mixing valve).


    Dual Mercury Master Technician- for Mercury Outboards, Mercruiser and Mercury Racing at European Marine in Greenville, SC.
    Still consider myself a "Marine Apprentice" after 47 years (learn something new every day).
    Mercury Parts, Mercury Outboards, Smartcraft & Accessories, Injector Service, TDR Reeds- BBC Sponsor

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    #16
    EuropeanAM Thanks for your thoughts. I am concerned about this as most of my buddies have blown their motors at one point and need new/remanufactured power head.


    You wrote: "A) or D) (None of the above- maintain the system by inspecting hoses and replacing mixing valve)."


    Is the "mixing valve" you mention above a component within the oil injection pump assembly itself? I dont see anything else mentioned in the "oil injection flow system" pic below. I assume mixing valve is within pump assembly.... I do see two screws on top of the pump assembly but when I look up the part number for the pump assembly it can only be purchased as an assembly.
    Please advise a little more detail on mixing valve and thanks again. pump with text.JPGoil flow.jpg
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    #17
    +100 to DONS advice , the mix valve/check valve is on bottom of vst that output hose connects to change it now , always insure your warning horn and system is in order it is a very reliable system ,in 1985 when this system came out i was very scared of it also i was informed the pump was tested equivalent to one million hour by MERC MARINE ....... (wizard ) ,,,, its what i do
    .................................................. ...the scariest thing in life is the unknown ...................................

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    #18
    Message received Change mix check valve as shown in pic green arrow for those who may reference this thread in the future.

    As for “wizarding” ….You and European are diagnosing a problem via a few pics/a few sentences. That is pretty fricking wizard like to me!!
    Very generous of you guys to help us DIYers get sh*t done. I am an appreciative guy ….PM me a venmo contact and I buy you both Starbucks or something as a gesture. Your help is worth much more obviously but a gesture never hurt…..check valve.JPG

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    #19
    thats it ... glad to help just a thanks goes along way with me
    .................................................. ...the scariest thing in life is the unknown ...................................

  20. Mercury 3L/4 Stroke/Verado Moderator EuropeanAM's Avatar
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    #20


    Fresh clear lines at the same time is wise... as well as a Service Manual to use as your guide for bleeding the air from the oil injection system (need to supply pre-mix fuel while doing this).


    Dual Mercury Master Technician- for Mercury Outboards, Mercruiser and Mercury Racing at European Marine in Greenville, SC.
    Still consider myself a "Marine Apprentice" after 47 years (learn something new every day).
    Mercury Parts, Mercury Outboards, Smartcraft & Accessories, Injector Service, TDR Reeds- BBC Sponsor

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