Mercruiser 3.0 cutting out in gear?

Discussion in 'Gas Engines' started by jln8383, Jul 17, 2010.

  1. jln8383
    Joined: Jul 2010
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    jln8383 Junior Member

    I have a 99 bayline with the 3.0 mercruiser in it. It seems to run very well with the ear muffs on it and when in nuetral as well it will start and rev up just fine. I put it on the water yesterday and it ran great all morning no problems. After lunch we got on it and it didnt want to idle and it really doesnt want to get up on plain the engine starts to cut out. It will eventually go and get up on plain and once you get it going it runs fine until you have to stop again. It seems like it usually happens during the hottest part of the day cause as the sun started heading down and it cooled off last night it ran fine again. This happens with it a lot and im lost with it. I have changed the spark plugs and ran a fuel system cleaner through it. The temp gauge always stays the same it doesnt seem to be overheating. I have read some stuff on fuel vapor lock or maybe the impeller not working properly but im not sure. Im lost can anyone PLEASE HELP?
     
  2. gonzo
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    gonzo Senior Member

    It can have a variety of problems. Very likely it is a combination. It can be the fuel filter, the fuel pump, the carburator, the fuel line, the anti-siphon valve. Vapor locks can be a problem with fuel injection but not as likely with a carburetor. What color and condition were the old sparkplugs in? That will tell you a lot about the engine.
     
  3. jln8383
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    jln8383 Junior Member

    What is the Anti-syphon valve? The old spark plugs actually looked pretty normal cause that's what I was hoping is that they would give me a clue, but they were normal. Think it could be a fuel pump? Thanks for your help!
     
  4. gonzo
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    gonzo Senior Member

    The anti-siphon valve is inside the fitting where the fuel line attaches to the tank. It has a ball with a spring. The valve opens with about 1 PSI of suction. It prevents the fuel from being siphoned into the bilge if the hose disconnects itself from the engine or gets broken. A faulty or clogged valve can give you intermittent problems.
     
  5. jln8383
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    jln8383 Junior Member

    huh very interesting. I did not know about this valve. Sometimes it smells like fuel in the bilge would it cause that too?
     
  6. gonzo
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    gonzo Senior Member

    That is very dangerous. Do you run the blower before starting? The fuel pump may have a ruptured diafragm.
     
  7. jln8383
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    jln8383 Junior Member

    ya I always run the blower everytime. So ur leaning towards the fuel pump then?
     
  8. CDK
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    CDK retired engineer

    Vapor lock could very well be the problem.

    On this engine Mercruiser uses a plastic hose to feed vapor from the carb back to the fuel pump. With age, the pump capacity decreases and on hot days it just mills vapor around, causing the fuel level in the carb to drop, so the engine stalls. Once at speed everything seems fine until the throttle is pulled back.

    Of course the fuel return line should lead to the tank, so this problem would never occur, but Mercruiser deemed that to be too expensive for their cheapest engine....
     
  9. jln8383
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    jln8383 Junior Member

    huh OK well that's what it seems to do. Once I get it going its fine but as soon as I stop again it wont run. So how do i fix vapor lock?
     
  10. CDK
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    CDK retired engineer

    Several solutions are possible, in order of preference:

    -Put a T-piece in the fuel line between the pump and the carb and install a thin copper tube to the top of the fuel tank or to the fuel filter/water separator (if there is one). To the fuel tank is the best. The diameter must be substantially smaller than the tube going to the carburetor, otherwise nothing arrives there.

    -Install a new fuel pump to resolve the problem for the next few years.

    -Keep a large bilge blower switched on at all times or enjoy the engine sound with the cover open.

    -Use the boat in cold weather only.

    I would start by unscrewing the fuel tube from the carb cover and see if there is a very dirty tiny filter in there. I've owned Merc engines with and without it.
     
  11. jln8383
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    jln8383 Junior Member

    Ya I cleaned the little filter inside where the fuel line screws into the Carb, it wasn't very dirty. So a new fuel pump would fix it you think and also the gas smell I'm getting? I read if there is any fuel at all in the little clear tube coming from the Carb that the fuel pump needs to be replaced, not sure why or how that works but I was going to check that out. Thanks for everyone's help I really appreciate it!
     
  12. gonzo
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    gonzo Senior Member

    The pump has a double diafragm..When the first breaks, the fuel gets trapped in between and sucked by the carburetor.
     
  13. powerabout
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    powerabout Senior Member

    Fuel return lines on gas engines are illegal under USCG rules thats why you dont see them.
     
  14. CDK
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    CDK retired engineer

    Yes, I've read that somewhere on a forum, but have not been able to pinpoint where it says so (and why). There sure cannot be a technical reason when every passenger car is equipped with one.

    While searching I did find a Mercruiser document with several "cool fuel systems", so the problem does exist and so does the solution.
    But not for free of course!
     

  15. gonzo
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    gonzo Senior Member

    Cool fuel applies only to fuel injection. It has a cooler in the water line coming from the impeller.
     
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