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  1. #1

    How The Cooling System Works

    Alright I ma having a problem with my ski and I can't seem to figure out where it is coming from. It is a 2000 Yamaha GP1200R. We recently rebuilt the whole top end, replaced all the oil and fuel lines, replaced the 3rd piston and honed out the cylinder. We replaced all the gaskets and when we went to crank it everything worked fine.

    The problem I can't seem to figure out is that when I hook up the garden hose or when I put it in the lake the engine gets water in the engine some how. I am baffled as too how this is happening? We were sure to crank the engine before we turned on the garden hose as to not back water up into the engine.

    We are going to take it apart this weekend again but I am not really sure what to look at. I am going to replace the head gasket again just to make sure it isn't that and we are going to examine the head for cracks. But outside of that I am lost.

    What I am looking for is a diagram or someone to explain to me the exact flow of water from when it enters the ski to when it comes out. It is an open-cooling system. I know where it comes in and I know that it exists through the exhaust but what I am confused about is the path it travels when it gets into the manifold and the head. If some one could explain that, that would be awesome. Or if anyone knows of something else to check please let me know. I am getting very frustrated with it.


  2. #2
    Happily Self-Employed WFO's Avatar
    Join Date
    Nov 2005
    Houston, Texas
    There have been a couple threads recently with the same problem- "water entering motor after rebuild"

    You probably have the cyl base gaskets on backwards

  3. #3
    Happily Self-Employed WFO's Avatar
    Join Date
    Nov 2005
    Houston, Texas
    Water flow 1200 and 1300 power valve motors:

    The exhaust manifold, U-pipe, and stinger pipes are jacketed, so the water travels around the outside to keep them cool.
    Water travels from the pump, thru the transom plate and enters the engine via the water log on the exhaust manifold. From the exhaust manifold, it enters the cylinders at the bottom, and travels upwards into the cylinder head. From the head, it travels to the U-pipe via the water hoses on top of the head. Water then travels thru the U-pipe into the stinger pipe. There are 2 rubber boots that connect the U-pipe and the stinger... a grey one that connects the exhaust part, and a big black one that connects the outer part of the pipes. Water travels thru the stinger and sprays into the water box, where it exits the boat with the engine exhaust.

  4. #4
    if the engine were pressure tested, wouldn`t of it revealed a leak?

  5. #5
    Steveard's Avatar
    Join Date
    Jul 2010
    Portland, Oregon
    WFO nice post on the cooling system flow and yes if he had pressure tested it properly it would have failed.

  6. #6
    Quote Originally Posted by Steveard View Post
    WFO nice post on the cooling system flow and yes if he had pressure tested it properly it would have failed.
    yup that`s what I was getting at! leak right past the base gaskets if they are installed wrong...if not, then there are other issues that need to be found...

  7. #7
    Alright thanks for the suggestions guys. I agree I should have pressure tested it, I didn't. Amateur mistake for sure. We are going to take the head back off and replace the gasket. I will do a pressure test this time and see what it looks like.

    Thank you WFO for the explanation of exactly how the cooling system works. I was wondering that even before this problem popped up.

    Hopefully its something as stupid as the gasket being put on wrong. I will let you know.

    Thanks Again.

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