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  1. #1
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    1998 Mercury Promax 300 Fuel Delivery Issue?

    I took my boat for an easy run today, as part of my regimen of running it every 45 days or so during winters here on Folsom Lake, CA. I was simply cruising it at 4000 rpms, give or take. No WOT bursts or anything. It was running smooth for about 10 minutes, then suddenly surged two or three times, then suddenly cut out. Upon restarting it, it started and idled for a few seconds, then died again. On the third try, it did not catch at all. It almost feels like it ran out of gas, even though I had 3/4 of a tank from a fill-up before winter, with Marine Stabil added.

    I previously ran it in January, and it ran fine. The boat is garaged year round, so it is not exposed to the elements unless I'm out on the water.

    Upon launching today, I squeezed the primer bulb as I normally do, and it initially felt soft as usual, and firmed up as I primed. But, later, after the engine cut out, I squeezed it to see if I had gas in the line, and it seemed quite firm, almost too firm. Not firm like hardened rubber, but as if it had quite a bit of pressure in it. FWIW, the bulb and some lines were replaced about two years ago, so they're good, but I thought I would mentioned how the bulb seemed over pressurized in case it might be a clue.

    Is it possible that a fuel filter can cause something like this? Last time it was changed was two years ago. If I recall correctly, my filter looks similar to an oil filter for a car. Do I just unscrew the thing to remove it, or must I relieve fuel pressure somehow before removing it? Is there something specific that I should look at on the filter? Like can it be emptied or something to see if might be clogged?

    Any tips on how to test the fuel pump? Or anything else I might look at that might be simple enough to at least determine what might be wrong, like line connections, or...? This time of year is so inconvenient for taking it somewhere, as I suspect a back log of people trying to get their boats ready for spring fishing.

    I'm pretty good with cars, but have always had this motor serviced professionally, aside from real simple things, but I'd be willing to get my hands a bit dirty so resolve this.

    Thanks in advance.
    Last edited by Basnova; 02-20-2020 at 04:42 PM.
    1998 Viper Coral 201
    1998 Mercury Promax 300 with Sportmaster

  2. #2
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    If it has the merc grey fuel line feeding the motor , I would check to see if the inner wall has released from the outer wall causing a restriction in fuel flow. hf...

  3. #3
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    He is correct, sounds like fuel line is coming apart internally. Not uncommon at all. I have never heard of a 1998 300 promax. Don,t just replace fuel line, probably pieces are stuck in fuel system.

  4. #4
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    Thanks for the replies, guys. Am I correct in assuming that the firm primer bulb pressure is a clue? I had lines replaced two years ago, which is why I am asking. I also assume I would need to disconnect the lines and look in them. Do I need to relieve pressure before doing so, and if so, any tips in that regard? I have a service manual, and regarding the fuel filter, it just says to remove it when talking about inspecting or replacing it, but makes no mention of relieving fuel pressure. On my Vette and Suburban, any fuel line disconnects call for relieving pressure, which is why I ask.
    @half fast: sounds like you speak from experience here. If I go look at it, and don't see any Grey lines, because they were recently replaced, would they still be something suspect to you?
    @billybob: wouldn't pieces get caught in the fuel filter? Also, do you mean you never heard of a Promax 300, or didn't know they made one in 1998?

  5. #5
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    Try running it on a 5 gal remote thank with a known good primer ball and fuel line, check valves in those balls can do strange things.

    Dave
    1980 Cougar 19 tunnel,90 2.4L Bridgeport EFI in middle of restoration.
    1988 BAJA Sunsport 186, 96 225 Pro Max
    79 12' Auminum, 95 Merc 9.9
    RIP Stu
    "So many idiots, so few bullets"

  6. #6
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    Quote Originally Posted by Dave Strong View Post
    Try running it on a 5 gal remote thank with a known good primer ball and fuel line, check valves in those balls can do strange things.

    Dave
    Thanks for that suggestion, Dave, good idea. Do you happen to know if there is a special procedure for relieving fule line pressure before disconnecting lines?

  7. #7
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    Quote Originally Posted by Basnova View Post
    Thanks for that suggestion, Dave, good idea. Do you happen to know if there is a special procedure for relieving fule line pressure before disconnecting lines?
    No real pressure just the fuel in the line.

    Dave
    1980 Cougar 19 tunnel,90 2.4L Bridgeport EFI in middle of restoration.
    1988 BAJA Sunsport 186, 96 225 Pro Max
    79 12' Auminum, 95 Merc 9.9
    RIP Stu
    "So many idiots, so few bullets"

  8. #8
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    I had a stalling problem with my 1 year old 300XS so I replaced all of my fuel lines with ethanol proof linerless and then installed a fuel pump rebuild kit that I ordered online. After 40+ hours since, it has only stalled 1 time.

    "Confidence" is that wonderful feeling you had right before you fully understood your true situation.- Unknown
    I don’t know what effect these men will have upon the enemy, but, by God, they terrify me.- Duke of Wellington
    “I am not afraid of an Army of lions lead by a sheep; I am afraid of sheep lead by a lion.” – Alexander the Great
    Whoever said the pen is mightier than the sword obviously never encountered automatic weapons.-Douglas MacArthur

    Allison GSE 300XS

  9. #9
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    Seeing as how my fuel filter is mounted right on my engine, I checked that first, since it's the last thing in the fuel line, aside from a very short hose from there to the motor. Although the filter was installed in 2016, when I unscrewed it, it was full of fuel, and the fuel looked clean. When I looked inside the filter, it looks brand spanking new.

    I'm suspecting it's the fuel pump. I see two wires going to to it. I was thinking maybe I should put a voltmeter on those contact points to see if it's even getting any power. Maybe I should see if I can find a line on the output side of it, and disconnect it to see if it's pumping at all. I'll take a pic of the fuel pump so you guys can see it. It's definitely different-looking, being mounted to the block of the motor rather than in-line somewhere.
    Last edited by Basnova; 02-25-2020 at 06:00 PM.
    1998 Viper Coral 201
    1998 Mercury Promax 300 with Sportmaster

  10. #10
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    I rebuilt the Diaphragm fuel lift pulse pump, not the electric VST pump.

    "Confidence" is that wonderful feeling you had right before you fully understood your true situation.- Unknown
    I don’t know what effect these men will have upon the enemy, but, by God, they terrify me.- Duke of Wellington
    “I am not afraid of an Army of lions lead by a sheep; I am afraid of sheep lead by a lion.” – Alexander the Great
    Whoever said the pen is mightier than the sword obviously never encountered automatic weapons.-Douglas MacArthur

    Allison GSE 300XS

  11. #11
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    Quote Originally Posted by JR IN JAX View Post
    I rebuilt the Diaphragm fuel lift pulse pump, not the electric VST pump.
    Thanks for the clarification. Please excuse my ignorance, but does my motor also have both of these?
    1998 Viper Coral 201
    1998 Mercury Promax 300 with Sportmaster

  12. #12
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    Quote Originally Posted by Basnova View Post
    Thanks for the clarification. Please excuse my ignorance, but does my motor also have both of these?
    Yes, there is a flat diaphragm pulse pump that lifts the fuel from the tank and fills the VST tank. The VST electric pump boosts the fuel at high pressure to the injectors. The pulse pump has a fuel line the connects to the tank.

    Many modify theirs with a boat mounted electric pump that takes the place of both pumps.

    "Confidence" is that wonderful feeling you had right before you fully understood your true situation.- Unknown
    I don’t know what effect these men will have upon the enemy, but, by God, they terrify me.- Duke of Wellington
    “I am not afraid of an Army of lions lead by a sheep; I am afraid of sheep lead by a lion.” – Alexander the Great
    Whoever said the pen is mightier than the sword obviously never encountered automatic weapons.-Douglas MacArthur

    Allison GSE 300XS

  13. #13
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    Quote Originally Posted by JR IN JAX View Post
    Yes, there is a flat diaphragm pulse pump that lifts the fuel from the tank and fills the VST tank. The VST electric pump boosts the fuel at high pressure to the injectors. The pulse pump has a fuel line the connects to the tank.

    Many modify theirs with a boat mounted electric pump that takes the place of both pumps.
    Thanks for that. What I must be seeing on the side of the engine block is the VST. I don't see anything else inline prior to that, and certainly don't see anything in the tank. I'll post a pic of the thing on the side of the block.
    1998 Viper Coral 201
    1998 Mercury Promax 300 with Sportmaster

  14. #14
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    Mercury Mercruiser Marine - PUMP KIT-FUEL (#14360A78)
    https://www.google.com/url?sa=i&url=...AAAAAdAAAAABAv

    "Confidence" is that wonderful feeling you had right before you fully understood your true situation.- Unknown
    I don’t know what effect these men will have upon the enemy, but, by God, they terrify me.- Duke of Wellington
    “I am not afraid of an Army of lions lead by a sheep; I am afraid of sheep lead by a lion.” – Alexander the Great
    Whoever said the pen is mightier than the sword obviously never encountered automatic weapons.-Douglas MacArthur

    Allison GSE 300XS

  15. #15
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    Quote Originally Posted by JR IN JAX View Post
    Mercury Mercruiser Marine - PUMP KIT-FUEL (#14360A78)
    https://www.google.com/url?sa=i&url=...AAAAAdAAAAABAv
    Thank you for that. Unfortunately, I don't think that's my problem. From what I can see, that unit is before the fuel filter, and since my fuel filter is full, I think the little unit is fine. I thought this behemoth was the fuel pump, which is after the fuel filter:
    Click image for larger version. 

Name:	Mercury Promax 300 Fuel Pump.jpg 
Views:	58 
Size:	402.0 KB 
ID:	459365
    If this isn't the fuel pump, what is this?
    1998 Viper Coral 201
    1998 Mercury Promax 300 with Sportmaster

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