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Thread: 40hp outboard motor over heating

  1. #1

    Default 40hp outboard motor over heating

    I have a 1993 Yamaha 40hp outboard motor with a jet unit on it that just started dieing on me. I let it sit for 5 minutes and it fires right up and runs great for five of ten minutes and then slowly loses power and dies again. I removed the thermostat and cleaned it (had a small pebble and grass in it). I then placed the thermostat in a glass of hot water and it opened right up so I assumed it was OK. Took the boat out again and it still has the same problem. I checked the plugs and there is no water in the cylinders or in the engine housing. Next I squirted water in the in the internal opening that the thermostat fits into and water poured out the bottom of the jet unit so I don't think the internal water pathways are clogged but I don't know much about the make up of the internal water jacket system so I very well could be wrong about that. The top end of the engine was rebuilt 2 years ago. Any help on what to check next (I will install a new thermostat) or on how to flush out the cooling system would be greatly appreciated.

  2. #2
    Moderator Paul H's Avatar
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    Is anything coming out of the "pee hole" There should be a small tell tale flow that lets you know the water pump is working.

    The impeller on the water pump may have gone south, as they wear out over time. That or the water passages are plugged.

  3. #3
    Member Soundfisher's Avatar
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    Why do you think that it is overheating? Is water peeing out of the motor? If so,I think that it may be dieing because lack of fuel. Crack open the fill cap on your tank a little,leave it and then run it and see if that helps. It may be creating a vacuuum and not letting fuel flow. After it sits for a while the vacuum is released and then it starts. Hope this helps. Let us know.

  4. #4

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    Paul and Soundfisher thanks for the replies. Yes there is plenty of water coming out the pee hole. I was told that the engine could be circulating water and peeing but if the thermostat wasn't opening, no water would circulate through the engine. Soundfisher you may be right about the vapor lock but I'm using the same gas can and hose that I always have used so why would it all of a sudden start vapor locking?

  5. #5

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    Sounds like your carb fills with gas over time, then runs out as you run the motor. I'd be looking for fuel or vent blockage. Can't believe it's overheating to the point of not running in only 5 minutes. I had wasps build a mud-dobber nest in my 3 wheeler gas tank vent line when I was a kid, took me forever to figure out what was going on, but had the exact symptoms you are describing.

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    Something very simple but often over looked as a brain fart, check the vent on your gas can if you don't have internal tanks.

  7. #7
    Member akjeff's Avatar
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    Default Fuel lines???

    Check for deteriorated or cracked fuel hoses. Check inside and outside the cowling. A cracked fuel hose will cause air to be sucked in preventing fuel flow. I had this happen to my chainsaw. Found out it is somewhat common.

  8. #8
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    Default Fuel feed problems

    Can easily be diagnosed by the primer bubble..

    If the primer bubble is sucking down flat, you have a tank vent or flow restriction problem. If the primer bubble has air in it there is a hole in the feed line prior to the primer bubble and you are sucking air, if you can pump the primer bubble and fuel is definatetly flowing through it you have a fuel pump problem, or a hole after the primer bubble, which will be leaking fuel while you hold pressure on the bubble (can be double checked by pumping while running, and motor should not die). If the primer bubble is solid, you have a different problem.

    Next???

    1. When engine dies, use a spark checker to see if you have ignition on all cylinders. Spark should be able to jump 1/4 inch gap or so and be blue in color, with a decent audible snap.
    2. If spark is weak or erratic on 1 cylinder, suspect bad ignition coil or wiring,
    If spark is weak or erratic on all cylinders suspect bad stator windings or wiring,
    3. If spark is good on all cylinders, find mechanic.

    I suspect a fuel problem or a stator problem, since 1 misfiring cylinder will cause a rough run, and usually not die.

    P.S. an overheating motor will feel extremely hot and shoot steam from the pee-hole. If left ignored it will usually die and never restart due to seizure.
    Last edited by Chris_Stout; 10-13-2008 at 00:02. Reason: Overheating

  9. #9

    Default Thanks for all the help

    After reading all the posts I am starting to think the problem may be due to a spark or a fuel problem and not over heating as the motor did not feel extremely hot and has not shot steam from the pee-hole.

    However, the primer bubble is not sucking down flat, and the gas tank is an original Yamaha metal tank and does not have a vent in the cap. I have not been able to find a hole in any gas hose, but I will look again.

    I think the spark is OK and the motor appears to firing on all three cylinders, but I did notice a kind of a reddish brown color on the plugs that I have not seen prior to this problem. I will check the plug fire strength next.

    Again thanks for all the help. Now, how about making this snow and ice go away for a couple of weeks so I can get this thing figured out. I am not ready to pull the sled out yet.

  10. #10

    Default Check the gas cap

    My dad had a 40 hp yamaha that had the same symptoms, but it was an easy fix. The factory gas cap may not look vented but it is. The passages in his gas cap became clogged over time and it would die after several minutes of run time. The easiest way to check it is to run the motor until it dies and then open the tank. If it makes a sucking noise when you unscrew the cap that is your problem. Good luck.

  11. #11
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    FWIW, I've never seen a Yamaha "just die" from overheating. They usually go into a safe mode that limits RPM to just above idle. The same safe mode applies when the oil tank is low. The safe mode won't propel you but it will allow you to maintain directional control.

    Do you use a fuel filter? The plugs' color would make me think contaminated fuel. Perhaps your metal tank needs replacement. Plastic tanks are cheap and reliable.

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    Default It sounds like water too me

    I have lots of experience to these kind of things, so here it is.
    Plugs color change could be running hot?
    Checking spark-good idea.
    Motor into limp mode-wont happen an this older style has no limp mode. However it does have a sieze mode, that is bad so check compression on all 3 cyl's.
    ***Most important take a good beem flashlight, shine in fuel tank and evaluate your fuel. Sounds like water in fuel system, and water will not burn. This water will be seperated from your fuel and have a jelly like appearance to it.
    Your fuel tank must vent.

  13. #13
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    Quote Originally Posted by Paul H View Post
    Is anything coming out of the "pee hole" There should be a small tell tale flow that lets you know the water pump is working.

    The impeller on the water pump may have gone south, as they wear out over time. That or the water passages are plugged.
    You are both right and wrong! My 70hp yamaha was peeing just fine but after 5 minutes or so would over heat when i was trolling. High speed would run all day just not troll down for more than 5 minutes. I would let it sit for a while and it would troll great for 5 minutes then same thing all over again. Found out it was the water pump impeller. I got that fixed and was good to go!

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    Default Yamaha 40

    I've had the same problems as you have. As the others stated, check your fuel vent, re-check your thermostat for more debris, change the water pump, the sqeeze bulb on your fuel line has a internal check valve that my be wearing out and not keeping a good prime,or your fuel pump could be getting weak.Your engine does have a safe mode to prevent engine overheating. When my 40hp was giving me fits as your was, I found very small black deposits in the engine fuel filter. What it turned out being was a the old (8 yrs old) fuel line that was slowly eroding and the rubber was actually entering the filter and some even went into my carbs and at times would partally plug and kill the engine at idle and made it run rough at higher RPM's. Once I had the carbs cleaned and replaced the old fuel hose, I had no more issue's. good luck

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