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Mercury outboard overheating

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  • Mercury outboard overheating

    I have a 200 merc ('93) which is fine at idle but temp increases as rpm goes up. Water pump, housing, plugs, etc all changed with no difference. I tried new stats with no change. I pulled the stats out with no change. Now I am wondering about this poppet valve. If I have no stats in now, will the decrease in water back pressure cause this valve to close more...and if I can get to this valve(s) and remove or replace it, will I be better off running with stats or without.
    Also, does anyone have any idea where this poppet valve is on this motor? Someone told me on the starboard side below the block but how do you get to it?

  • #2
    Re: Mercury outboard overheating

    Assuming your engine is mounted on the boat correctly, the poppett is on the starboard side. I'm not exactly sure about your engine but I believe there is a hose coming out the top of the motor that goes down and feeds the poppett. You should be able to locate this hose and follow it down to the poppett. You have to find some creative ways to get your hand down in there to loosen the bolts and remove the parts. It is doable so don't give up. Maybe a mirror and shop light. Socket or screwdrivers with flexible shaft or universal joint work good. There are about 10 parts in there you need to change. I don't think you can buy a packet with all the parts already there so you have to get the individual part #'s on each of these. I wouldn't recommend changing the carrier. You can go to the mercury outboard website and enter your seriel # in the parts express and get the schematics and part #'s. Your local boat shop might have all these also, but go look at the picture anyway. I'm probably gonna get corrected on poppett theory but you asked so I'll try to answer your questions on the thermostats. First thing I know is you should use the thermostats as they control water pressure below the thermostat temp which I think is 143F but check that. Without them your engine will run rough at low speeds because it is too cold. Then as your water pressure increases from about 6-9psi you'll see on your pressure gauge the pressure fall back a little and then pick back up. This is your poppett opening. Now it's the poppett spring versus the water pump determining how much water your getting flushed through the head and it's not controlled by temperature.


    • #3
      Re: Mercury outboard overheating

      The easiest way to access the poppet valve is to remove the lower part of the cowling. There are 4 small bolts/screws that you remove,2 in front, 2 in the back, then tap the lower half gently w/rubber hammer. Then it very easy to get at the poppet. There is nothing in there that will jump out at you, but the spring does have a light load on it. When you put it all back together, make sure that you screw the screw completely in the carrier so the poppet diaphram will seat properly.. You definately need your stats installed.


      • #4
        Re: Mercury outboard overheating

        Thanks for the help. Was able to locate the poppet and replaced it with new one today but overheating is unchanged. If anyone can think of anything else...(not pump, housing, plugs, poppet, thermostats) let me know. Even tried to flush engine with salt-away several times.
        Idles at 170 and under load increases to over 200. If I don't idle it back down, alarm goes off and temp continues to rise.


        • #5
          Re: Mercury outboard overheating

          LDS, I think i have the same engine as you, and I am having the exact same problem. I just posted this yesterday, and slepinin referred me to this thread. My motor is a 1995 Mercury 200hp 2.5L Offshore.

          Yesterday I was out on my boat. Temp was great at idle with no alarms, idled out for about 35 minutes. As I brought it up on plane, the constant steady alarm began to ring. One steady alarm - Beeeeeeeeeeeeeeeep! Check my temp gauge and it is just up over 3/4 (usually it stays in the center). Ease back the throttle and the alarm stops, and temp comes back down to the center. Never heats back up at idle. I have great water flow out the back with a 2 month old impeller. Thinking it was a stuck thermostat, I pull both thermostats out of the top of the heads and try running motor without them in to rule them out. Still get same problem when I throttle up.

          So you replaced the poppet valve, this is where i need to start. Is it on the port or starboard side of the motor. How did you track it down?

          Please remember to post any further findings, and what resolves it when you get to that. I will do the same.




          • #6
            Re: Mercury outboard overheating

            This is probably out of place. As I know almost nothing about these engines and I appoligize up front if it is not even close. I had a Toyota truck once that ran fine drove fine. but if you went over say like 50 miles an hour it would gradually overheat. go slower it would cool off. turned out that the compression was so so and the rings were letting exhaust gasses back into the engine block and heating up the oil. Which could not disapate the heat very well. Talking like 40-50 lbs on compression test. thing was they were all even so the motor did not stumble. was a weird problem but none the less that was the problem. Like I said not sure if that is relatable or not..


            • #7
              Re: Mercury outboard overheating

              The poppet is on the starboard side. See my post up a little ways. Keep us posted on the out come.
              I changed my poppet and the motor is running a little on the warm side, but enough to set off the alarm. I took the poppet back out yesterday and reassembled it, really did not see anything wrong. The new spring was painted black, but 95% of the paint is gone, so water is moving through the poppet. Ran the motor on the hose and everything looked good. Was going out tomorrow, but small craft warning already posed for 15-25 tomorrow, so will wait a couple of days.


              • #8
                Re: Mercury outboard overheating

                Do you have a water pressure gauge installed? With one you can see the pressure changes assoicated with the poppet valve opening/closing. If the poppet valve is stuck closed your engine will overheat at RPMs above 2500-3000.

                Did you replace all the pressure relief valve components, you should? I.e., spring,grommet, diaphragm,etc..


                • #9
                  Re: Mercury outboard overheating

                  my money is on the poppet, bet it aint seating right, spring should be preloaded when installed in block, should have bout 5 to 9 pounds of pressure when assembled. if it dont seal the block, the motor Will get hot when medium idle. should idle with 3 pounds pressure on water pressure gauge, good luck ron
                  me and forest , we be mates, 374vee


                  • #10
                    Re: Mercury outboard overheating

                    I have a similar problem with a 250 EFI Mercury(1996). Replaced water pump impeller, t'stats, poppet valve and still get overheat alarm after running a while at 3000 RPM. Have water pressure and motor pees a steady stream.
                    I understand the motor has a filter screen somewhere in the water system but the powerhead has to be removed to get at it.
                    Any ideas? Short of tearing down the entire engine I am at a loss.


                    • #11
                      Re: Mercury outboard overheating

                      I have been fighting the same battle with my 2004 150hp carb model it started after running across a sand bar. I replaced the water pump, cleaned the water tube (clogged with sand) pulled both stats and flushed back through the engine.It runs cool up to about 2000 rpm but starts to overheat when it comes up on plane. Cools right down when you back off. I will try the poppet tonight and reply back. Does anyone else know of any other places that might have clogged short of a complete disassembly?


                      • #12
                        Re: Mercury outboard overheating

                        As presiously suggested, a water pressure gauge is an invaluable tool for troubleshooting cooling system problems. Reduces the frustration of repairing/replacing parts that do not need repairing/replacing and the frustration of not quickly finding the actual cause...

                        1996 Mercury 150HP V6 carbed outboard motor and Maxum 1900XR bowrider boat


                        • #13
                          Re: Mercury outboard overheating

                          Problem solved.
                          Last edited by BikerBen; June 27th, 2009, 07:25 AM. Reason: Problem solved


                          • #14
                            Re: Mercury outboard overheating


                            What was your problem, and how did you resolve it?



                            • #15
                              Re: Mercury outboard overheating

                              I'll throw my hat in the ring......I have a '92 175hp 2.5

                              Had the same overheating problem. Compression test showed no problems. Stats were changed and rechecked. I had the lower unit 'completely' rebuilt . Mid section was removed where we found the exhaust diverter had gotten so hot the thing was starting to melt and cracked, replaced that. We found a bunch of sand and crap on top of the midsection (pic) and figured that must be the problem. It was all flushed and cleaned and put together. I ran it EZ with a few full throttle passes. The temp alarm did not go off, but I spun a hub on my prop after around 45 minutes. I did not hit anything and I believe it is caused by hot exhaust gases. I do not believe the problem is completely solved. I am installing a temp gauge to see what temps I am running (I believe the overheating horn goes off at 243 degrees so I could be running at 242 degree and no alarm......who knows?) as I did not have a heat gun to check nor did I feel the telltail to see how hot the water was.. The poppet valve had been checked and seemed to be working fine but in desperation I will be replacing the entire poppet assembly.

                              I will follow this thread and follow up with my results. I'm a little tired of this also