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  1. #1
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    Merc 2.5 efi, fuel pressure drop

    Hi,

    I'm currently struggling with some fuel pressure issues with my 260hp. I swapped out the gold regulator with an adjustable fuel pressure regulator (with vacuum line and Gauge mounted). I still see pressure drop when i rev the engine fast, and I see 10psi drop when pushing the engine hard. As a quick fix i raised the pressure to 49psi, so that I'll get 39psi when pushing the boat throughout the rpm. I know that it probably run pretty fat on the lower rpms though, so I should get this sorted out.
    Let me shoot in that I started to see pressure drop on the old gold reg, in the end of last season. Have been rock steady before that.

    I have a few questions, If someone feels like educating me, please feel free to do so

    * Is it possible to manipulate the vacuum hose on the regulator, to rule out faults with the regulator? What does this hose do/ how does this work?
    * The vacuum hose to the brucato PCU, what does this do? How does it operate?
    * I have a bosch 044 pump. I have replaced the relay (40A) and replaced the wires to bigger ones. The pump is only 3years old. How would I know if the pump is failing?


    Any help is truly appreciated

    Thanks
    Christian
    Best regards
    Christian


  2. #2
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    take your pump to an efi shop and ask for a flow test

  3. #3
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    Change your fuel filter. Is your vent line open?

    The vacuum hose hose on the regulator should only drop pressure 1-2 lbs at idle

    The vacuum hose on the ecu is a completely separate system, has nothing to do with pressure. Only tells the ecu what load the engine is under, combined with rpm it uses the two inputs for proper fueling

    You could pull the return line and pump exactly one gallon into a marked jug. Time how long it takes. Should be under 1 minute, been a long time but I think it may even be 45 seconds

    I'm guessing your fuel filter is either clogged or not up to the task
    Erik Kiser

  4. #4
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    Thanks for your help and ideas, guys. I will check if the filter is clogged, or just replace it at once.

    I have this waterseperator filter:
    https://www.parker.com/literature/Ra...RAC_Series.pdf

    And it should be good enough for the flow requirements for my motor, right?

    "The vacuum hose hose on the regulator should only drop pressure 1-2 lbs at idle"
    Does this mean that the vacuum hose only have an effect on idle, and not through out the rpm range?

    "The vacuum hose on the ecu is a completely separate system, has nothing to do with pressure. Only tells the ecu what load the engine is under, combined with rpm it uses the two inputs for proper fueling"
    Ok.. I only asked since I was curious about how to check if the MAP sensor is working or not. If I squeeze the vacuum hose tight, will that give me some response on idle, or will i have to have put some load on the engine?


    thanks
    Christian


    Best regards
    Christian


  5. #5
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    The vacuum hose on th regulator only effects it at idle, when the engine is pulling the most vacuum. It wouldn't cause what you're experiencing

    With the engine idling preferably under a load you can just unplug the ecu vacuum hose from the horn. Engine should go full rich
    Erik Kiser

  6. #6
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    Thanks for your help Erik!

    I've tried to swap out my pump, and put in a spare 984 pump. Still the same issue, fuel pressure drops 10 psi at WOT. Also drops 4-5psi at fast rev at Idle.
    So i checked if my tank line was restricted, and put the inlet hose directly in the tank. NO CHANGE..

    I then swapped out my EBAY adj.regulator with the original bosch gold reg. And now my pressure is pretty stable. Although the pressure is 34psi and not 39 :S
    This is the same result when trying 2 different gold regs.

    Could it be that my 10micron filter is restricted, causing the lower pressure? My gauge is electrical, but I cross checked the reading with the gauge on my previous regulator, and they where showing the same values.

    I have my water seperator filter on the High pressure side of the pump, to prevent any restrictions on the pumps sucsion side. Would this be a problem?


    thanks
    Christian
    Best regards
    Christian


  7. #7
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    I would ditch the water separator. Someone may remember but I think Mercury put out a bulletin years ago to not run them on high perf engines but I may be imagining that. Put a good 10 micron or smaller fuel filter inline, I use the Fuelab 6 micron filter. There are several made with replaceable elements, Aeromotive comes to mind, skip the cheap Chinese eBay stuff. Use a good regulator, either use a known good Mercury or name brand aftermarket. A couple hundred dollars spent on the fuel system can save you thousands in repair cost. Lastly, you need some way to verify your gauge is correct. Use a mechanical gauge to verify. I'm assuming you have 3/8" fuel line and a 5/16" return?
    Erik Kiser

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  9. #8
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    Ok, so I'll take your advice and ditch the water seperator. I have already ordered a Bosch inline filter: https://www.ebay.com/itm/Bosch-04509...72.m2749.l2649

    I'm a little unsure about the flow rates on this filter, so I hope it'll be sufficient.

    What about filter before the pump? Would you recommend a 30-100 micron filter?
    https://www.ebay.com/itm/AN8-100-32M...72.m2749.l2649

    As for my fuel lines, I've tried to follow the instructions from Diamond Marine 101 guide, while I use metric hoses, I have:

    * 13mm (1/2") from tank to pump.
    * 10mm (3/8") from pump to filter.
    * 10mm from filter to fuel-rail.
    * 8mm (5/16") return to tank.
    Best regards
    Christian


  10. #9
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    Sjefen,

    I have heard that the Bosch 984 is not good at pulling fuel, it is a pusher. Do you stantion pipe in your gas tank? Possibly with a screen at the bottom? This may be creating a condition that would lead to pressure drop. Bosch and Diamond marine recomend that the fuel gravity feed the low pressure side of the the pump. It should be noted that this configuration would not pass coast guard inspection, which is why some people feed the high pressure pump with a low pressure pump I suspect.
    Whitney Woolard
    Oregon/Washington
    '96 Allison XB2003 w/Mercury 2.5 260ss

  11. #10
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    @Cervelo777
    Thanks for your reply. I have found out that my original pump isn't a 044, but a 957 pump. I tried to swap it out with a 984, as I mentioned above, with same results ( pressure dropping). I've also heard that all these high pressure pumps doesn't like pulling fuel, but It has worked just fine in the past. So I'm looking at some sudden changes that are causing this :S As you mention, I have the fuel tank pipe with a screen on the bottom end, which is described in Diamond marines' 101 as a bad Idea. I could try to rebuild this though. The pump is mounted at the bottom level of the fuel tank.

    But as i said to Erik earlier in this thread, I've got the issue more or less under control now. I swapped out the adj.reg to original gold regulator, and Now have a stable pressure. But it is low, 34 psi instead of 39..

    So what I'll do, with your' advice on hand:

    * Ditch the water seperator
    * Install a 8micron high flow filter after the pump
    * Install a 100 micron high flow filter on the pumps inlet side.
    * Increasing the AN fittings from AN6 to AN8.
    * Rebuild the fuel pipe in my fuel tank.
    * Buy a Aeromotive adj.fuel regulator
    * Swap out my pump for a 044?? Or will this only be a AMP sucking overkill??

    Thanks for all your help!

    Christian
    Last edited by Sjefen; 07-26-2018 at 05:49 AM.
    Best regards
    Christian


  12. #11
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    Christian,

    I had almost the exact same experience. I ended up setting my Aeromotive adjustable at 42 lbs at idle, and it runs at 37 pretty steady through out the rest of the RPM range. (At least that is what my digital dash gauge tells me, which could be the liar in the whole deal) I considered using the 044 also, but don't feel like I can afford the extra juice it draws, (I would rather save it for trolling motor, Electronics, etc.) if it was more of a race boat I would have definatly gone to the 044.
    Whitney Woolard
    Oregon/Washington
    '96 Allison XB2003 w/Mercury 2.5 260ss

  13. #12
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    It may be as simple as this:

    Your new regulator is bad
    Your old regulator is good
    Your gauge does not show correct pressure, so the 34 psi on the gauge is actually 39 psi

    Try a different gauge as a next step.
    Markus' Performance Boating Links:
    www.toastedmarshmallow.com/performance

  14. #13
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    After that, we can get into moving things around.
    Markus' Performance Boating Links:
    www.toastedmarshmallow.com/performance

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