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1988 Johnson 48 SPL Cooling Control

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  • 1988 Johnson 48 SPL Cooling Control

    Trying to understand how the cooling circuit works...can't seem to find the right information. When running at lower rpm e.g. 1500 the engine temp comes up to around 140F but when running at higher rpm e.g. 2500 the engine temp indicator plummets to the lowest level which is about 100F. Since the river water is 45F I suspect the engine temp drops lower. I'm sure the bypass valve is opening to cause this temp drop. How is the cooling system supposed to work to maintain a reasonable operating temperature when there is a bypass valve that seems to void the temp control provided by the thermostat? I appreciate a response from anyone who can explain how this is supposed to work.


  • #2
    Re: 1988 Johnson 48 SPL Cooling Control

    First off, welcome to iboats.

    Your pressure and temperature controlled cooling system contains the pressure relief valve (aka poppet), springs, and t stat. The circulation of water through the cooling system by the water pump (impeller) is controlled by the balanced action between the pressure control valve, water pump pressure and the t stat. Water circulates in the powerhead and absorbs heat, and when the power head and cooling system temps reach around 145 F, the t stats open, allowing heated water to pass through the water discharge. The thermostat is constantly monitoring the temp and opening and closing as necessary to maintain the proper running temp. Now as the engine speed is increased, water pump pressure opens the relief valve. This bypasses the t stat, allowing the engine to run cooler when it is running at higher speeds.

    There are recircilating systems as well, with dual water lines.
    Chris

    Top Secret Files:

    http://forums.iboats.com/showthread.php?t=299680

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    • #3
      Re: 1988 Johnson 48 SPL Cooling Control

      Thanks for the explanation. I was hoping there was some way to maintain a reasonable operating temperature when the engine was operating at higher rpm. When the river water temperature is 45F it's going to run quite cool at higher rpms. I guess it was designed that way. Would there not be an advantage in fuel economy if the operating temperature was 140Fat all rpms?

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      • #4
        Re: 1988 Johnson 48 SPL Cooling Control

        Another thought, has anyone ever tried or considered removing the bypass valve and machining the head to accept a second thermostat in it's place to provide more cooling flow at higher rpm and still maintaining operating temperature?

        Comment



        • #5
          Re: 1988 Johnson 48 SPL Cooling Control

          there is not room to get 2 stats in there, even machining. when was the last time you change the tstat and poppet. the spring could be bad especially if used in salt or brackish water.
          FLORIDA GATORS
          TEBOW Country



          Please, NO PM's (Private Messages) regarding boat/engine problems. they will not be answered.
          That is what these forums are for. Post your questions, in the appropriate Forum.

          Comment



          • #6
            Re: 1988 Johnson 48 SPL Cooling Control

            I agree with TD, inspect your spring, and t stat for that matter. It may be stuck open.
            Chris

            Top Secret Files:

            http://forums.iboats.com/showthread.php?t=299680

            Comment



            • #7
              Re: 1988 Johnson 48 SPL Cooling Control

              The thermostat is new, installed in the course of doing a rebuild, rebore at .020 over etc. both cylinders...but the bypass valve spring was used as is and the motor was used in salt water before I obtained it. I guess if that spring has been corroded it's characteristics would change and allow the bypass to open at a lower pressure. I'll change the spring and hope that helps. I really don't like running an engine cold. The t stat was also checked in a boiling water bath and begins to open at 130F and full open at 160F.

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              • #8
                Re: 1988 Johnson 48 SPL Cooling Control

                should not start opening until 140 full open 160.
                FLORIDA GATORS
                TEBOW Country



                Please, NO PM's (Private Messages) regarding boat/engine problems. they will not be answered.
                That is what these forums are for. Post your questions, in the appropriate Forum.

                Comment


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