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Normal running temperature of outboard engines?

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  • Normal running temperature of outboard engines?

    Hi all,I'm curious, what is the average running temperature of a warmed up Outboard motor? I have a Merc 200 from 75'. I was thinking of adding a thermal switch to my motor in case of overheating but I need to know what the normal (at the heads) temp of the motor is. Anyone have #'s. Thanks again.Adam

  • #2
    Re: Normal running temperature of outboard engines?

    I should search old posts before I start a new one. I found the answer posted by Kclost.Thanks. Adam"KCLOST Senior Chief Petty Officer Member # 11685 posted 07 May, 2004 01:50 PM --------------------------------------------------------------------------------It depends on what you are measuring, Water or Metal.... Your temp. sensor should be measuring metal on the cylinder head, unless someone installed a different style at a later time...Anyway, assuming you have the original style measuring metal, 180F-190F is not out of the question at WOT speeds. Your overheat sensor is set up to go off at 240F by the way.... "


    • #3
      Re: Normal running temperature of outboard engines?

      Enough said. The overheat sensor you have is the key! Do you have an inline 6 or a V-6? Depending on what it is, can determine what sensor you may have... My post above was for a later year motor (if I remember correctly), so let's make sure of what you have first...


      • #4
        Re: Normal running temperature of outboard engines?

        KCLOST,We have done this before,and I hate to do it again..... but I have to.Sorry. As far as a Johnson outboard goes,163 degrees is overheated,reading on the engine surface with a wax stick or pyrometer.I would imagine 240F would be a melt down.Can Mercs run this much hotter?DHP


        • #5
          Re: Normal running temperature of outboard engines?

          inline 6? V6? I wish. No its a Mercury 200 20hp. I was thinking of putting a thermal kill switch on the heads just in case of overheating. There are a few switches that trip at 240F maybe one that trips at 200F. I figured this would cutoff the electrics before the engine seizes or something.


          • #6
            Re: Normal running temperature of outboard engines?

            Way lower! The larger Merc's have 143°F thermostats. Over 170°F is overheating. If it's too hot to hold your fingers on the head for almost all water cooled outboards, it's too hot.

            I don't respond to Private Messages PM's that are motor questions.

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

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            • #7
              Re: Normal running temperature of outboard engines?

              Trust me, on at least the V6 motors of the late 70's to early 90's, even on lower HP motors of that time frame. The overheat sensor is anywhere from 240F to 285F in some cases. This sensor measures the Metal temp of the head just near one of the top cylinders. Underneath the cylinder head cover, not far away from the combustion chamber. It never touches water.Water temps will never get that high however. But 180F-190F metal temps is not out of the question. And is normal for those motors.The problem is, you can't measure the metal yourself where the overheat sensor or temp. sensors are located. They are fixed against the head, underneath the water jacket cover . And the cover has water flowing through them which cools them down to a lower temp, compared to where the sensor is measuring. Just because a thermostat is rated for 143F does not mean that the water will not exceed that. It has to go over 143 to open it, at higher rpms and increased metal temperatures that water can get hotter, how much I don't know as I've never been able to measure it at high speeds. But my numbers of 180F+ refer to aluminum head temps. near the combustion chambers. There is a big difference between water and metal temps! Especially where the sensor is located.Boatster, I thought you had a Merc 200hp... On that 20hp you need to see what type of overheat sensor you have and what temp it is rated to go off.


              • #8
                Re: Normal running temperature of outboard engines?

                I have a 150 Optimax from 2000 with smartcraft and am reading 159 degrees when trolling and 89 degrees when running at W.O.T. I assumed this was water or "coolant" temp. not head temp. Is this correct? Does this sound about right as far as operating temp when trolling or running full out?


                • #9
                  Re: Normal running temperature of outboard engines?

                  ATF, I'm not really familiar with the optimax's. But that sounds good to me.. Again this is water temp readings....89F does seem low though????Where's Trumanlake when you need him?????


                  • #10
                    Re: Normal running temperature of outboard engines?

                    i have to agree with you kc. the way our sensors work the cylider head temp is going to jump right up there (heat and power- directly porportional). but if we had a way to measure the actual water temp after it left the block i would assume that it would be slightly over the thermo temp (approx 150 or there abouts @ partial throttle) but once the poppit opens i would imagine it would drop considerably, because of the amout of water passing through the block. but that is a fine line also, because too much flow and you have no heat transfer to the water. the poppit is very critical for wot. can't figure out how i missed this the last 2 days. sorry


                    • #11
                      Re: Normal running temperature of outboard engines?

                      the more flow the better for heat transfer!
                      if you could get to turbulent flow that would be even better.
                      i threaded a brass 3/8" pipe nipple thru the water jacket into the head and attached a radiator fan switch which you can get at various temps. another post gives the autozone stock numbers and switch temps.