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  1. #1
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    Unhappy Mariner fuel starvation hesitation

    I have a 1994 Mariner 75hp outboard. Runs great for a while (maybe a half hour; maybe an hour), then starts hesitating. I press in the choke, and it will speed up most of the time, but it will slow down again. Usually happens above 2500-3000 rpms, but will stall at idle also when this happens. Had to limp home today (surge-hesitate, surge-hesitate) by pressing the choke off and on. Fuel is visible in the clear fuel filter between the tank and the bulb. Iíve rebuilt the fuel pump, replaced the fuel hose including the bulb, removed and cleaned the fuel tank and pickup tube, replaced the hoses in and out of the fuel pump, replaced the spark plugs, run Sea Foam in the gas. This started about mid-summer. Help!! Ideas anyone? Thanks.

  2. #2
    Lieutenant Junior Grade hkeiner's Avatar
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    Default Re: Mariner fuel starvation hesitation

    Mercury published the following Service Advisory (#2001-17) describing how to confirm fuel starvation:

    http://www.marinepartsman.com/Mercur...2001/EN_17.PDF


    Replacing a lot of parts and doing a lot of tasks to fix a problem (such as fuel starvation) that does not exist can be frustrating. If my motor had your symptoms, I would follow the Advisory to first confirm that fuel starvation is occurring. If it is fuel starvation, then you can consider the components upstream of the fuel pump that might be causing this to occur. If there is no fuel starvation, then you can consider the components downstream of the fuel pump, such as the carbs. For example, here is a discussion where the motor was thought to have a fuel starvation problem and it turned out to be gummed up carbs instead.

    http://forums.iboats.com/mercury-mariner-outboards/1988-mercury-100hp-w-acceleration-problems-215710.html

    These are my suggestions. Others on this forum may have additional or better suggestions.
    Last edited by hkeiner; April 1st, 2008 at 12:43 PM.
    1996 Mercury 150HP V6 carbed outboard motor and Maxum 1900XR bowrider boat

  3. #3
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    Default Re: Mariner fuel starvation hesitation

    If the carbs were gummed up, wouldn't I have the problem all the time? When I first go out, it runs like a champ. By the end of the trip, I was limping along by pumping the choke. ???

  4. #4
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    Default Re: Mariner fuel starvation hesitation

    Inboard tank or portables?

  5. #5
    Lieutenant Junior Grade hkeiner's Avatar
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    Default Re: Mariner fuel starvation hesitation

    If the carbs were gummed up, wouldn't I have the problem all the time? When I first go out, it runs like a champ. By the end of the trip, I was limping along by pumping the choke. ???
    My suggestion was to first use a vacuum meter to confirm fuel starvation rather than assume it based upon the way the motor runs. Doing this test will point you in the right direction either way. I have found that randomly replacing parts and performing repairs based upon a hunch (educated guess) can often take longer and cost more than using a methodical troubleshooting approach. Just a suggestion though.

    Good luck.
    1996 Mercury 150HP V6 carbed outboard motor and Maxum 1900XR bowrider boat

  6. #6
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    Default Re: Mariner fuel starvation hesitation

    Inboard tank. The see-through fuel filter shows gas.
    I'll try the vacuum test. Since it runs fine sometimes -- usually when I first go out, couldn't the vacuum test look OK sometimes and then go bad later? Thanks.

  7. #7
    Lieutenant Junior Grade hkeiner's Avatar
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    Default Re: Mariner fuel starvation hesitation

    couldn't the vacuum test look OK sometimes and then go bad later?
    The vacuum measure is only meaningful if observed when the motor is actually bogging. If the vacuum is within spec when the motor is running fine and it does not increase when the motor is bogging, then you do not have a fuel starvation situation with the fuel line. If the vacuum goes up on the gauge during engine bog, then you do. Once you know this, you can then consider what further troubleshooting steps to perform. The troubleshooting steps and corresponding potential fixes are different depending on whether you have a fuel starvation situation or not. The procedres in the Mercury Advisory also will determine whether the fuel pump is working OK or not.

    Good luck.
    Last edited by hkeiner; March 5th, 2008 at 01:53 AM.
    1996 Mercury 150HP V6 carbed outboard motor and Maxum 1900XR bowrider boat

  8. #8
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    Default Re: Mariner fuel starvation hesitation

    You can have a restriction all the time and have varied rpms just as you are describing. You start out with full float bowls, your float bowls are slowly emptying as your fuel pump cannot keep up (per a restriction). So you start to run out of gas. You bog down and run at much lower rpms, using less gas in the process. Your float bowls start to fill up again and rpms pickup and you repeat this process over again. If you pull the choke and your rpms pickup that tells me you are running lean. Maybe a fuel restriction, bad fuel pump, or maybe most likely in your case an air leak.

  9. #9
    Lieutenant Junior Grade hkeiner's Avatar
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    Default Re: Mariner fuel starvation hesitation

    You can have a restriction all the time and have varied rpms just as you are describing.
    Indian Ground is absolutely correct. The vacuum reading may be outside the spec when the motor is bogging or running fine, or it may be outside the spec only when the motor is bogging. Either way, the vacuum test is a good way to confirm fuel starvation. Just don't check it only when the motor is idling or at slow speed, as any restriction in the fuel system may not show up in the vacuum test at a lower level of fuel demand.
    1996 Mercury 150HP V6 carbed outboard motor and Maxum 1900XR bowrider boat

  10. #10
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    Default Re: Mariner fuel starvation hesitation

    While the engine is bogging, have someone pump fuel bulb. If the engine takes off while you're pumping the bulb, you're one step closer.

  11. #11
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    Cool Re: Mariner fuel starvation hesitation

    It's interesting you talked about pumping the fuel bulb. I forgot to mention that I did that, and the speed picks up. After I do that, one time it might continue OK after, and at another time, it will bog back down again after I stop pumping. What does that indicate? I plan on doing the vacuum test this weekend.

  12. #12
    Lieutenant Junior Grade hkeiner's Avatar
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    Default Re: Mariner fuel starvation hesitation

    That is similar to the symptom another guy had and he found that it was a gummed up carb. The squeezing of the bulb apparently forced extra fuel through the gummed up passages/jets. Your cause could be different, of course, but the fuel starvation test is what helped him figure out his problem.

    My point is that it is easy to jump to the wrong conclusions by interpreting symptoms. Diagnostic troubleshooting using meters and gauges is usually more definitive.

    Below is the link to the same thread I provided earlier. Good luck.

    http://forums.iboats.com/mercury-mariner-outboards/1988-mercury-100hp-w-acceleration-problems-215710.html
    1996 Mercury 150HP V6 carbed outboard motor and Maxum 1900XR bowrider boat

  13. #13
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    Default Re: Mariner fuel starvation hesitation

    You need to know that if you keep running the engine that way instead of getting it figured out, you'll burn pistons. Don't pussy foot around, get it fixed.

    I had a simular problem in a v6. To track it down positively, I put a pressure gauge (15 lb) right on the line to the carburetors. When I was having trouble, the pressure would drop. It turned out to be all of these:

    1. a little sludge in the fuel, plugging the filter
    2. fuel pump
    3. Bad hose (aftermarket)
    4. Plastic aftermarket fuel connector

    Fix was:
    1. Throw out cheap mercury filter, put in a Raco seperator
    2. Clean fuel tank
    3. put in pump kit
    4. Replace cheep aftermarket fuel line parts with the good stuff.

    This is a v6, so it drinks a lot at WOT.

    hope it helps,
    John

  14. #14
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    Default Re: Mariner fuel starvation hesitation

    Yeh it can be a lot of things. I'd start with the cheapest stuff, depending on the outcome of your vacuum test.
    quicksilver carb cleaner. Use as directed on the can
    If your fuel line are perished, just replace them, they're not expensive.
    If you have a fuel filter in the line, replace it
    If you have fuel strainers in your carbs, clean them
    Again, depending on the results of the vacuum test, I'd rebuild the fuel pumps after that. I'd probably just rebuild them anyways depending on when they were rebuilt last. The parts are cheap enough.

    There's also some instructions on this board on a hectically strong blend of Seafoam+fuel and 2 stroke. I do it once a year on my engine. I can't find seafoam here in south africa, so I use "add contents of this can to a tank of fuel" carburettor cleaner. But I make a strong mix like the suggested seafoam mix.
    It basically came down to about 3.5 liters of fuel + can of carb cleaner (0.5 liters) + enough 2 stroke for the lot (and just a drop extra for love). And then I just high idle the mix through the engine (2000-2500rpm).

  15. #15
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    Default Re: Mariner fuel starvation hesitation

    Well, this is what I found. Main problem was the fuel pump. I rebuilt it again, and found that the first time I did it I didn't secure the little plastic pins that hold in the check valves. It's a little odd in how you break the stem off the top of the pin, and after you put the plastic washer and pin in place, you push the stem into the pin to force out the two wings of the pin to secure it. That didn't sink into my mind the first time, and the pins were floating loose in the pump. Runs tremendously better, but I still can be going fine for a hour or so at various speeds, and then the motor will start to bog down as if it was starving for fuel. I ran Seafoam through it twice (1 can to about 7 gallons of gas), but still have the bogging down on occasion, although not nearly as bad. Maybe I should use a stronger mix. I hate the take those carbs apart. I can see that leading to a costly screwup. Thanks everyone for your suggestions.

  16. #16
    Lieutenant Junior Grade hkeiner's Avatar
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    Default Re: Mariner fuel starvation hesitation

    What does the vacuum gauge show when the motor is bogging? Is it constant or does vacuum increase during engine bog?
    1996 Mercury 150HP V6 carbed outboard motor and Maxum 1900XR bowrider boat

  17. #17
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    Default Re: Mariner fuel starvation hesitation

    Unfortunately, when I got a gauge, I got a fuel pressure gauge instead of vacuum gauge. When I got ready to do the test, that's when I realized what I had, so I went on and rebuilt the fuel pump. Also, the hose fittings they gave me were the wrong size. (That's the problem when you're shopping 1 1/2 hours away from your boat, and you're assuming the hose sizes are all the same.) Fuel pressure gauge and correct hose size fittings are my next acquisitions. I'd like to get this thing resolved before I put the boat up for the winter, but time's awasting.

  18. #18
    Lieutenant Junior Grade hkeiner's Avatar
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    Default Re: Mariner fuel starvation hesitation

    Fuel pressure gauge and correct hose size fittings are my next acquisitions.
    I presume you meant fuel vacuum gauge. Anyways, you can still do some effective testing with a pressure gauge by connecting it between the fuel pump and the carbs. It is hooked up in the same manner as the vacuum test. Connect a "T" fitting in line between the fuel pump and the first carb and connect the pressure gauge onto the third connection.

    If the pressure is below specs or if you see it drop during engine bog, this would confirm that fuel supply is an issue and you can go from there. If the fuel pressure is within specs and it does not drop during engine bog, then you know you have to consider those things that are downstream from the fuel pump, such as the carbs or ignition. The shop manual for your motor will specify the fuel pressure specs. As an example, the attached thumbnail shows the specs for my motor, as taken from my Mercury shop manual. Your motor's specs, of course, may be different.
    Attached Images Attached Images
    1996 Mercury 150HP V6 carbed outboard motor and Maxum 1900XR bowrider boat

  19. #19
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    Default Re: Mariner fuel starvation hesitation

    Atleast you did find something wrong with the fuel pump. You can verify correct operation of the check valves by blowing and sucking on the "in" and "out" hose of the pump while it's off the motor. check that those check valves seal properly. if you disconnect your fuel line from the tank you can blow and suck on the end where it goes to the fuel pump's "in". Check for vacuum leaks.
    Easiest technique is to draw a vacuum on the valves or hose, and closing the end by sticking your tongue into it. It should pretty much keep your tongue sucked fast.

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