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Mercury outboard overheating

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  • #16
    Re: Mercury outboard overheating

    Problem NOT solved. After replacing the impeller / water pump, both thermostats, the poppet and spring, and cleaning the cooling water strainer, I thought things looked good while running the engine at the dock. The water temperature stabilized at about 134 degrees F, water pressure increased as I revved the engine, peeing looked strong, etc. However, today I took the boat out, and as soon as I increased the speed to about 10 mph, the overheat alarm sounded until I dropped back to idle speed. The engine temperature stabilized at about 175 degrees F. I slowly limped back to port scratching my head. It seems like the overheating problem only occurs when the boat is moving. At port, I revved it to about 3000 rpm with no problem. While moving, the alarm sounded at under 2000 rpm.

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    • #17
      Re: Mercury outboard overheating

      It's not getting enough cooling water, or there is restriction. The pressure gauge is the best diagnostic for that.

      What is the water pressure when it overheats?

      If it's high, there's restriction in the base plate, plugged with debris, or maybe even wrong one.

      If it's low, you're sucking air, either engine too high, or an air leak below the water pump. If it's always low there could be more problems.

      Hope it helps
      John

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      • #18
        Re: Mercury outboard overheating

        I leave the dock and have to cruise along for awhile at under 6mph in a no-wake zone. The water pressure is about 3.5 psi. If I throttle up to about 7 or 8 mph the overheat alarm sounds. As soon as I throttle back down to idle, the alarm stops, and if I run the pee into my hand, it is only lukewarm. The really odd thing is that I can't replicate the problem with the boat tied to the dock. It won't happen in neutral at least up to 3000rpm, and in forward gear up to about 1500 (I don't want to tear the cleat off the boat). However, when I'm under way, the alarm will sound at well under 1500 rpm. Can I possibly have a temperature sensor problem?

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        • #19
          Re: Mercury outboard overheating

          Yes you could. If the temp sensor is going off a little soon, and you are running a little lean at 8 mph, which I would have to assume is very heavily loaded, it could alarm. I think that could make it overheat anyway.

          Is there a chance you're overpropped?

          John

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          • #20
            Re: Mercury outboard overheating

            Not the prop. I've been running with the same prop for the past four years. I'll check the sensors tomorrow.

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            • #21
              Re: Mercury outboard overheating

              I wonder if a loose connection at one of the three temperature sensors could trigger an alarm, especially since the alarm is not triggered unless there is actual physical movement of the boat. Also, it doesn't seem as though the motor is actually overheating since the water "peeing" out is only lukewarm.

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              • #22
                Re: Mercury outboard overheating

                I leave the dock and have to cruise along for awhile at under 6mph in a no-wake zone. The water pressure is about 3.5 psi. If I throttle up to about 7 or 8 mph the overheat alarm sounds.
                A big part of troubleshooting with a water pressure gauge is to monitor the changes in PSI at various RPMs and then consider the troubleshooting chart to either eliminate or narrow down the possible causes. If the PSI is acting normally at all RPMs and you detemine that the motor temp is OK (using alternative means such as a hand held IR temp meter or wax temp stick), you can pretty much narrow down the problem to your sensor or wiring. A multimeter will help in troubleshooting that stuff.

                I suggest that you report back on the PSI readings at various RPMs and the motor temp when using alternative means to measure. This information will help those giving advice on what repairs to try next.
                1996 Mercury 150HP V6 carbed outboard motor and Maxum 1900XR bowrider boat

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                • #23
                  Re: Mercury outboard overheating

                  Fighting same problem (with Merc dealer working on it) with a 2003 125 Carb. (have posted). Replaced alarm switch (t-stat switch with single wire) today and new one did the same thing...I guess it is definitely getting hot.

                  Have also lowered prop pitch and put in another new poppitt. All other parts are new.

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                  • #24
                    Re: Mercury outboard overheating

                    Many of you may have the same gasket failure under the powerhead.

                    The water passage is next to the exhaust passage, as RPM raises, exhaust bleeds into the cooling system, water pressure remains about the same bby cooling is radically reduced until engine speed (and exhaust pressure) falls low enough for full water flow again, and the engine cools.

                    Lift the powerhead and change the gasket pictured in mosi's second pic

                    The exhaust adaptor plate that fits here also has a gasket on the motor side of the plate, change both gaskets.
                    Last edited by CharlieB; July 5th, 2009, 12:16 AM. Reason: Added statement

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                    • #25
                      Re: Mercury outboard overheating

                      Thread move up!

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                      • #26
                        Re: Mercury outboard overheating

                        I haven't yet solved my overheating problem, but I now have a readable water pressure gauge and temp gauge. They're in an LCD display which quickly becomes unreadable when exposed to sunlight, but it's good enough for troubleshooting if I cover it up until I want to read it. I find that my water pressure never goes beyond 4-5 psi, and the temperature slowly creeps up as I'm moving along at channel speeds at about 1000 rpm. As soon as I throttle up, the overheat alarm sounds, not surprisingly because at that point, the temperature is already near 180 degrees F, and the alarm sound at near 190. However, if I put my hand in the pee stream, I find that it's only lukewarm, and if I take the motor out of gear, and run it at higher rpm (about 2000 rpm) the temperature quickly drops down to the 140's. I've removed the thermostats and run water up the water pipe successfully, put in new thermostats (even though the old ones still worked properly), replaced the poppet and spring, rebuilt the water pump, even checked out the oil injection system for leaks, and there's new oil in the lower unit. Now where do I look?

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                        • #27
                          Re: Mercury outboard overheating

                          (continued) A couple things I forgot to mention...I cleaned the air compressor strainer screen, and the pee comes out nice and strong.

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                          • #28
                            Re: Mercury outboard overheating

                            I'm having the same problem. (overheat alarm comes on above idle and shuts off at idle). Merc 2003 150 saltwater carburated V6. I put a new water pump and new stats and it runs great on the hose but overheats in the water. I haven't checked the popet valve yet but i will as soon as I get off duty wendnesday and i'll post results.

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                            • #29

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                              • #30
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                                Re: Mercury outboard overheating

                                Ran the engine again after a cool down. The telltail is flowing water fine and it appeared to have a clog that passed. pressure is 5 psi to 8 depending on rpm. I have a warning light on the gauge that looks like a crank and piston. It is on when I turn the key in the on position prior to cranking and stays on. It came on after the temp light last run it the water but went out after the start during the run with the water hose after checking screen but came back after the cool down indicating a problem (possibly a sensor). Engine appears to be running fine other than the lights and the temp.

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