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  1. #1
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    Low fuel pressure - HELP!!

    2004 175 DFI Model 1175D83CY - Serial 0T972330 System Smartcarft ECM
    I found water in the fuel tank and proceeded to clean the entire system.
    Cleaned the tank
    Cleaned the lines
    Cleaned the vapor separator
    changed the fuel filter
    It starts but idles rough. As if miss firing in a one cylinder or two.
    Checked coils - All good.
    Removed and cleaned all spark plugs ( I was in there anyway)
    Cleaned the fuel injectors (as best as possible at home) checked ohms - all good.
    Cleaned the air injectors
    (as best as possible at home) checked ohms - all good.
    Cleaned and checked tracker valve, fuel pressure regulator, air pressure regulator. All good.

    Air pressure reads 76 PSI
    Fuel
    pressure also reads 78 PSI

    I am testing the low and high pressure fuel pumps.
    The high pressure pump seems ok. If I disconnect it, the engine will not start.
    The low pressure pump, when disconnected does not affect the motor. Starts and runs same.....poorly....
    Is that my problem??? Before I buy a new pump, how do I test it? I do not hear it run when the key is turned on... dam buzzer.

    What am I missing?
    Thanks in advance for any suggestions and guidance.


    Last edited by Flipper2022; 09-25-2022 at 12:35 PM.

  2. Mercury 3L/4 Stroke/Verado Moderator EuropeanAM's Avatar
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    #2
    Better check for a leaking seal at the top of the HP fuel pump... but a good test is to check/record both Air and Fuel pressures:

    -At idle

    -Under Acceleration

    -5/10/15 seconds after engine is shut down (restart for each time period).

    Both must be recorded at precisely the same time, for each above instance. Glycerin-dampened DUAL pressure gauge set required.


    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).
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    #3
    Thank you EuropeanAM !! Always helpful.
    I will check the seal at the HP pump.
    Sadly, I only have one gauge.
    In the test you mention. Am I looking for the pressures to maintain first and then slowly decrease evenly... maintaining the 10 PSI differential?
    Also, my motor has the air and fuel outlets (for measuring pressure) on the port rail. I assume I still should be looking for air to be 79 +/- 2 PSI and fuel 89 +/- 2 PSI.
    I am getting 76/77 in air but highest fuel pressure has been 80 PSI.
    Thank you again for your comments.

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    #4
    Some auto parts stores will loan out fuel pressure gauges for free, might give that a try.
    Mercury 2002 200hp Carbed Saltwater
    OT566776
    2002 Cape Horn 22' Bay

  5. Mercury 3L/4 Stroke/Verado Moderator EuropeanAM's Avatar
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    #5
    Correct- but don't "look for" pressures. RECORD the ACTUAL OBSERVED pressures.

    And yes- both must be taken at the same time, WITH A GLYCERIN-DAMPENED GAUGE SET (unfortunately, very fuel loaner tools will have this type of gauge).


    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).
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    #6
    I will have to sort the gauges out. Looking in the internet, that is a lot of money for a rarely used tool....

    While I figure that one out.....
    I do have one question. I want to verify the HP pump is putting out the 80 PSI it is supposed to have.
    Can I put my gauge, in-line, on the output of the HP pump to the starboard rail to get the pressure when I first turn the key and then while the engine is running?
    Any reason why I would not see the 80 PSI there? Also, can someone confirm the 80 PSI is supposed to remain fairly steady while running?

    The fuel pressure at the port rail (Yes, that is where the fuel test point is in this model) is the result AFTER the fuel has passed the fuel pressure regulator and the tracker valve. I want to isolate the location where the fuel pressure drops.

    Thank you all....

  7. Mercury 3L/4 Stroke/Verado Moderator EuropeanAM's Avatar
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    #7
    That's really putting the cart in a different county than the horse. And unless you isolate and plug-off pathways the fuel travels, moving the gauge from one point to another point in the same circuit will have little affect on the pressure reading.


    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).
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    #8
    Ok, I got gauges now. Glycerin filled and two of them so I can take the measurements at the same time for fuel and air. 70 bucks instead of 100s....
    I still plan to test the fuel pumps themselves. The service manual provides direction about how many amps each pump should be pulling according to the state of the pump. Me being a bit incredulous, I will test the amps and the pressure coming out of each pump.
    I will report the results.... thank you for the help.

  9. Mercury 3L/4 Stroke/Verado Moderator EuropeanAM's Avatar
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    #9
    Be careful- the HP pump is capable of more pressure than the fuel pressure specifications.

    I would still recommend you start with accurate air/fuel pressure test results as suggested earlier, and provide the results.


    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).
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    #10
    Ok, will do it that way. Thanks !!!

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    #11
    Ok, I got to take the readings as suggested.
    On the readings below, keep in mind the needle is bouncing a bit so it is a best estimate on where the needle bottoms.
    I am also testing out of the water on a hose using ear muffs.

    Idle - 600 RPM - Air 72 Fuel 85

    @ 1200 RPM Air 75 Fuel 85
    @1700 RPM Air 75 Fuel 85

    Shut down test - starting, running for a few seconds and then turning off for each reading.
    @ 5 seconds after turn off - Air 65 Fuel 75
    @ 10 Seconds after turn off - Air 65 Fuel 75
    @ 15 Seconds after turn off - Air 62 Fuel 68

    Also, I took the head off from the compressor. Cylinder wall looks good. Reeds look good on both sides. "O" rings all look good.
    Checked Fuel pressure at Port Rail on key turn on - Pressure jumps to about 50 PSI and quickly settles to below 20 PSI

    Still idles rough.

    The service manual says the fuel pressure at the Port Rail should be 89 +/- 2 PSI and the air pressure should be 79 +/- 2 PSI.
    Ok, I am a little below that but from the readings it looks like I am maintaining the 10 PSI differential that is required. Am I wrong?
    I am beginning to suspect either coil (s) or Injectors again.
    Direct injectors near impossible to test because they are buried beneath wires and hoses. Alternative way of testing?????

    One last question that came up last night. The belt seems loose. About 1 to 1.5 inch deflection while running.
    Would a loose belt reduce air pressure output from the compressor ? Seems logical...

    Looking forward to comments and ideas. THANKS !!!!
    Last edited by Flipper2022; 09-29-2022 at 10:15 AM.

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    #12
    if water got to vst you likely have some pump problems , how many hours since belt changed , possible but dout thats your problem
    .................................................. ...the scariest thing in life is the unknown ...................................

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    #13
    Thank you. The belt is new. And I checked the tensioner and also seems to be strong and the base where the spring engages is also good. Maybe the rough idle makes the belts bounce more than normal.
    The VST did have water in it and I am considering changing the HP pump. However, that would not address the air pressure being at 75 PSI max pressure at idle instead of 80. I would think that if the air pressure comes up to 80, then the fuel pressure will come up to 90 as it is supposed to do.
    Maybe the air compressor rings need to be changed.......
    Any ideas?

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    #14
    Ok, this is where I am as of today.
    I decided to test my theory about the air pressure being low. So, I connected my shop's air compressor and set it to 80PSI. Then disconnected the air compressor from the Opti and used the HP line from the compressor to the starboard rail to inject the 80 PSI from my shop. I tested the Air pressure regulator. Prior to getting to 80 PSI, no air leaked from any part of the rails or injectors. Neither from the excess air pressure hose on the port rail. When I pass 80 PSI, the regulator opens and lets the air out as it is supposed to do.

    With the air at 80 PSI, the motor ran 100 % better. So much better that the deflection I was seeing on the belt (about 1.5 inches) went away.
    It would still skip now and then but a marked improvement. By the way, I did this in idle and at about 1200 RPM. I did not want to go passed that.

    The plan now is to replace the compressor piston rings. I already took it apart. It all looks good considering it is a 2004. I will also replace all the "O" rings and anything else that could benefit from a replacement. I hope new piston rings will increase the air pressure from 75 PSI to the required 80 PSI +/-
    I am thinking of running SeaFoam injector cleaner as I continue my testing.
    Does anyone have experience and results from doing that? THANKS !!!

  15. Mercury 3L/4 Stroke/Verado Moderator EuropeanAM's Avatar
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    #15
    The earlier-style compressors parts (nearly all) supersede to a new compressor.

    Make sure you check the reeds/plate very carefully. Leaky reeds will result in lackluster pressure (especially at idle or low RPM's).


    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).
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    #16
    Thanks for the feed back.
    Air compressor piston and cylinder walls are in good shape. So is the connecting rod and bearings.
    The reeds are also good. Checked the stand open specification of 0.020 in and they are spot on.
    I can only believe the rings are not doing a great job pushing only 72 to 75 PSI.
    I got the new rings yesterday and will try to install and test between today and tomorrow. I am also replacing all the "O" rings.
    I'll report air and fuel pressure test results.
    Any comments about using SeaFoam to clean the injectors????

  17. Mercury 3L/4 Stroke/Verado Moderator EuropeanAM's Avatar
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    #17
    Not going to do a lot to help with injectors- and I prefer to avoid the addition of "canned" alcohol to systems that may already have an alcohol problem.


    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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    #18
    While I wait for parts, I thought I would provide an update.
    The air compressor rings have been replaced and it is pumping great. Solid pressure.
    I did find out that the air regulator is letting air out right about 76 to 77 PSI. A little too early but I think it is still ok.
    The fuel pressure is 10 PSI above air
    The idling is much improved but still has a "Skip" every 10 second's or so... not smooth
    I decided to check the reeds and air handler.
    Surprise !! Clearly I have blow back because there is oil in the air handler. I took the reeds out and inspected them. About 70% were partially open. Many within the Mercury specification ( 0.010). I seriously wonder about that speck.... Shouldn't the reeds be 100% closed? I have decided to change all the reeds.
    After some research and reading lots of opinions about reed manufacturers, I decided to get them from Boyesen. Part B26413D for my motor.
    I was able to get them and the gaskets from Pro-Marine.
    I am hoping this will resolve my idling issue.
    I would like the air pressure to come up BUT - Should I invest $275 on a new regulator to gain 3 to 4 PSI?
    Would like opinions on that........

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    #19
    some stand off is acceptable ( rather not be ) but ..... oil in plenum is normal for optis ,,,,, no my opinion
    .................................................. ...the scariest thing in life is the unknown ...................................

  20. Mercury 3L/4 Stroke/Verado Moderator EuropeanAM's Avatar
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    #20
    Reeds- MUST not be pre-loaded. A good seal is critical, BUT even at .010" standoff the reeds can seal QUITE well.

    Oil in the intake is NORMAL and I'd be quite concerned if there WASN'T any oil there.

    Strongly recommend TDR Reeds, I wouldn't use a two-stage reed on that engine for ANYTHING. You'll not like the results.

    Air pressure is well outside of specifications. Verify the same pressure when you swap Air/Fuel gauges around- if it is really that low, the regulator is not functioning properly.


    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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