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Overheating Problem did what damage?

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  • Overheating Problem did what damage?

    2004 25HP Yamaha four stroke outboard model number - F25ELHC
    Motor is mounted on a 16' 9" 2004 Smoker Craft - Motor has appr. 450 hours

    Has been a great running motor for many years with nothing but preventative maintenance things completed each year as required. Water pump was last replaced in the fall of 2015. This past week the boat was warmed up normally and nothing appeared wrong. Boat did have a normal water stream while sitting at the dock. After leaving the marina heading across the lake at full throttle (approximately 23 mph) it started feeling like the boat was loosing power. I was thinking it was just the wind and then I started noticing it was really starting to lose power for sure. Looked back and saw some steam and then it shut down. Removed the cover and it was definitely hot. After getting back to the marina and allowing the engine to cool four about 4 hours, it was restarted and was determined to have a water pump failure. But did appear to be running okay. So now what and what else was hurt by the heat buildup? After a visual inspection of the motor I did not see any paint peeling or any other heat marks. Decided to replace the Water Pump Assembly and Thermostat Assembly. After these replacement the motor ran well in the bucket of water at the shop and had a good stream of water. However, at the marina the motor ran fine at low idle but when I tired to apply more power it did not like it. The motor would basically vibrate hard and just not ran very well - could not accelerate at all. Back to the shop. Next we replaced the plugs because it was acting like it was fouling. Back down to the marina and we still had the same problem. Now what? Basically the shop I am using has a bunch of great people but they are not a Yamaha shop. So it was decided to leave it for the weekend and make some calls on Monday. Well, I was supposed to be fishing in a tournament this weekend and could not so a bunch of my fishing friends got together and started helping with their ideas and thoughts. We checked the fuel filter and just looked things over when someone asked what about the temperature sensor (thermistor)? Could it be bad (shorted or open)? I then measured the unit with a ohmmeter and I found that it was shorted out - zero ohms. I looked up some information on it and if I am understanding it correctly 0.0 ohms would tell the motor (computer or CDI unit) that it was still very hot therefore would still be locked in "Limp Home Mode". I also noticed that when trying to run it in the marina if the rpm's was at 1200, 1500, or even 1850 it would run great and was very smooth running. However, once I tried to go above 2000 rpm's the motor would just start cutting out again. Could this be the reason why it is not running correctly? I no this is very long but I wanted to make sure you had the complete history. So what do you think? If this is it, have I missed anything else? And what does CDI stand for in the service manual? My assumption is it's the Computer. Assuming this does fix the problem will the motor be okay after all of this? Thank you for your time and comments.

  • #2
    try running with the temp sensor/switch disconnected.
    it should not read 0 ohms when cold.


    • #3
      The temperature sensor should read approximately 1250 ohms at 68 degrees F.

      The resistance of the sensor decreases as the engine temperature increases. If the CDI sees a
      resistance of 0 ohms it will definitely put the motor into RPM reduction mode (2000 ~ 3000 RPM).

      Perhaps excessive temperature has damaged the sensor or the wiring related thereto.

      CDI stands for Capacitor Discharge Ignition. It this instance CDI stands for the brains of the motor. The computer. The device that will put the motor into RPM reduction mode if and when it gets information that tells it the motor is too damn hot.

      Hopefully all that is damaged is your sensor. Install a new one and see what happens because you know that yours is out of specification. You might be lucky.
      Last edited by boscoe99; 05-21-2017, 08:50 AM.


      • #4
        I would have expected a simple switch not a sensor -

        impressive engineering esp for 2004, on a little 25 horse carbureted

        guess the "computer" is also using it for - spark advance?

        more importantly -

        shouldn't there have been an audible alert for the overheat?

        hard to read the post - but I don't think I saw that


        • #5
          tiller model , probably does not have a buzzer

          pull the plugs to look for water in cylinders.
          overheat can warp a head and blow a head gasket
          Last edited by 99yam40; 05-21-2017, 10:02 AM.


          • #6
            Yep. Tiller model. F25ELHC. No buzzer.

            Light for low oil pressure only. Of course the light is on the front of the motor, behind the operator. I suppose the thinking is that the operator will feel the RPM reduction and then look at the light. Light on = low oil P. No light = must be over temperature.

            Of course folks ignore buzzers and lights so I suspect they can ignore the motor going into RPM reduction mode.

            No one ever said any two Yams operated the same.


            • #7
              Hopefully no damage. Reading the post, it appears this may the first warning he has gotten on a 13 year old motor. Not bad. As mentioned, many people get multiple overheat warnings with no apparent damage. I wonder if the op could post pics of the failed water pump parts? Would be interesting.
              Last edited by pstephens46; 05-21-2017, 11:08 AM. Reason: Harassed by damn yellow flies


              • #8



                • #9
                  Those yellow flies are vicious sob's. The itch is far worse than the bite. Lasts longer also.


                  • #10
                    Originally posted by fairdeal View Post

                    Yes that is correct. Sitting in garage, typing on iPhone and under attack. Made a couple errors.

                    Their bodies litter the floor at my feet. They are slow but draw blood with their bite.


                    • #11
                      I would like to know how they can land on you without you feeling it? Their slicing into you is painless I understand because of some spittle or other such nonsense that they use prior to making their cut.

                      With Ebola, mosquitos, yellow flies, ticks and fleas it is a wonder that we are not all dead by now.


                      • #12
                        One of many enemy bodies...


                        • #13
                          I love Beaufort (buuufort) and Savannah, and the coast in general, but damn the critters that are there. Everything is looking to eat something else.


                          • #14
                            The various enemy combatants cycle during the year.

                            Sand Gnats - November-May
                            Yellow Flies - May-June
                            Mosquitoes - every month not listed above
                            Fire Ants - year round


                            • #15
                              So here is the latest on the problem,

                              So here is the latest,

                              First off, thank you to all of the good information I have received to date. Yes this particular engine does not have a buzzer for either the temperature or oil because it is a tiller unit. Also because it is a tiller unit the water stream is behind you to the right side of the motor as you steer the boat looking forward with your left hand. Not the best design in the world. Why they don't put a water stream on both sides is beyond me. So yes I should have been paying closer attention to the fact that I felt a lack of power and I should have looked back to see if the water was coming out. This is my first boat and I have had it for 5 years now and have not had one problem. Everyone has told me that these engines are bolt proof and I don't have to worry about anything just use it and go fishing. Well I have learned alot of things after all of this that is for sure.

                              So after putting everything back together and putting the temperature sensor back in the housing of the motor, I verified I still had the same problem. The interesting thing was when I started the motor up I was able to increase the rpm's above 2000. I did not go any higher then 3200 rpm's and then I went back to idle speed. The next time I increased the rpm's I could not go above 2000 this time. Motor was still in "Limp Home Mode". My assumption was earlier the CDI unit was not receiving the input from the temperature sensor or the engine was not warmed up enough. But the good thing was I still had the failure. Next I shut the motor off and disconnected the temperature sensor. Restarted the motor and it ran fine at idle and I was able to increase the rpm's above 2000 rpm's and went as high as 3500 rpm's. The motor was not in "Limp Home Mode". So I think this is the problem. I will hopefully get a new sensor tomorrow and install it and then I hopefully will be able to get back on the water. This brings me to my next set of questions. What next???

                              We did check the compression of both cylinders and we got a reading of 110. I was told that is good. Not sure what the 110 meant but I am trusting the technician. I have replaced the water pump, thermostat and plugs so far. I have checked the fuel filter, oil level, oil color (not milky white or anything indicating water and oil have mixed) and have looked over the complete motor. So have I missed anything?

                              What steps should I take to gradually start breaking the motor back in and what signs should I be looking for in the future????

                              Again, thank you for all your help and I am sorry this is so long.